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  • International Union of Crystallography (IUCr)
  • 1980-1984  (2,506)
  • 1984  (1,211)
  • 1982  (1,295)
  • 1
    Electronic Resource
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    Editorial. Division of Acta Crystallographica (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 1-2 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739482000011
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  • 2
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    On the evaluation of root-mean-square errors in atomic coordinates in protein crystallography (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 432-438 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The calculation of standard deviations for atomic coordinates in human deoxyhaemoglobin A on the basis of various reciprocal-space residuals has shown the resulting values to be in good agreement with each other. Evidence is presented that such calculations may be more reliable than is commonly accepted. It is also shown that Wilson's statistics may be applied successfully to low- and high-angle protein diffraction data.
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    URL: http://dx.doi.org/10.1107/S0567739482000977
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  • 3
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    The origin of the crystallographic pedigree (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 443-453 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The origin of the crystallographic pedigree [Haisa (1978). Acta Cryst. A34, 753-759] is shown to be the Fm3m family, by examining crystal structures of chemical elements and compounds. The family is maintained by making the most of the relevant symmetry sites over the structures of a (Strukturbericht A1), ab (B1), ac (C1), abc (L21), abd, ae, abe, ace (J11), abce (J21), abcde, bcef (D89), abcef (H58), cdf, bdef (D8a), abdef; acefh (D84), ai (D2f), abcgj, abdfk and abcejl types in Wyckoff notation. The descent in site symmetry from Oh to its subgroups may be caused by the balance of the interactions between the neighbors of atomic contents which can be estimated from the lifting of the degeneracy of the nearest-neighbor distances, or the coordination index. When the interactions become out of balance, a descent of the space group through Pm3m, Im3m and P63/mmc to their subgroups occurs to form the crystallographic pedigree.
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  • 4
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    The use of Karle–Hauptman determinants in small-structure determinations. II (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 464-470 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: This paper presents the results of an investigation into the usefulness of Karle-Hauptman determinants for the solution of the phase problem in small structures. The algorithm is discussed in some detail. Results for five test structures are given. For all test structures starting sets of about 25 reflexions with an acceptable phase error of 25° or less could be obtained. Various attempts to extend these starting sets in order to solve the structures are reported. In the writing of the program compatibility with the MULTAN system has been the authors' intention.
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    URL: http://dx.doi.org/10.1107/S0567739482001016
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  • 5
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    The kinematical theory of X-ray spherical-wave diffraction (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 454-463 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is shown that the Fraunhofer approximation, used in the kinematical theory of X-ray diffraction, may fail for a wide variety of crystals with different perfection. The kinematical theory describing the diffraction pattern in the general case is developed. The case of spherical-wave diffraction by a plane parallel crystal is considered in detail. The intensity distribution and the diffraction line width are ascertained to be essentially dependent on the region of diffraction in which the observation plane is located. On the other hand, the diffraction pattern geometry is independent of the diffraction region and is determined only by the crystal structure and the optics of diffraction. The geometry of the diffraction pattern recorded by the divergent-beam method is analysed in detail.
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    URL: http://dx.doi.org/10.1107/S0567739482001004
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  • 6
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    A density-modification method for the improvement of poorly resolved protein electron-density maps (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 21-29 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An efficient computer procedure has been developed for the extraction of regions of contiguous, well-connected high density from a three-dimensional electron-density map. This procedure may be used to generate an extended model volume, from a smaller volume based on a starting atomic model. The starting model may, for example, only include main-chain atoms for a protein, or may omit uninterpretable segments of chain. The procedure may also be used to extract regions of contiguous density where no model exists. The extended model volume is used to produce a properly scaled model electron density. Calculated structure factors are obtained from the scaled model electron density by fast Fourier transform, and combined, with appropriate weights, with the existing phase information to give improved phase angles. Calculation of a new electron-density map with the new phase angles initiates the next step of a cyclic procedure which converges rapidly. The procedure has been applied to the structure determination of tyrosyl-tRNA synthetase. It has led to identification of most of the available amino-acid sequence in the electron density, and a revised tracing of the main polypeptide chain. Evidence for improvement in phase angles is obtained from electron-density difference maps for substrate and inhibitor binding, in which a reduction in background density is observed.
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    URL: http://dx.doi.org/10.1107/S0567739482000059
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  • 7
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    Lattice dynamical calculations of the mean square amplitudes of crystalline biphenyl (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 29-33 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The molecular mean-square-amplitude matrix of crystalline biphenyl, C12H10, is calculated with an intermolecular potential of the atom-atom type. The effect of the presence of the low-lying torsional mode, which interacts with translational modes, is discussed. The Born S-matrix method is used, and proves to be an excellent approximation, which takes about of the computer time of the exact calculation. The resulting amplitudes are in fair agreement with experiment, and show that the extremely high amplitude of libration about the long molecular axis may be satisfactorily explained without assuming a double-well shape for the torsional potential.
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    URL: http://dx.doi.org/10.1107/S0567739482000060
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  • 8
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    Experimental correction for primary and secondary extinction. I. Method (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 642-647 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The energy transfer equations are used to solve secondary extinction in a parallel-sided crystal slab in Laue geometry. Practical formulas are given for determination of the refiectivity per unit length, σ(ε), from the measured absolute powers of the diffracted and direct beams at rocking angle ε = θ- θB. The calculation assumes non-divergent beams, and the experimental tolerances are considered in detail. The primary extinction factor is defined by yp(ε) = σ(ε)/σkin(ε), where σkin is the kinematical reflectivity. When yp 〉 0.5 it may be approximated by yp ∼ exp[-(αδ)2], where α is about 0.5 and δ is the average size of the coherent domains when measured in units of the extinction distance Λ, δ = D/Λ. The extinction distance can be varied by changing the X-ray wavelength or polarization, and yp(ε) can be determined from two measurements.
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    URL: http://dx.doi.org/10.1107/S0567739482001326
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  • 9
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    Treatment for the intensity problem of n-beam kinematical reflections in a dynamical formalism (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 41-48 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The use of exponential functions as an approximation for reflection powers leads the intensity problem of n-beam kinematical diffraction to an eigenvalue problem. It is solved in the way similar to an n-beam Borrmann dynamical diffraction problem. Besides the intensities of Bragg-reflected beams, the excitation of modes concerning the intensity attenuation is also calculated for highly absorbing infinitely thick crystals. Experiments of multiple reflection from GaAs, InAs and InP single crystals for Cu Kα radiation were carried out. Comparison between the experimental and calculated reflected intensities for several 2-, 3-, 4-, 5- and 8-beam cases are given and discussed.
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    URL: http://dx.doi.org/10.1107/S0567739482000084
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  • 10
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    On the application of phase relationships to complex structures. XXI. An extension to the MAGEX procedure (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 683-685 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In the previously described MAGEX procedure [Hull, Viterbo, Woolfson & Zhang Shao-Hui (1981). Acta Cryst. A37, 566-572] trial phases found from a ψ map were refined by a parameter-shift process which maximized a function depending on the overall satisfaction of the triple-phase relationships. Advantages have been found in using a different function which has a somewhat stronger physical basis. Two algorithms are described which may be used either for centrosymmetric structures or for individual reflexions which can take only restricted phase values.
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  • 11
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    Low-resolution neutron diffractometry with a position-sensitive multidetector (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 670-679 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A data reduction system for neutron crystallography using a two-dimensional planar multidetector is described. The method is based on an a priori calculation of the intensity distribution for each reflection for any crystal orientation. The orientation matrix of the crystal has to be known. From the calculated intensity distribution a mask is derived for each reflection. The detection elements of the detector which are inside the mask are summed to determine the intensity of that reflection and the cells outside are used to determine the background. The size of the mask is adapted to the relative height of the measured intensity of the reflector compared to the background. The theoretical intensity distribution for each reflection is calculated in reciprocal space from the primary-beam divergence and wavelength spread and the crystal mosaic spread, on the basis of the kinematic theory of diffraction. The derivation of the necessary equations, i.e. the determination of the resolution function of the instrument, is presented. Some typical results of data collection are also presented. The advantages of this data collection method are the safe determination of weak reflections, the easy discrimination of reflections which nearly overlap, the determination of crystal mosaic spread, and the discrimination of inelastic scattering in the proximity of intense reflections.
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    URL: http://dx.doi.org/10.1107/S0567739482001363
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  • 12
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    High-resolution electron microscopy of McGillite. II. Polytypism and disorder (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 695-702 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A new series of polytypes of a sheet silicate mineral, mcGillite, has been found using high-resolution electron microscopy. Their crystal structures are described in terms of the stacking sequence of the one-layer monoclinic structure [Iijima (1982) Acta Cryst. A38, 685-694]. Multiplicity of the polytypes results from regular occurrences of the 120° rotation twinning on the basal (001) plane. The two- and three-layer polytypes found in the present study are both monoclinic and are different from two-layer schallerite and three-layer friedelite, belonging to the family of manganpyrosmalite and being reported as trigonal.
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    URL: http://dx.doi.org/10.1107/S0567739482001405
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  • 13
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    Determination of the position of even the lighter anomalous scatterers in a crystal structure by means of a two-wavelength technique (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 710-717 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: For identical anomalous scatterers among a majority of normal scatterers the structure-factor amplitudes are estimated from intensity data obtained either with two wavelengths at one side of the absorption edge or with one wavelength at either side. Direct methods (or a Patterson synthesis) will allow the localization of these anomalous scatterers. Test calculations on the known structure of the iron-containing protein ferredoxin, simulating the effects of synchrotron radiation, show the feasibility of the procedure.
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  • 14
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    A comment on the method of determination of the polarization ratio for crystal-monochromated X-rays by Vincent & Flack (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 739-740 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: By reference to previously unpublished tests, the validity of the experimental procedure proposed by Vincent & Flack [Acta Cryst. (1980), A36, 614-620] for the measurement of the polarization factor is questioned.
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    URL: http://dx.doi.org/10.1107/S056773948200148X
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  • 15
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    Electric field gradients and charge density in corundum, α-Al2O3 (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 733-739 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The charge density in α-Al2O3 has been refined with respect to X-ray structure factors [complete spheres for both Mo Kα and Ag Kα radiations with (sin θ/λ)max = 1.19 and 1.495 Å-1 respectively] and electric-field gradient tensors at both atomic sites, using Hirshfeld- type deformation functions. Atomic charges from a κ refinement are + 1.32 (5) for Al and -0.88 (8) e for O. The charge distribution of O is polarized towards the four Al neighbours, and metal-metal bonds are clearly absent. Quadrupole coupling constants and asymmetry parameters of the field-gradient tensors cannot be determined from the structure factors. They define the quadrupolar deformations near the atomic centers, and the X-ray data define the charge density between the atoms. The orientational parameters of the tensors and the signs of their largest eigenvalues can be qualitatively retrieved from the X-ray data. The refinement of anisotropic secondary-extinction parameters may, however, destroy this information. Refinement with respect to the field gradients affects mainly the quadrupolar deformation terms, and has little influence on the X-ray scale factor (i.e. monopolar terms) and computed electrostatic fields (i.e. dipolar terms). Properties of the charge density with different angular symmetries are thus only weakly correlated.
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  • 16
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    Electron microscopy of Li2O–Al2O3–SiO2 glass ceramics (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 741-743 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A combination of high-resolution electron microscopy and electron diffraction methods is used to obtain microstructural information of lithium-bearing glass ceramics, Li2O-Al2O3-4SiO2 with TiO2 as a nucleating agent (system A), and of the commercial system 0.68Li2O3 Al2O3 6.1SiO2 0.13ZnO 0.03Na2O 0.01K2O 0.11TiO2 0.077ZrO2 (system D). The experiments reveal the presence of small amounts of γ-spodumene in system A. In system D, the volume fraction of residual glass is estimated and the microstructure of various nucleating agents is elucidated.
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    URL: http://dx.doi.org/10.1107/S0567739482001491
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  • 17
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    Notes and News (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 749-750 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    URL: http://dx.doi.org/10.1107/S0567739482001533
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  • 18
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    A matrix method for lattice symmetry determination (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 748-749 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A general approach for the determination of metric lattice symmetry has been successfully tested on 3̃0 000 lattices from the National Bureau of Standards Crystal Data File. The central focus of the method is on the determination of matrices relating any primitive cell of the lattice to itself rather than on determining reduced cells or conventional cells. The method can conveniently be used in routine structure work as it readily detects the highest possible metric symmetry within any specified range of cell-parameter errors.
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  • 19
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    Matrix isolation spectroscopy by A. J. Barnes, W. J. Orville-Thomas, A. Müller and R. Gaufrés (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 751-751 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 20
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    High-resolution electron microscopy of the incommensurate structure in Sr2Nb2O2 (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 780-789 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The incommensurate structure of strontium diniobate, Sr2Nb2O7, is formed below 488 K with a one-dimensional lattice modulation along the [100] direction. A model of the atom displacements due to this lattice modulation has been proposed from an analysis of the systematic extinction of the extra reflections due to the incommensurate structure. High-resolution electron microscopy clearly reveals the lattice modulation in the incommensurate phase. The observed images are well interpreted by image simulation of the proposed model.
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  • 21
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    The temperature dependence of the Debye–Waller factor near a phase transition: CsPbCl3 (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 790-796 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: By fitting effective one-particle potentials to Bragg neutron diffraction intensities measured by Sakata, Harada, Cooper & Rouse [Acta Cryst. (1980), A36, 7-15] for the cubic perovskite caesium lead chloride, CsPbCl3, it has been established that, contrary to the results of Sakata et al., the effective one-particle potentials for the Cl ion are neither independent of temperature nor of a highly anharmonic form. To the resolution limit of the data, collected out to sin θ/λ = 0.58 Å-1, the one-particle probability density function of the Cl ion is singly peaked. It is proposed that the strong temperature dependence of the effective one-particle potentials, and the accompanying anomalous temperature behaviour observed for the mean-square displacements of the Cl and Cs ions above the anti-ferrodistortive phase transition temperature, result from soft-mode vibrations. The temperature dependence of the mean-square displacement in the presence of soft modes is derived and it is shown for the first time that a significant temperature-invariant component will be the main additional effect at temperatures well above the critical temperature. The Cl and Cs ions in CsPbCl3 obey this derived mean-square displacement relationship.
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  • 22
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    The zeolite A debate: how wrong is a nearly correct space group? (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 821-825 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The apparent contradiction between NMR and X-ray results concerning the Si, A1 distribution in zeolite A is quantitatively analysed. It is concluded that the Fm{\bar 3}c structure model, based on X-ray refinements, is compatible with the NMR data if a cautious chemical-shift assignment is applied. The appropriate use of crystallographic approximations and their symmetry is discussed.
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  • 23
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    A theory for the evaluation of small-angle scattering diagrams of quasi radially symmetric particles considering polydispersity and deviations from radial symmetry (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 835-840 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A general theory has been developed describing the scattering intensity of a polydisperse dilute ensemble of particles. These particles are assumed to be mainly radially symmetric. Deviations from radial symmetry are treated quantitatively using an expansion of the electron density in terms of multipole components. The particle radii are assumed to have a Gaussian-like distribution. The electron density of the particle core is allowed to be different from that of the solvent. As a practical application of this theory a Fortran 77 program was written which determines the radial electron density profile, the standard deviation of the radii, and additional parameters describing the deviations from spherical symmetry directly from the measured intensity.
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  • 24
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    Theoretical considerations on phase determination by three-beam interference (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 841-848 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: For φ-scan experiments rotating the crystal through a three-beam case the rocking-curve: profiles have been calculated on the basis of the dynamical theory of X-ray diffraction. Divergence and wavelength spread of the incident beam have been taken into account. It is shown that for centrosymmetric crystal structures the asymmetry of the profiles is related to the sign of the triple product of the structure factors F({\bar {\bf h}})F(g)F(h - g) which are involved in a three-beam case. If anomalous absorption ('double Borrmann' effect) can be neglected the typical asymmetry is independent of the diffraction geometry (Laue or Bragg case). For Laue geometry and thick crystals (μ0 t 〉 1) corrections are necessary. For centrosymmetric structures it may be possible to determine the phase sum of a triplet by inspection of the rocking curve without computer calculations. The general features of the rocking curve are discussed and a physical interpretation is given.
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    The reflectivity of a pyrolytic graphite monochromator (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 859-863 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is shown that a pyrolytic graphite monochromator crystal scatters X-rays as a mosaic crystal in accordance with the Darwin formulism, both for reflecting power and integrated intensity, over a range in wavelength from 0.5 to 1.54 Å. The scattering cannot be considered kinematic and, from estimates of the reflectivities parallel and perpendicular to the diffraction plane, polarization ratios are calculated which are in accord with published values. A simple description of the scattering process is given. The variation of polarization ratio with mosaic spread is discussed.
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    The electronic state of O2− in MgO studied by the Compton profile method (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 155-161 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The electronic state of the O2- ion in MgO is studied in detail by measuring the isotropic Compton profile and profiles in crystal directions [100], [110], and [111]. The theoretical profiles are calculated using linear combinations of local orbitals orthogonalized to each other symmetrically. The orthogonalization is performed with the 'exact' cluster method. Nine different local wave functions of the O2- ion have been used as the basis of the calculations. The experimental and theoretical profiles as well as their Fourier transforms are compared with each other. The orthogonalized Watson +1 potential-well wave function and the orthogonalized LCAO band function of Pantelides, Mickish & Kunz [Phys. Rev. B. (1974), 10, 5203- 5212] were found to describe best the ionic state of O2- even though neither of the functions gives complete agreement with the experiment.
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    Six regular polygons at a four-connected vertex (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 165-166 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Six angles are formed between four straight lines meeting at a point in space. Since there is one relationship between them. only certain combinations of six regular polygons will fit together at such a point. Some of these are enumerated.
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    An X-ray diffraction study of lithium (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 163-165 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An accurate X-ray diffraction investigation has been carried out on two single crystals of lithium. The data from crystal 1 were obtained at temperatures of 248 and 296 K, and those for crystal 2 in the temperature range 293 to 423 K. The measured integrated intensities have been analysed for the presence of anharmonicity in the atomic vibrations by using a temperature factor based on a one-particle potential of the form V(u) = ½ αu2 + γu4 + δ(u4x + uy4 + uz4 - u4). An average value for the parameter δ of 290 ± 140 eV nm-4 was found. No significant value of γ was found in the analysis of data from crystal 1, but a value of -360 ± 40 eV nm-4 was obtained from the higher-temperature data of crystal 2.
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    Structural studies on A-cation-deficient perovskite-related phases. I. ThNb4O12, thorium/vacancy ordering in slow-cooled samples (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 177-186 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The A-cation-deficient perovskite, Th0.25NbO3, i.e. ThNb4O12, when slowly cooled from the melt, presents an interesting hierarchy of ordering phenomena. The associated diffraction effects have been studied with electron microscopy/diffraction and X-ray diffraction techniques. Three main types of ordering processes occur, with different degrees of long-range order. A primary ordering of thorium atoms into alternate (001)p planes of A-cation sites (cell ap × bp × 2cp, P4/mmm) exhibits well-established long-range ordering. A secondary ordering of thorium atoms within the (001)p planes (cell 3\sqrt{2\alpha_{p}} × \sqrt{2b_{p}} × 4cp, Immm) is short range in nature and gives rise to superlattice reflections in the form of diffuse rods directed along g(110)p and g(\bar 110)p. The length of the rods corresponds to correlation lengths of only 20-30 Å between {110}p planes of thorium atoms, and the thorium/ vacancy ordering is adequately described by a sinusoidal modulation model, with accompanying modulated displacements of niobium and oxygen atoms. Thirdly, a system of octahedral tilts about [110]p or [\bar 110]p axes is described by a cell \sqrt{2\alpha _{p}} × \sqrt{2b_{p}} × 2cp, Pmam. Independent models for thorium/ vacancy ordering and octahedral tilts have been refined with the intensities of the corresponding groups of satellites measured from precession photographs. The complete model for ThNb4O12 can be described in the unit cell 3\sqrt{2\alpha _{p}} × \sqrt{2b_{p}} × 4cp, P2mm. The superlattice reflections arising from the octahedral tilts are split into groups of four satellites in the form of crosses, owing to microdomain formation in ThNb4O12, with domain boundaries parallel to (100)p and (010)p and with average widths of 2̃5 Å.
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    The effect of crystal bending on direct phasing of electron diffraction data from cytosine (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 207-211 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Limiting conditions for the crystal structure analysis of organics using electron diffraction intensity data from elastically bent microcrystals are shown for a representative aromatic structure, cytosine, C4H5N3O. In a projection down the longest unit-cell axis, the normalized structure-factor magnitudes are greatly changed by slight bends, making the diffraction data useless for crystal structure analysis. This alteration of intensity is less severe for a projection down the shortest cell axis and allows a correct structure analysis for bends comparable to those measured experimentally. The correct crystal orientation, moreover, is only achieved by epitaxial growth and not solution growth.
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    Pseudoatom expansions of the first-row diatomic hydride electron densities (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 225-239 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Representations of high-quality molecular electron densities are studied. An evaluation of restricted radial functions is made using a least-squares figure of merit, the molecular dipole and quadrupole moments, the electric fields at the nuclei, the electric-field gradients at the nuclei, an approximate energy and difference-density maps. For the heavy atom, a satisfactory representation has a fixed core function with a variable population and requires optimized dipolar core polarization functions, and an additional monopole term. The heavy-atom valence regions, and the H require expansions to at least the quadrupole level, with one Slater-type radial function per multipole and all exponents optimized. Additional valence radial functions and higher multipoles are required to give completely satisfactory difference-density maps but do not consistently improve the physical properties.
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    A procedure for joint refinement of macromolecular structures with X-ray and neutron diffraction data from single crystals (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 239-247 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A procedure is presented for the stereochemically restrained least-squares refinement of macromolecular structures with neutron and X-ray diffraction data from single crystals. This procedure has been tested by refining a model of ribonuclease A using neutron data to minimal spacings of 2.8 Å and X-ray data from within 2.0 Å spacings. Joint X-ray and neutron refinement is well conditioned and tends to avoid false minima that may occur when a medium-resolution structure is refined solely with the neutron structure factors.
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    Toward a secondary-extinction correction (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 248-252 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The treatment of secondary extinction currently in general use by crystallographers is due to Becker & Coppens [Acta Cryst. (1974), A30, 129-147]. It is derived from approximate solutions to the Darwin transfer equations developed by Zachariasen [Acta Cryst. (1967), 23, 558-564]. Among the approximations invoked by Zachariasen is that the effects of scattering and absorption are to be treated separately. A scheme is here proposed which makes that unnecessary. It is further suggested that progress in the correction of experimental data for secondary- extinction effects is more likely if one begins with the exact solution to the transfer equations due to Werner [J. Appl. Phys. (1974), 45, 3246-3254]. That solution has so far not penetrated the diffraction literature, probably because it is in the form of a difficult infinite series of Bessel functions. In one special case, which nevertheless has attracted the general attention of those interested in this subject, it is shown that the Werner series may be summed, yielding an integral equation involving only the zero-order Bessel function. Excellent closed-form approximations to this integral, accurate to about 1% or less, are displayed.
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    On the co-existence of structurally different regions in the low–high-quartz and other displacive phase transformations (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 252-256 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The mechanism of the low = high quartz phase transformation is discussed in terms of tilting of [SiO4] tetrahedra and formation of Dauphiné twin domains below the transformation temperature. The low-quartz domain boundaries have a finite thickness. Within the domain wall the tilt angle changes gradually from + φ to -φ. Depending on temperature, the gradual change may be static or there may be a change of the average value of φ due to a dynamical process. Around the center of the wall between the low-quartz domains the structure resembles that of high quartz. The co-existence of low- and high-quartz-type regions in a temperature range below the transformation temperature is a necessary consequence of the formation of coherent domains. It is concluded that analogous ranges of coexistence of low- and high-symmetry regions exist in many, if not all, displacive phase transformations in which the phases involved have subgroup-supergroup symmetry relations and, therefore, form twin domains (dielectric, ferroelastic phase transformations, etc.).
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    Non-periodic central space filling with icosahedral symmetry using copies of seven elementary cells (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 257-264 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is shown that copies of seven elementary cells suffice to fill any region of Euclidean three-dimensional space. The seven elementary cells have four basic convex polyhedral shapes and three of them appear in two different sizes. The space filling is non-periodic, has a central point, and preserves the full icosahedral group.
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    The analysis of powder diffraction data (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 264-269 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A comparison has been carried out between the results of analyses of several sets of neutron powder diffraction data using three different methods: the Rietveld method [Rietveld (1967). Acta Cryst. 22, 151-152; (1969) J. Appl. Cryst. 2, 65-71], a modification of the Rietveld method to include off-diagonal terms in the weight matrix [Clarke & Rollett (1982). Acta Cryst. Submitted] and the SCRAP method, which involves the estimation of observed Bragg intensities [Cooper, Rouse & Sakata (1981). Z. Kristallogr. 157, 101-117]. Two simulations have also been carried out to demonstrate the way in which the results can differ in more extreme cases. This study has confirmed that the values of the estimated standard deviations given by the Rietveld method are not reliable and that, of the methods considered, only the SCRAP method will in general give reliable values for the estimated standard deviations of the structural parameters.
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    Symmetries and properties of non-rigid molecules edited by J. Maruani and J. Serre (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 88-88 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    On integrating the techniques of direct methods and isomorphous replacement. II. The first applications (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 294-300 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: With error-free diffraction data from the protein cytochrome c550 from Paracoccus denitrificans, having molecular weight Mr ∼ 14 500, space group P212121, and a single PtCl2-4 derivative, estimates (0 or π) of the three-phase structure invariants are obtained by recently secured direct methods employing the six-magnitude first neighborhood [Hauptman (1982). Acta Cryst. A38, 289-294] and compared with the known values. The comparison shows that these methods are capable of estimating reliably several tens of thousands of those invariants having the extreme values 0 or π, approximately. It is therefore anticipated that direct methods will play an increasingly important role in the determination of macromolecular structures.
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    High-resolution images of ordered alloys by high-voltage electron microscopy (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 310-317 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The image formation of high-voltage, high-resolution electron microscopy of ordered alloys has been studied on the basis of many-beam dynamical diffraction theory. It is revealed that superstructure images are observable for a rather thick crystal when nearly kinematical relationships hold among certain beams of the superlattice reflections; these beams are almost in phase and have amplitudes proportional to their structure factors. Thickness dependences of the phase differences and the scattering amplitudes are calculated for the superstructure of D023 type of the gold-based alloys Au3X (X = Mg, Zn and Cd). The results are discussed in connection with the difference in atomic scattering factors of the constituents X. The contrast of the superstructure image is discussed in terms of the amplitude-phase diagram of the superlattice reflections.
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    Appreciation of Co-editor's Service (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 167-167 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Molecular dynamics simulation of crystalline naphthalene (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 297-305 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A realistic molecular dynamics calculation for naphthalene is presented and a comparison is made between the simulation and the crystallographic data. The molecular dynamics sample consists of 4096 molecules arranged in a volume with cyclic boundary conditions, each molecule being associated with one processing element of the ICL DAP computer. The potential functions used are the same as those used for lattice dynamics, and are developed from atom- atom 6-exp functions. A comparison with a lattice dynamical result establishes the correctness of the program. Temperature is introduced in the usual molecular dynamics way, resulting in a true modelling of anharmonic behaviour. The calculation proceeds at zero pressure throughout, yielding the temperature variation of the crystalline unit cell, the mean molecular orientation and the rigid-body thermal vibration tensors T and L. These are compared with the recently reported results measured by neutron scattering from powder samples and analysed through constrained refinements. The variation of unit-cell volume with temperature is particularly close to the experimental result, though discrepancies begin to be significant in measurements involving the orientational behaviour of the molecules.
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    Points, lines and walls in liquid crystals, magnetic systems and various ordered media by M. Kléman (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 309-310 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Union Office Computer (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 309-309 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Errors in bond lengths from using wrong scattering factors (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 308-309 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The incorrect assignment of atom type leads not only to incorrect thermal parameters but in unfavourable circumstances, such as having a limited data set or through overweighting low-order reflections, to incorrect bond distances.
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    Point defects and defect interactions in metals edited by J.-I. Takamura, M. Doyama and M. Kiritani (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 310-310 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Vectors and tensors in crystallography by D. E. Sands (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 312-312 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    On the solution of the phase problem. II. Seminvariant distributions fitted by comparing their function values (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 340-346 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An a posteriori method of the determination of a correct set of structure-factor phases based on a comparison between the trial and theoretical distribution functions of semivariants, using the χ2 test, makes possible the full utilization of a priori structure information contained in the phase relationships. It is expected that the application of this method should raise the efficiency of existing direct methods.
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    Lattice-dynamical evaluation of temperature factors for aromatic hydrocarbons, including internal molecular motion: a straightforward systematic procedure (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 350-356 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: As a part of a systematic project of evaluating temperature factors for molecular crystals in a harmonic approximation, following the Born-von Kàrmàn procedure, application to essentially 'rigid' aromatic hydrocarbons has been reconsidered. In this approximation, where 'rigidity' just implies non-mixing between internal vibrational modes and external ('lattice') modes, contributions of the former to the temperature factors of the various atoms have been added to the corresponding contributions of the latter modes. Following Gwinn's method for normal coordinate analysis in the molecule, and Pawley's method for calculating the lattice modes, it is possible to write highly efficient routines, so that practical application to the various molecules becomes easy and straightforward, the final result being reached in a quite reasonable computing time, with a 'usual' crystallographic input. For intermolecular interactions, Williams IVa potentials have been used, and a Califano-Neto harmonic force field has been adopted for calculating in-plane internal vibration modes; a suitable value for torsional force constants has been assigned for calculating the out- of-plane modes. For these calculations, which have been carried out on benzene, naphthalene (h8 and d8), anthracene, phenanthrene and pyrene as examples, the relative importance of intramolecular contribution can be seen in all these compounds, with respect, for instance, to the ring members (here only carbon atoms), or to the peripheral members (here only hydrogen atoms). Because of TDS influence upon the experimental results, the agreement with the observed B's is generally less good than for simpler models, where the internal modes are neglected. For H atoms, the calculated internal contributions to the B's are too high in the case of benzene and naphthalene, and about right for pyrene. The situation is discussed in terms of the effect of packing upon internal modes, and relative non-rigidity of pyrene with respect to smaller molecules.
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    Vibrational smearing of deformation densities in diatomic molecules. An application of Hartree–Fock–Slater SCF calculations (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 372-377 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The smearing of the deformation density in the diatomics H2, CH, Bell, CO and N2, caused by internal vibration, is calculated using a large number of LCAO Hartree-Fock-Slater electronic wave- functions. Both the effect of anharmonicity and the thermal population of rotational and vibrational states at 300 K are considered. The main conclusion is that the effect of the smearing is very small for these molecules. Compared to the effect of the anharmonic zero-point vibration upon the charge deformation density, the influence of including higher states at 300 K is negligible.
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    A study of generalized intensity statistics: extension of the theory and practical examples (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 362-371 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Generalized probability density functions, cumulative distribution functions and moments of the normalized structure amplitude |E|, depending on space-group symmetry of the crystal and on the composition of the asymmetric unit, were extended to include the tenth moment of |E| and five-term expansions. The formalism was also simplified and is presented in a concise and unified form. The equations linking the formalism to practical problems, the composition and space-group terms, are discussed from a practical point of view and a convenient implementation of the above statistics in a computer program is indicated. The generalized cumulative distributions of |E| and of the normalized intensity z = |E|2 are compared with corresponding distributions based on five published structures, each containing one outstandingly heavy atom (Pt, Rh and Br) and about twenty light ones in the asymmetric unit, excluding hydrogens. These examples indicate that the above formalism is a valuable tool for resolving space-group ambiguities which cannot be treated by conventional methods because of effects of atomic heterogeneity. N(z) distributions for a structure belonging to the space group Fddd show that the theoretical expressions correctly predict the existence of different intensity distributions in reflection subsets with hkl all even and hkl all odd for this space group.
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    Characteristic Debye temperature of ammonium fluoroberyllate, (NH4)2BeF4 (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 388-390 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: From the elastic stiffness constants Cij of ammonium fluoroberyllate at room temperature, wave velocity surfaces have been calculated and are shown in three different sections (XY, YZ, ZX). The mean sound velocity in the crystal has thus been determined and used to calculate the Debye temperature of (NH4)2BeF4. The Debye temperature so determined is 381 K. Some of the bulk elastic properties of polycrystalline (NH4)2BeF4 have also been calculated using the above Cij values.
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    Anatomy of a Bragg reflexion and an improved prescription for integrated intensity (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 378-387 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Using a narrow aperture in front of a detector, the intensity distribution of a single Bragg reflexion has been determined experimentally in terms of two variables, the angular setting of the crystal and that of the detector aperture. The resultant two-dimensional distributions, corresponding to the main scan procedures, ω, ω/θ, ω/2θ, show in a direct pictorial manner the interaction of the X-ray source, the spectral composition and the mosaic (and fragment) composition of the crystal. The clarity of presentation contrasts with earlier studies where the various components were projected onto one dimension. The two-dimensional presentation allows a clearer appreciation of (a) the rôle of the various components of the experiment and (b) the significance and function of the various scan procedures, ω, ω/θ, ω/2θ, corresponding to σ = 0, 1, 2 respectively, σ being the 2θ:ω scan ratio. This study leads to a new improved prescription for the measurement of integrated intensity which is more compatible with the spatial distribution of the Bragg reflexion. It also points to the existence of an inbuilt systematic error source in the conventional prescription, only made evident as a result of the extension into the second dimension. The possibility of a different procedure for the measurement of integrated intensity - the 'slice' ω/2θ scan - is indicated. This scan has potential advantages, one being that it would allow a more precise specification of background, hence bypassing the problem of variable truncation and leading therefore to the establishment of improved accuracy in structure factors. This study should also prove of interest for the future application and exploitation of linear and two-dimensional counters in single-crystal studies.
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    A diffraction pattern caused by thermal diffuse scattering of X-rays in a pyrolytic graphite crystal (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 390-391 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A defect line across the 002 diffuse spots scattered from a pyrolytic graphite plate has, for the first time, been clearly observed in X-ray films taken with unfiltered radiation from a copper target. The line was caused by the 002 Bragg reflection of the thermal diffuse scattering.
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    Nomenclature and generation of three-periodic nets: the vector method (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 42-50 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Three-periodic nets are connected graphs which permit embeddings having a threefold periodicity. To many crystal structures such nets can be meaningfully assigned and used to express the topology of the structures. It is shown that such a net can be fully characterized by a finite graph in which the edges are labelled in a suitable way. The reversal of the process of assigning a labelled finite graph to a given net can be used to generate nets of real and hypothetical crystal structures in a systematic fashion.
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    Comments on The rigid-body vibrations of molecules in crystals by Burns, Ferrier & McMullan (1967) (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 393-395 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The rigid-body analysis of the thermal vibrations in seventeen molecular structures by Burns, Ferrier & McMullan [Acta Cryst. (1967), 22, 623-629] has been repeated and major discrepancies have been observed. The sources of errors have been diagnosed and the relevant comments together with the new list of results are reported.
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    Theory of internal strain measurement in zinc-blende-structure crystals and a reanalysis of work on gallium arsenide (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 116-120 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The internal strain parameter (or the bond-bending parameter) of zinc-blende-structure materials can be obtained from the stress dependence of the intensity of weak reflections in uniaxially stressed crystals. The theory, including dispersion and temperature effects, is presented and applied to the reanalysis of earlier work on gallium arsenide. The results are inconsistent, leaving the bond-bending parameter of this material unknown.
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    Radii of gyration and scattering curves of hollow bodies of homogeneous electron density: errata (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 395-395 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Equations (15), (16), (17), (18), and (20) in the paper by Hiragi & Ihara [Acta Cryst. (1981), A37, 378-382] are incorrect. These equations should be as follows: \ matrix { F = 4A^{2}C \Biggl [ \sum_{k=l}^{n} f \Biggl ( s_{1},\varphi - {{2\pi}\over{n}}k \Biggr ) \Biggr ] \Psi _{c}, \cr f(s_{1},\beta) = \Biggl [s_{1}^{2}A^{2} \Biggl ( \tan ^{2} {{\pi}\over{n}} \sin ^{2} \beta - \cos ^{2} \beta \Biggr ) \Biggr ]^{-1},}\eqno (15) \matrix { x \Biggl { \exp(is_{1} A \cos \beta) \cr x \Biggl [ i \cot \beta \sin \Biggl ( s_{1} A \tan {{\pi}\over{n}} \sin \beta \Biggr ) \cr - \tan {{\pi}\over{n}} \cos \Biggl ( s_{1} A \tan {{\pi}\over{n}} \sin \beta \Biggr ) \Biggr ] + \tan {{\pi}\over{n}} \Biggr }, } \eqno (16)V = 2\pi ABC, \eqno (17)F = {{4\pi ABCJ_{1}(s_{1}, K)}\over{s_{1} K}} \Psi _{C} \eqno (18)F = 4\pi ABC{{\sin (sL) - sL \cos (sL)}\over{(sL)^{3}}} \eqno (20) Equations (15) and (16) in the original article lead to incorrect scattering intensity, whereas equations (17), (18), and (20) give the correct but unnormalized value. The figures in the article were calculated with the correct equations and hence need no alteration.
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    Fifty Years of Electron Diffraction (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 397-397 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Crystals: growth, properties and applications. Vol. 4 edited by H. C. Freyhardt (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 397-397 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Structural phase transitions. I edited by K. A. Müller and H. Thomas (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 397-398 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Proceedings of the VIth international conference on Raman spectroscopy: linear and nonlinear processes edited by W. F. Murphy (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 398-399 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Sputtering by particle bombardment. I. Physical sputtering of single element solids. Topics in Applied Physics. Vol. 47 edited by R. Behrisch (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 399-399 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Photoelastic and electrooptic properties of crystals by T. S. Narasimhamurty (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 399-400 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Improvement of electron microscope images by the direct phasing method (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 408-413 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The resolution improvement and/or phase correction method devised for X-ray crystallography has been developed within the weak-scattering approximation for electron crystallography. Here the information of electron micrography and electron diffraction can be effectively combined. The usefulness of this method to improve the resolution and to improve the image quality has been demonstrated by test calculations. This method will be effective for radiation-sensitive materials, because it can work with only one micrograph and the diffraction data.
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    A measurement of charge asphericity in iron metal (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 725-729 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Electron-density distribution in metallic iron has been determined based on the reflection data measured up to sin θ/λ = 1.36 Å-1 with Mo Kα radiation using a spherical specimen of 0.186 (6) mm in diameter. Fe is cubic, Im3m, a = 2.8652 (1) Å, V = 23.522 (3) Å3, Z = 2, μ(Mo Kα) = 30.35 mm-1 (T = 297 K). Final R was 0.007 for the 37 unique reflections, the internal agreement factor for the equivalent reflections being 0.013 for the 527 observed reflections. In the difference synthesis a positive peak of 0.7 (1) e Å-3 was found in the [111] direction at 0.32 (1) Å from the iron nucleus and a trough of -0.6 (1) Å-3 was found in the [100] direction at 0.54 (1) Å from the nucleus. Charge asphericity in iron metal, presumably due to the 3d electrons in t2g orbitals, is essentially the same as that of the other 3d b.c.c. metals, V and Cr.
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    Adjustable resolution Bragg reflection systems (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 502-507 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In this paper is described the realization of an idea due to J. W. M. Dumond who, almost 50 years ago, suggested that Bragg diffraction peaks might be made narrower by multiple reflection [Dumond (1937). Phys. Rev. 52, 872-883]. The optical elements which result belong to a family of harmonic free monochromators, X-ray and neutron polarizers whose properties make feasible many new Bragg diffraction optical systems.
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    Short-range-order structure of amorphous hydrous erbium oxide (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 264-269 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The hydrous lanthanide oxides comprise a complicated series of both crystalline and amorphous compounds. The amorphous short-range-order (SRO) phase of hydrous erbium oxide is the subject of this work. A thorough discussion of the instrumental arrangement and electronic counting chain for the X-radiation diffraction data collection is included. The scattering profiles for hydrous erbium oxide are shown and details of the ensuing data correction procedures are given. A radial distribution analysis of the calculated interference function, Si(S), was performed. SRO interatomic distances and the first-shell coordination number are reported. The mean Er-O bond distance is found to be 2.36 Å and the erbium metal is nine coordinated. Comparison of the experimental radial distribution curve to ones calculated from crystalline systems lend credibility to the experimentally determined distances.
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    Improvement of protein phases by coarse model modification (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 269-277 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A procedure is suggested for the refinement of a set of protein phases and for its extension to a higher resolution, which is a development of the approach of Agarwal & Isaacs [Proc. Natl Acad. Sci. USA, (1977), 74, 2835-2839]. A new set of phases is obtained by combining the starting phases with those calculated from a stereochemically non-conditioned coarse 'atomic' model which is automatically constructed and subjected to a least-squares refinement in reciprocal space. The method has been tested with actinidin data generated from atomic coordinates. Starting from the phases calculated to 3 Å resolution and the amplitudes calculated to 2 Å resolution a new set of phases was obtained with a mean error of 31° for 12 713 non-centrosymmetric reflections in the range to 2 Å. The refinement of the phases to 3 Å, resolution for γ-crystallin IIIb from calf lens and its extension to 2.7 Å resolution resulted in a noticeable improvement in the electron density map.
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    The two-wave X-ray field calculated by means of integral-equation methods (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 283-291 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The problem of calculating the two-wave X-ray field on the basis of the Takagi-Taupin equations is discussed for the general case of curved lattice planes. A two-dimensional integral equation which incorporates the nature of the incoming radiation, the form of the crystal/vacuum boundary, and the curvature of the structure, is deduced. Analytical solutions for the symmetrical Laue case with incoming plane waves are obtained directly for perfect crystals by means of iteration. The same method permits a simple derivation of the narrow-wave Laue and Bragg cases. Modulated wave fronts are discussed, and it is shown that a cut-off in the width of an incoming plane wave leads to lateral oscillations which are superimposed on the Pendellösung fringes. Bragg and Laue shadow fields are obtained. The influence of a non-zero kernel is discussed and a numerical procedure for calculating wave amplitudes in curved crystals is presented.
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    The temperature dependence of the anharmonic properties of crystalline naphthalene-d8 (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 803-810 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The temperature variation of the unit cell and molecular orientation in perdeuteronaphthalene is measured with considerable accuracy by neutron diffraction from a powder sample in the temperature range from 5 to 293 K. Measurements were done on the D1A and D1B instruments at ILL Grenoble. Results were obtained through constrained refinements using the program EDINP. The fundamental problem of background scattering is scrutinized, concluding that even with such a problem it is possible to obtain good measurements of rigid-body thermal-motion tensors by this technique. These results are of importance for the lattice dynamical studies of this system.
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    Isamu Nitta, 1899-1984 (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 313-313 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    On the solution of the phase problem. I. Utilization of a priori structure information in direct methods (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 338-340 
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    ISSN: 1600-5724
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The aim of the phase determination is formulated as a minimization of a function of estimated errors in the phases. It is concluded that a good phase-determining procedure in direct methods has to fulfil the following requirements. 1. The probability distributions of seminvariants used should contain the maximal amount of a priori structure information. 2. The whole information contained in probability distributions should be employed. 3. The choice of the starting set and the set of phase relations used should minimize Σ|FH|2 var φH. The procedures that are able to fulfil these requirements are described in the following papers [Hašek (1984). Acta Cryst. A40, 340-346, 346-350, 350-352].
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    Crystallography, geometry and physics in higher dimensions. II. Point symmetry of holohedries of the two hypercubic crystal systems in four-dimensional space (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 331-337 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The interpretation of physical properties of incommensurate modulated crystals leads to the use of their point groups and their total character tables in their superspaces. Examples are chosen of point groups of holohedries of the two hypercubic crystal systems - primitive and body-centred- in four-dimensional space. A geometrical presentation is given of the point group - including its character table - of the primitive hypercubic crystal system, as it is useful for the prediction and simplification of tensorial physical properties of the corresponding crystals. Through geometrical considerations, the exceptional splitting of the hypercubic family of \bb E4 into two crystal systems is easily proved. Finally, the two different relations - according to the parity of n - existing between the point group of the primitive hypercube of \bb En and its subgroup of rotations are explained.
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    On the solution of the phase problem. IV. Distributions fitted using the Kolmogorov test (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 350-352 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The proposed method of determination of a correct set of phases is based on a comparison between the trial and theoretical distributions of seminvariants using the Kolmogorov test. If the Kolmogorov test is restricted to a single region of magnitudes where only a small variance around the mean seminvariant value is expected, then the test is reduced to a simple rule. The smaller the number of seminvariants differing significantly from the expected mean value, the more probable the set of phases. In this simple form the Kolmogorov test has been used since the very beginnings of direct methods. In spite of the fact that the method seems to be less efficient than the distribution fitting using the x2 test [Hašek (1984). Acta Cryst. A40, its simplicity and low claim on computing time enables one to survey a large number of trial sets and so to increase the power of the method based on a combination of the Kolmogorov test with the x2 test, or with the low-order distribution moment test.
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    A comparison of two methods for the calculation of pseudorotation parameters (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 364-366 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A comparison of the two methods available for the calculation of exact values of pseudorotation parameters P, τm, i.e. the Fourier-series (FS) method [Rao, Westhof & Sundaralingam (1981). Acta Cryst. A37, 421-425] and the least-squares (LS) method [Jaskólski (1983). J. Chem. Educ. 60, 980-981] reveals that they yield identical values for τm and P. The variances σ2(τm) and σ2(P) calculated by these methods are, however, different since they have different interpretations.
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    The Δω, Δ2Θ two-dimensional nature of Bragg X-ray reflexions from a small single crystal: its influence on the assessment of structure factors (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 355-363 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: By its two-dimensional nature, the Δω, Δ2θ intensity distribution of a Bragg X-ray reflexion has greater angular resolution and greater information content than the corresponding one-dimensional Δω reflexion profile. It allows for the measurement of integrated intensity, exactly and equally truncated, over the full range of θ. Also, it is potentially correctable point by point for extinction and simultaneous diffraction. Consequently, it has inherent capabilities for the estimation of structure-factor values with improved accuracy. To realize this potential, it is necessary to identify and appreciate the various factors which, convoluted together, determine the 2D distribution. Among these factors, important ones are the crystal mosaic/fragment distribution, μ, the X-ray source emission distribution and the wavelength distribution. By first treating the situation for a hypothetical point source, the relation of the reflectivity (or 'level of interaction') with the μ distribution is highlighted. Extension to a real source indicates the probable need for deconvolution in practical cases to extract meaningful estimates of the μ distribution and hence the reflectivity distribution, the most significant measured quantity for accurate structure-factor evaluation. The 2D distribution is discussed in relation to single (H) scattering, multiple (H.{\bar H}) scattering (extinction) and simultaneous (H. K) scattering.
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    A general rule and many formulas for the evaluation of triplet phase invariants from isomorphous replacement and anomalous dispersion data (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 374-379 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: On the basis of a common characteristic observed in previously derived formulas for the evaluation of triplet phase invariants from either isomorphous replacement data or anomalous dispersion data, it has been found possible to combine mathematical expressions, certain differences of magnitudes, arising in the analysis of the two techniques to form a myriad of new mixed formulas. The common characteristic is that the various types of differences of magnitudes that are involved in the formulas are all definable in terms of the heavy-atom structure. The formulas involve the mixing of terms arising from several isomorphous derivatives or from a combination of such terms with various types of terms arising in anomalous dispersion or the mixing of various terms arising in anomalous dispersion alone. The evaluation of the triplet phase invariants is facilitated by the use of a simple rule, called the General Rule, that is generally applicable to the case of one predominant type of anomalous scatterer. In the case of more than one predominant type of anomalous scatterer, a slightly more complicated calculation is required and is described. Test calculations show that a very large number of invariants may be evaluated by these means with reliabilities that are potentially high, but depend, of course, on the reliability of the experimental data. A benefit from having the large variety of formulas is that triplet phase invariants can be evaluated at many points throughout the range -π to π and their reliability is enhanced because much information is obtained from only the largest differences of magnitudes.
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    Polarization phenomena of X-rays in the Bragg case (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 394-398 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Using a double crystal diffractometer with an additional crystal between the two crystals used as X-ray-optical polarizer and analyzer the phase relation between mutually perpendicularly polarized wave fields is examined in the Bragg case. The additional crystal is a (110)-surface oriented silicon crystal adjusted for the symmetric 220 Cu Kα1 Bragg case. In the case of coherent excitation of both σ- and π-polarized wave fields in the silicon crystal it is experimentally shown that a unique phase relation exists. In a further experiment it is demonstrated that in the case of incoherent excitation the rocking curve is the sum of the σ- and π-polarized intensities without any phase relation. Owing to the different half width of these two curves, step-shaped flanks of the rocking curve are found at excitation with unpolarized radiation.
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    Multiple Laue rocking curves (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 445-450 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Multiple Laue rocking curves of perfect crystals show a narrow central peak with a width of some 10-3s arc and an additional oscillatory structure. The finite structure of these curves is analyzed for two- and three-crystal Laue arrangements. These profiles can be used for precise determinations of structure factors and for an extension of small-angle scattering experiments to the extreme small-angle regime where large objects and long-range particle correlations become visible. An extremely high angular resolution can be achieved without significant reduction of the intensity, owing to a decoupling of the angular resolution from angular width of the beam. The analytically calculated rocking curves are compared to numerical results and to experimental results and show good agreement with both.
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    Investigation of a silicon single crystal by means of the diffraction of Mössbauer radiation (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 459-465 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Diffraction experiments with resonant γ-radiation were performed on a single crystal of silicon at room temperature. The high energy resolution of the Mössbauer effect was used to separate elastically and inelastically scattered radiation. A special experimental setup is described, which in addition allows the determination of the total thermal diffuse scattered intensity. Inelastically scattered radiation was observed in the whole investigated angular range. In the vicinity of Bragg reflections the ratio of the integrated intensities of inelastically to elastically scattered radiation was in very good agreement with the results of calculations using different computer programs based on first-order thermal diffuse scattering theory for a wide range of chosen scan lengths. The measured ratios of the elastically scattered radiation of different Bragg reflections were used to determine the temperature factor B.
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    An experimental determination of the anomalous dispersion factor f'' for sulfur (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 482-484 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The anomalous dispersion factor, f', of sulfur for Cu Kα radiation has been determined to be 0.60 (3) using measurements of Bijvoet differences of 175 selected reflections for a 6-(4-nitrobenzyl)thioinosine crystal.
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    Yield, flow and fracture of polycrystals edited by T. N. Baker (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 487-488 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Combining direct methods with isomorphous replacement or anomalous scattering data. I (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 489-495 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A direct-methods procedure has been proposed for separating the phase doublet resulting from the use of either isomorphous replacement or anomalous scattering techniques. The phase doublet is expressed as φH = φ′H ± |ΔφH|. Formulae combining the structure-factor relationships with the phase-doublet information are given. Problems concerning the practical applications are also discussed. A test calculation with the error-free data for the protein insulin showed a satisfactory result.
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    The relation of inner compressibilities to the pressure dependence of atomic position parameters in simple crystal structures (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 498-502 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The application of hydrostatic pressure to crystals in which some atoms occupy sites with polar point-group symmetry causes the position coordinates of these atoms to change. This phenomenon is here related to inner compressibility tensors. These arise naturally in the theory of inner elasticity, where they are linear combinations of the macroscopic linear compressibilities weighted by components of the internal strain tensors, and they indicate the ease with which the separation between pairs of sublattices may change under pressure. The form of the inner compressibility tensors is presented for eleven simple crystal structures involving up to four atoms in the basis. Finally, the inner compressibilities and con- straining equations for components of the internal strain are obtained from the pressure dependence of the structure of the elements As, Sb, Bi, Se, Te and I.
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    Graphic representation and nomenclature of the four-dimensional crystal classes. III. A notation for the crystal classes: erratum (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 723-723 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A printers error is corrected in Table 2 on page 407 of Whittaker [Acta Cryst. (1984), A40, 404-407]. The correct lines are: Family XVII; wx/yzw[0111]*w[0110]/z[0\bar 110]: wx[0110].
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    Crystal symmetry: theory of colour crystallography by M. A. Jaswon and M. A. Rose (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 726-727 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Indirect imaging edited by J. A. Roberts (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 728-728 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Les sous-groupes isomorphes d'un groupe d'espace de type p4. I. Détermination univoque (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 624-631 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: From a given standard setting (O, A, B) (conventional unit-cell origin and vectors) of a two-dimensional space group G(p4), it is possible, for each isomorphic subgroup g(p4), to select exactly one standard setting (o, a, b) subject to the following conditions. (1) Vector conditions: a = p1A + p2B, b = -p2A + p1B, P1 〉 0, P2 ≥ 0 (P1, P2 ∈ \bb Z). (2) Origin conditions: (a) if (P1 + P2) is odd, then the coordinates X, Y of o, with respect to (O, A, B), obey the next conditions: X, Y integers, 0 ≤ X 〈 GCD(P1, P2), 0 ≤ Y 〈 (P21 + P22)/ GCD(P1, P2), GCD = greatest common divisor; (b) if (P1 + P2)/GCD(P1,P2) is even, then 2X and 2Y are both even or odd, 0 ≤ X 〈 GCD(P1, P2), 0 ≤ Y 〈 (P21 + P22)/2GCD(P1, P2); (c) if P1, P2 are even and (P1 + P2)/GCD(P1, P2) is odd then 2X and 2Y are both even or odd, 0 ≤ X 〈 GCD (P1, P2)/2 and 0 ≤ Y 〈 (P21 + P22) /GCD(P1, P2). In any case there are exactly (P21 + P22) subgroups relevant to the same vector conditions. Tables of isomorphic subgroups p(4) are given for indices up to 25.
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    Orientation relationships between two crystal lattices: matrix description (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 642-645 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Orientation relationships between two crystal lattices are frequently specified in terms of parallel directions and planes in each lattice. The corresponding matrix, relating the vector bases of the lattices, can be obtained by a general method involving the metric matrices of the two lattices and the crystallographic indices of parallel planes and directions. Equivalent matrices can be defined by changing the lattice bases: different selections of the invariants of such matrices are indicated. Finally, criteria for choosing the 'best' matrix relating the two lattices are discussed in the context of phase transformations and of interfacial structure.
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    Crystals: growth, properties, and applications. Vol. 9. Modern theory of crystal growth edited by A. A. Chernov and H. Müller-Krumbhaar (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 727-728 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Graphic representation and nomenclature of the four-dimensional crystal classes. II. The individual symmetry operations (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 58-66 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The nature of the four-dimensional crystallographic symmetry operations is clarified by representation of their effects in hyperstereograms presented in the form of stereo-pairs. Appropriate graphical symbols have been devised to indicate the orientations of the corresponding symmetry elements. Typographical symbols have been devised for the operations themselves, and for their symmetry elements, which are adaptable for use in a system of symbolic nomenclature of the four-dimensional classes following the general principles of the Hermann-Mauguin notation.
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    High field magnetism edited by M. Date (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 88-88 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Determination of anomalous scattering lengths of samarium for thermal neutrons (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 99-102 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Anomalous scattering lengths of natural Sm for thermal neutrons with wavelengths between 0.827 and 1.300 Å have been determined by single-crystal diffraction from a Sm complex of known structure. 140 selected reflections were measured at a temperature of 37 K at each wavelength and b0 + b' and b" refined in each case. The values obtained are in good agreement with values obtained from a Breit-Wigner calculation using tabulated absorption resonance parameters for 149Sm. A value of b0 = 4.3 ± 0.2 fm is deduced from the diffraction experiments.
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    Lattice-image interpretation of a relatively-small-unit-cell crystal (1984)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 143-151 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Many-beam lattice images obtained at 200 kV from V2O3 crystals are discussed in comparison with those calculated in the Bloch-wave approach. The technique of optical diffractogram and equal-thickness fringes is utilized, if possible, to determine the defocus value and the crystal thickness which are the essential parameters for objective interpretation at atomic resolution. Whereas images observed in a thin region (∼ 50 Å) of crystal have been reproduced fairly well by simulation, there are others from thicker regions which are not always explained for lack of knowledge of the parameters. As for the effect of the partial coherence, the validity of the envelope-function approximation is examined with the aid of the first principle involving image-intensity summation, and under the experimental conditions used it proved to be satisfactory for qualitatively reproducing the observed image even for relatively thick (∼ 450 Å) regions.
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    Coordinated use of isomorphous replacement and layer-line splitting in the phasing of fiber diffraction data (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 417-425 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In order to calculate electron density maps from fiber diffraction patterns, the terms which superimpose because of cylindrical averaging of the pattern must be separated and their phases determined. This can be done using a technique analogous to the isomorphous replacement method of protein crystallography, but large numbers of heavy-atom derivatives are required. Because of the cylindrical averaging, the number of derivatives increases with increasing resolution. This paper describes a method for measuring the fine splitting of layer lines which occurs when a helical structure repeats approximately, but not exactly, in a given number of turns, and for using this as a source of phase information. The amount of phase information obtainable from each heavy-atom derivative is theoretically doubled since differences in both layer-line intensity and apparent layer-line position can be used, and this substantially increases the resolution attainable with a limited number of derivatives. The method is used to calculate an electron density map of tobacco mosaic virus at 6.7 Å resolution using only two derivatives instead of the four which would have been required using previously available methods.
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    Bragg diffraction from a material of circular cross section (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 425-432 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The problem of dynamical Bragg diffraction from a set of Bragg planes in a material circular in the diffraction plane is solved by a combination of a Riemann-function technique and numerical integration of the Takagi-Taupin equations. In regions affected by non-Laue surfaces the solution is compared with an approximate Green-function method based on truncation of small arcs of the circle. The bright-field and dark-field intensity profiles are determined only by the radius of the circle compared to the extinction distance, and on the absorption parameters. The dependence of the profiles on these parameters is studied.
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    Interference effects among variants (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 438-442 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: There have been several recent efforts to account for the broadening of the superstructure Bragg maxima and their displacements in reciprocal space observed for partially transformed b.c.c. solid solutions containing the ω phase. They have been concerned with intensity calculations from models for which the ω regions are defective in a variety of senses. All of the models include only one ω variant, while in fact the system must contain four equally likely variants. A method to correct the calculated intensity for interference effects among the variants, omitted from these models, is described. It is applied to a specimen model. Possible applications of the method are discussed. Though the concept is illustrated by its application to the ω phase, it is readily generalized to any multivariant system.
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    Cystalline modifications and structural phase transitions of NaOH and NaOD (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 470-476 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Structural work on the three modifications of sodium hydroxide is reviewed. The monoclinic and cubic modifications were determined with neutron and X-ray diffraction, respectively. The phase-transition temperatures were determined by specific-heat measurements. The cubic to monoclinic transition is a first-order transition with a freezing of the rotational motion of the OH (and OD) groups. The monoclinic axes a, b and c* tend to be oriented parallel to the original cubic directions [{\bar 1}{\bar 1}2], [1{\bar 1}0] and [ 111], respectively. The orthorhombic to monoclinic transition is a nearly continuous displacive phase transition with a soft acoustic shear mode. The order parameter is the homogeneous shear of the crystal in the a direction. Its temperature dependence is described within Landau theory.
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    Statistical approaches for the treatment of anharmonic motion in crystals. II. Anharmonic thermal vibrations and effective atomic potentials in the fast ionic conductor lithium nitride (Li3N) (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 568-576 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Results of X-ray diffraction experiments on lithium nitride (Li3N) in the temperature range between 294 and 888 K show strong anharmonic effects. The deviations from harmonicity cannot be interpreted by interstitial sites or split positions. The application of an anharmonic temperature factor which is based on the Gram-Charlier expansion leads to an excellent fit of the data, whereas an anharmonic temperature factor, based on the Edgeworth series expansion, cannot fit the measurements in a satisfactory way. The corresponding anharmonic probability densities and the effective one-particle potentials are presented. The activation energy of the ionic conduction in Li3N perpendicular to the c axis and the thermal expansion of the lattice constant are derived from the potentials. These results agree well with results obtained by other experimental techniques. Therefore it is concluded that the potentials derived from elastic scattering experiments are physically meaningful.
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    Electronic Resource
    Normalized structure factors. III. Estimation of errors (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 598-608 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A method for calculating the expected errors in |Eh| values is outlined. It is based on the precision of the measured data and the Wilson-plot parameters: and allows for errors arising from the use of the profile scaling function and/or the index rescaling procedure in the normalization scheme. Six refined structures are used to test the estimated errors in |Eh| against values deduced from a comparison with the 'true' normalized structure factor |\cal Eh|.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739482001259
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