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  • International Union of Crystallography (IUCr)
  • 1980-1984  (2,432)
  • 1983  (1,137)
  • 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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  • 25
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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 
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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 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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    Intensity distribution in powder X-ray diffraction from nonstoichiometric titanium sulfide containing stacking faults: errata (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 269-269 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: All exp (-i2πζ, exp(-iπζ/2), exp(-iπζ and exp(-i3πζ/2) in equations (21) in Onoda, Saeki & Kawada [Acta Cryst. (1980). A36, 952-957] are mis-expressed and to be replaced respectively by exp(i2πζ), exp (iπζ/2), exp (iπζ) and exp (i3πζ/2).
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    Sovremenaya kristallografia. Tom 3. Obrazovannie kristallov (Contemporary crystallography. Vol. 3. Crystal growth) by A. A. Chernov, E. I. Givargizov, K. S. Bagdasarov, L. N. Demyanets, V. A. Kuznetsov and A. N. Lobachev (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 272-272 
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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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    A technique for the calculation of multislice transmission functions for crystals with large (or infinite) repeat distances in the beam direction (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 282-286 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A partially analytic technique for the calculation of electron transmission functions used in multislice calculations is developed. This development utilizes the fact that atomic scattering amplitudes are generally available as fitting parameters to four Gaussians. The result is especially applicable to calculations with a large or infinite repeat distance in the incident-beam direction and initial test calculations give a time saving of a factor of four. Sample results are given for the calculation of images from an inclined stacking fault in gold.
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    Absorption and secondary extinction (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 326-332 
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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 is made of the treatments of absorption in the theories of secondary extinction by Werner [J. Appl. Phys. (1974), 45, 3246-3254] and by Becker & Coppens [Acta Cryst. (1974), A30, 129-147]. The Werner approach treats absorption and extinction together, whereas Becker & Coppens assume a prior correction for absorption, and make an approximate allowance for the effect of absorption on the extinction by modifying the parameter representing the effective crystal dimension. By consideration of the form of the corrections for a specially simple crystal shape, it is shown that the Becker & Coppens method predicts slightly greater extinction than does the unified treatment of Werner. However, the difference is small, and likely to be unimportant in practice The Werner solution of the Hamilton-Zachariasen transfer equations is exact, but not easy to use. The Becker & Coppens results are approximate, but they are presented in a form convenient for computation.
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    Generalized relations between d-orbital occupancies of transition-metal atoms and electron-density multipole population parameters from X-ray diffraction data (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 377-387 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Generalized relations relating multipole population parameters to d-orbital occupancies of transition-metal atoms are presented. The relations are cast in the form of a 15 × 15 matrix which reduces to smaller size for site symmetries higher than {\bar 1}. The matrix takes into account differences in normalization of density functions and atomic orbitals. The expressions are applied to diffraction data on Cr(CO)6, Mn2(CO)10, Co2(CO)8, Co3(CO)9CH, Co(NH3)6Cr(CN)6 and Co(NH3)6- Co(CN)6. In all cases crystal-field destabilized orbitals are depopulated relative to stabilized orbitals. Results are in almost quantitative agreement with theoretical populations; small remaining discrepancies are to be analyzed further for their possible significance. Occupancy of e'g orbitals in the last two compounds indicates a degree of covalency of the metal-ligand interaction in these low-spin complexes which is larger for the cyano ligand than for the ammonia molecule in agreement with generally accepted conclusions based on spectroscopic data. Very little difference is observed between the relative orbital occupancies of the Cr(CN)3-6 and Cr(CO)6 species.
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    Conditions for direct structure imaging in silicon carbide polytypes. Erratum (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 422-422 
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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 result of a printer's error, Figs. 9(a) and 9(b) in Smith & O'Keefe [Acta Cryst. (1983), A39, 139-148] have been transposed, so that (a) is labelled (b) and (b) is labelled (a).
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    Report of the Executive Committee for 1981 (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 481-499 
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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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    Diffraction studies on non-crystalline substances edited by I. Hargittai and W. J. Orville-Thomas (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 503-503 
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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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    Statistical modification of anomalous-scattering differences (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 512-515 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Statistical methodology originally introduced by French & Wilson [Acta Cryst. (1978). A34, 517-525] for the treatment of negative-intensity observations is shown to be useful for the estimation of anomalous-scattering differences. The general principles of this method are described, along with applications to data from a lead derivative of the enzyme carboxypeptidase A.
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    Debye–Waller factors of zinc-blende-structure materials: dynamic deformation effects (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 533-538 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A discussion is given on simple grounds, not explicitly involving the dynamical deformation formalism, of the implications of having the outer part of the electron distribution vibrating significantly differently from the core. The use of a lattice dynamical shell model to represent this effect is examined in some detail, with particular reference to the framework within which such a model gives meaningful results. Predictions are given from the 14-parameter shell models and the 11-parameter valence-shell models for the difference between Debye-Waller B values of the shells and cores and also for the effective Debye-Waller factors for the deforming ions in 14 zinc-blende-structure compounds. It is concluded that the effective X-ray Debye-Waller B at very small sin θ/λ is typically several percent smaller than the core B value, owing to a very substantial reduction in the mean-square displacements of shells compared with cores. Results are also given for the 15-parameter deformation-dipole models for eight materials. These show effects broadly comparable in magnitude to the shell models but more varied in detail. Notably, some models show for the first time a larger rather than smaller Debye-Waller factor for the deforming ion.
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    Moments of the probability density function of R2 approached via conditional probabilities. II. Completely correct and completely incorrect models in space group P\overline{1} (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 562-565 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: With the help of conditional probabilities formulas are derived for the first and second moment of R2 as a function of the size of the model. The formulas are valid in the space group P{\bar 1} for two extreme cases, viz completely correct and completely incorrect models. Incorporation of the observed intensities enables one to obtain accurate a priori estimates of (R2) and σ(R2). The theory agrees very well with simulated experiments.
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    Thirteenth General Assembly and International Congress of Crystallography (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 597-597 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Developments in high power lasers and their applications edited by C. Pellegrini (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 191-191 
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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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    The consequences of the neglect of TDS correction for temperature parameters (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 202-207 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The importance of carrying out TDS corrections is emphasized. Their relative effect on the derived temperature parameter values is discussed and shown to rely primarily on the experimental conditions and not on the softness of the crystal.
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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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    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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    Tensor properties and rotational symmetry of crystals. III. Use of symmetrized components in group 3(3z) (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 245-251 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: 'Symmetrized' components are introduced in place of the standard ones to improve the method presented in paper I [Fumi & Ripamonti (1980). Acta Cryst. A36, 535-551]. These components, which are simply related to the standard ones, allow a further reduction of the computational task and also a further simplification of the results and of their use. This is illustrated by application to general two-dimensional tensors of ranks 6 and 8 and by particularization of the results to the cases of the third- and fourth-order elastic tensors.
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    X-ray diffraction from nonstoichiometric titanium sulfide containing stacking faults: errata (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 269-269 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Mis-expressed terms in equations (13) in Onoda & Kawada [Acta Cryst. (1980), A36, 134-139] should be corrected. All exp(--i2πζ, exp(-iπζ/2), exp(-iπζ and exp(-i3πζ/2) on page 137 are to be replaced respectively by exp(i2πζ), exp (iπζ/2), exp (iπζ) and exp (i3πζ/2).
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    Nonlinear phenomena at phase transitions and instabilities edited by R. Riste (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 270-271 
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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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    Sovremenaya kristallografia. Tom 2. Struktura kristallov (Contemporary crystallography. Vol. 2. Structure of crystals ) by B. K. Vainshtein, V. M. Fridkin and V. L. Indenbom (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 272-272 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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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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    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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    Correct calculation of scale factor between diffraction data sets of isomorphous crystals and its application to protein crystallography (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 693-697 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A correct calculation of the scale factor between the diffraction data of isomorphous crystals is shown on the basis of a statistics criterion. This scale factor requires no additional refinement, improves the signal-to-noise ratio of difference Patterson maps and Wilson's statistics and allows an estimation of the degree of non-isomorphism. The method has been tested for the 3 Å data sets of protein γ-crystallin crystals and a few of its heavy-atom isomorphous derivatives.
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    The local atomic order of a Ni–12.7 at.% Al alloy quenched from 1323 K (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 740-746 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: High-angle X-ray diffuse scattering has been measured in a volume element in reciprocal space for a Ni-12.7 at.% Al alloy single crystal quenched from 1323 K. The Cowley-Warren order parameters were determined after separation of the first- and second-order size effects, using a procedure based on the Borie-Sparks quadratic approximation of atomic displacements. The experimental order coefficients were used to reconstruct the real-space locally ordered distribution of atoms, from which the spectrum of nearest-neighbor atomic configurations was determined. Some 61% of the Al atoms were found to be in a planar ordered configuration consisting of a nickel atom and four nearest-neighbor aluminum atoms, the basic structural unit of Ni3Al. These planar elementary ordered units were, of course, distributed on the three equivalent {100} planes. While most of the ordered 'domains' consisted of from one to three Ni3Al rings, a few domains were found to contain on the order of a hundred such elementary ordered units. The larger ordered domains may be thought of as embryos of the ordered Ni3Al phase which have formed during quenching of the bulk alloy single crystal. There is evidence that the spatial distribution of ordered domains is not random.
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    Theoretical study of the influence of the width of the entrance slit on the contrast of dislocations in X-ray topography by means of simulations (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 767-772 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Experimental topographs may be simulated by addition of simulations where one point source is lit on the surface of the crystal. The accuracy of a varying-step integration of Takagi equations is good enough to allow such computations. It is shown that all parts of the contrast are sensitive to the width of the entrance slit and that accurate characterization of defects must take this parameter into account.
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    Extinction in an extended-face crystal of zinc selenide (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 548-552 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: X-ray intensity measurements from an extended-face single crystal of cubic zinc selenide obtained by Mcintyre, Moss & Barnea [Acta Cryst. (1980), A36, 482-490] have been reanalysed with a view to explaining the unresolved discrepancies between theory and experiment present in the original analysis of the most severely extinguished reflections. The results obtained are shown to complement the recent findings of a wavelength-dependent study using the same crystal specimen [Stevenson & Barnea (1983). Acta Cryst. A39, 538-547] and foreshadow the need to allow for the presence of the Borrmann effect.
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    Unbiased three-dimensional refinement of heavy-atom parameters by correlation of origin-removed Patterson functions (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 813-817 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A procedure is presented for three-dimensional refinement of heavy-atom parameters without the use of phase information in the methods of single and multiple isomorphous replacement. This procedure is based on the Patterson-function correlation method of Rossmann [Acta Cryst. (1960), 13, 221-226] except that the origins of the Patterson functions are now removed from this correlation and centric and acentric reflections are treated separately. The resulting procedure is shown theoretically and by application to three test cases to yield accurate and essentially unbiased estimates of the occupancies, thermal parameters, and positions of heavy-atom sites.
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    Second-order phase transitions and the irreducible representation of space groups by H. F. Franzen (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 824-824 
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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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    Current Crystallographic Books 1970 through 1981 (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 597-598 
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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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    COSTED Travel Fellowship Programme (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 598-598 
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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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    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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    Rigid-body motion of the S2O32− group in thiosulphates (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 647-650 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The recently published thermal parameters of Ba, Na, Mg, Ni, and NH4 thiosulphates have been analysed for the rigid-body motion of the S2O2-3 group using the TL and TLS treatments. In the cases of Ba, Na and NH4 the rigidity of the group has been established and justified by employing various rigidity tests. Librational corrections to bond lengths have been found to be appreciable. For the cases of Mg and Ni the rigidity has been observed, but due to high e.s.d.'s detailed analysis has not been made. Hamilton's test has shown that the TLS treatment is preferable to the TL in all cases.
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    A method for determining the charge in the peaks of deformation densities (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 899-901 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: If electron density distributions of different provenance need to be compared, the evaluation of the charge contents of peaks in deformation densities is useful (besides the usual comparison of density sections). Since deformation peaks are often of approximately ellipsoidal shape, a procedure is proposed in which a rotation ellipsoid is placed in the peak position so that it encloses the peak and separates it from the neighbouring peaks. The charge contents of the peaks are calculated by numerical integration where positive; zero and negative values of the electron density are treated separately. This can be achieved by a minor alteration of a standard Fourier program. Numerical results are presented for p-dicyanobenzene. The C-C bond peaks contain 0.19-0.26 electrons, and the C-N bond peak about 0.26 electrons.
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    Moments of the probability density function of R2 approached via conditional probabilities. IV. Influence of the elimination of (low-intensity) data on the applicability of R2 in automated structure evaluation (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 920-924 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The average value of the residual R2 and its spread σ(R2) is described as a function of a threshold a, below which Eo2 values are omitted from the data set. Theoretical expressions, valid for finite data sets in the space groups P1 and P{\bar 1}, are derived for (R2) and σ(R2) as functions of a for models containing atoms correctly as well as incorrectly positioned. Use of a threshold causes a decrease in the resolving power of R2-based strategies used in automated structure evaluations. Random elimination of Eo values gives rise to a larger loss of resolving power than does the elimination of small Eo values.
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    Prices of Acta Crystallographica and Journal of Applied Crystallography (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 948-949 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Symmetry of the lattice-energy functional of a molecular crystal (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 941-946 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In calculating the lattice-energy hypersurface by the systematic variation of the molecular rigid-body parameters and the lattice constants, the ranges to be scanned depend on the molecular symmetry and on the space group. A generalization of Hirshfeld's approach [Hirshfeld (1968). Acta Cryst. A24, 301-311] applicable to the case of variable lattice constants is suggested. The symmetry of the multidimensional parameter space is defined by the direct product of the molecular point group and a normalizer NA(F) of the space group F. The normalizer NA (F) is a group of affine transformations of the crystal axes that leave invariant the coordinates of equivalent positions. An asymmetric unit of the parameter space is obtained through keeping the lattice constants within such ranges that satisfy the Niggli reduced-cell conditions.
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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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    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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    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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    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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    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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    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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    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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    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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    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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    Coherent inelastic neutron scattering in lattice dynamics by B. Dorner (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 187-187 
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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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    The spindle stage: principles and practice by F. D. Bloss (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 189-190 
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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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    The short-range structure of Ti and Zr b.c.c. solid solutions containing the ω phase. III. Extension of the diffraction theory to partially transformed systems (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 287-297 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A refinement of earlier models used to compute intensity distributions for the diffuse to phase is described. The model includes both faulted to regions and untransformed b.c.c. regions. The diffuse intensity generated by this model is compared with experiment. A result of this calculation is that, unlike its predecessors, it causes the observed diffuse maxima under the sharp fundamental Bragg reflections. The model is shown to be compatible with measurements of the integrated intensities of the fundamental reflections. A correction to our single-variant intensity calculation to account for interference effects among the variants is displayed. Our result is compared with those of other structural models for the diffuse to phase, and the implications of its details are discussed.
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    X-ray attenuation coefficients of carbon in the energy range 5 to 20 keV (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 322-325 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Mass attenuation coefficients for pyrolytic graphite have been determined by an X-ray energy-dispersive method with a relative experimental error of about 0.5%. Results are given for the Fe, Co, Cu and Mo Kα1 and Kβ1 lines.
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    Determination of the diamagnetic susceptibility and the electron static polarizability of crystals from X-ray diffraction data (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 411-415 
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    ISSN: 1600-5724
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The relationships associating the structure amplitudes, measured in a diffraction experiment, with the diamagnetic susceptibility and the static electron polarizability of ions for crystals with the NaCl-type structure are obtained. The calculations, carried out for a number of alkali halides and alkaline-earth oxides, have shown a good coincidence with the results of magnetic and optical measurements. The accuracy of the obtained results is analyzed; it is shown that when using accurate diffraction data the diamagnetic susceptibility and the electron polarizability may be determined with an accuracy of about 1%.
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    Twelfth General Assembly and International Congress of Crystallography, Ottawa, Canada, 16-25 August 1981 (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 425-480 
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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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    Time-like perturbation method in high-energy electron diffraction (1983)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 576-584 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The small-angle approximation usually encountered in dynamical theories of fast electrons essentially leads to a transformation of the propagation-direction variable z into a time-like parameter [Berry (1971). J. Phys. C, 4, 697-722]. The three-dimensional stationary Schrödinger equation is then approximated by a two-dimensional 'time'-dependent equation which may be solved by using the standard time-perturbation techniques encountered in quantum mechanics. The basic idea of the present approach consists in studying the evolution operator U(z,z0) instead of the wave function. Depending on the choice of bases, the matrix elements of U(z,z0) represent either the transition probabilities of diffraction or the kernel function of the propagation issued from Feynman-path integral theory [Berry & Mount (1972). Rep. Prog. Phys. 35, 315-397; Van Dyck (1975). Phys. Status Solidi, 72, 321-336; Jap & Glaeser (1978). Acta Cryst. A34, 94-102]. Special attention is devoted to the so-called 'Bloch waves' and 'physical-optics' formulations which both correspond to the same perturbation expansion but with two different unperturbed 'Hamiltonians'.
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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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    ISSN: 1600-5724
    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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    ISSN: 1600-5724
    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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    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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    ISSN: 1600-5724
    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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    Normalized structure factors. III. Estimation of errors (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 598-608 
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    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|.
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    Treatment of equations of constraint in least-squares refinement (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 618-623 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: With a least-squares program organization, as described by Busing [Acta Cryst. (1971), A27, 683- 684], the constraints have to be put into a form where the dependent parameters are expressed by the independent ones and, possibly, by further constants. Difficulties may arise if (1) several linear or non-linear constraints refer simultaneously to several parameters, and (2) if the constraints are not linear and cannot be solved analytically for the dependent parameters. For both cases a solution is offered which is based on the application of the well known solution of linear equations. Non-linear constraints are linearized. If all constraints are linear, Busing's organization is retained; if they are (partly) non-linear, Busing's organization has to be changed in the main program, and the user's subroutine SETP has to be written according to a different concept. Hints concerning programming are given and some examples are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739482001284
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