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  • 1
    Electronic Resource
    Electronic Resource
    A review of my papers on crystal optics 1912 to 1968 (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1-9 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The theory of the diffraction of X-rays by crystals was developed by Ewald as part of a unified study of the interaction of light of all wavelengths with crystals, beginning with the work for his 1912 thesis and extending to his papers in 1968. The formulation of the problem in terms of the interaction of electromagnetic radiation with a periodic array of dipoles is placed in its historical perspective and is compared with Laue's version based on the assumption of a continuous electron density distribution. The Borrmann effect, hinted at in 1917, is derived readily from consideration of the dispersion surface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000024
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  • 2
    Electronic Resource
    Electronic Resource
    A solution of the X-ray `phase problem' (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 17-21 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The phases of X-ray reflections are lost in the diffraction process in conventional two-beam diffraction. It is shown that the phases are not lost when three beams diffract simultaneously, and that phase information can be extracted from knowledge of the spatial distribution of the diffracted intensities about the three-beam point. In centrosymmetric crystals, the distribution is a sensitive function of the invariant phase of the product of the structure factors involved in simultaneous diffraction. High-resolution divergent-beam photographs illustrating the intensity differences associated with positive and negative triplet-phase products are shown.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000048
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  • 3
    Electronic Resource
    Electronic Resource
    Tensorial covariants for the 32 crystal point groups (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 83-95 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The recently published tables of Clebsch-Gordan products are applied to derive the tensorial covariants (bases of irreducible or physically irreducible representations) for the 32 crystal point groups. Tensors of the following intrinsic symmetries in Jahn notation are considered: &egr; (pseudoscalar), V (polar vector), [V2], V[V2], [[V2]2], [V2]2, &egr;[V2], and &egr;V[V2]. With this choice the most important tensors of optical and other properties are covered. Explicit lists of covariants in components of these tensors are given for the non- centrosymmetric groups; with the use of parity arguments the lists also apply to centrosymmetric groups. Applications, especially for phase transitions with reduction of the point symmetry, are briefly discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000152
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  • 4
    Electronic Resource
    Electronic Resource
    Ferroelastic transformations in lead orthophosphate and its structure as a function of temperature (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 108-114 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In Pb3(PO4)2 there are in general triplets of crystallographically independent atoms of the same kind that are pseudosymmetrically related by a pseudo-threefold rotation and interchange identity in ferroelastic transformations. By means of a rotation of axes that takes into account the distortion of the real monoclinic lattice, the atomic coordinates in the ferroelastic transformations of Pb3(PO4)2 are calculated. The results of the refinement of the structure by means of the Rietveld technique, at several temperatures below and above the phase transition, are reported, as well as the atomic displacements involved in the ferroelastic switching as a function of temperature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S056773947900019X
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  • 5
    Electronic Resource
    Electronic Resource
    Conditional phase probability distributions of structure factors in a Karle–Hauptman matrix (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 105-107 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: From the joint probability distribution of all structure factors in a Karle-Hauptman matrix new phase probability distributions are obtained. These calculations lead to a reformulation of the maximum-determinant rule for phase determination. In addition a new function is derived whose maximum corresponds to the most probable values for the phases of an arbitrary subset of the structure factors in a Karle-Hauptman matrix. This function accounts for the interaction among phases in a Karle-Hauptman matrix through triple products and quartets simultaneously.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000188
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  • 6
    Electronic Resource
    Electronic Resource
    Use of the neutron time-of-flight technique for structure investigations (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 131-136 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The application of a time-of-flight neutron diffractometer for crystallographic investigations, designed on the basis of a pulsed neutron source, is discussed. The procedure for obtaining integrated intensities and introducing corrections is described. The results of a test experiment with perdeuteronaphthalene are given. The deuterium atom positions in the structure of La2Mg3(NO3)12.24D2O are also refined and compared with those obtained using X-ray diffraction techniques. The observed systematic differences in bond lengths and valence angles are explained by the differences between the 'X-ray' and 'neutron' coordinates of the light atoms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S056773947900022X
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  • 7
    Electronic Resource
    Electronic Resource
    Experimental electronic valence charge density of NaCl and SrCl2 along the nearest-neighbour axis (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 140-142 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Valence-electron charge distributions have been calculated by difference Fourier inversion of experimental structure factors. The contribution of core electrons has been subtracted by using a one-term Gaussian representation. The results are in agreement with theoretical charge distributions obtained by the local-orbitals Hartree-Fock method.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000243
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  • 8
    Electronic Resource
    Electronic Resource
    Transformations to optimize the superposition of similar structures (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 158-163 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A simple, efficient and general method is described for finding the linear orthogonal transformation to superpose two similar structures given by sets of equivalent points, usually atomic position vectors. Formulae are also given for extracting the independent variables of rotation from the resulting transformation matrix. In addition, general transformations are derived, both in the case of proper rotation and in the case of rotatory inversion, to convert to a molecular frame of reference based on the superposition axis of symmetry.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000279
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  • 9
    Electronic Resource
    Electronic Resource
    Diffraction profiles from small crystallites (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 171-176 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The diffraction profile for a small crystallite has been obtained as the orientational average of the diffraction intensity given by Ino & Minami [Acta Cryst. (1979), A35, 163-170]. The formula obtained is a type of Debye interference function modified by a function {cal V}(r) (the self-convolution of a crystal shape function) and is expressed as a sum over all the atomic distance vectors in the crystal structure. Since the set of the vectors has Laue symmetry (the order of the group: L), the summation can be simplified to a sum over a reduced range corresponding to 1/L of the original range, while the {cal V}(r) is changed to {\bar v}(r) = ΣLp = 1= {cal V}(Rpr)/(LVt) (Vt: volume of the crystal; R1, . . . , RL: element of Laue symmetry group). Once the {\bar v} function is determined, the profile for a complicated crystal of any size and any crystal system can be systematically and efficiently calculated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000292
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  • 10
    Electronic Resource
    Electronic Resource
    The structure of ordered Ag3Mg: an ordinary long-period alloy as opposed to periodically antiphased alloys (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 181-188 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A necessary distinction is established between two kinds of long-period ordered structures: (1) periodic antiphases (PAP), whose principal characteristic feature is a continuous variation of antiphase half-period M as the composition changes; (2) 'ordinary' long-period structures, which show, as the composition changes, a series of phases stable in small composition ranges, every phase displaying a different, but well-defined, crystallographic structure. It is shown, through electron diffraction studies, that the ordered Ag3Mg alloy belongs to the second kind, unlike standard PAP alloys such as AuCu, AuCu3, Au3Cu, Cu3Pt and Cu3Pd. It is stressed that standard PAP structures can always be obtained by heating, above some transition temperature, a L 12 (or L 10) phase which always exists at lower temperature. This is not the case for ordered Ag3Mg. In this connection, the theory about stabilization of long-period structures is discussed [Sato & Toth (1965). Alloying Behaviour and Effects in Concentrated Solid Solutions, pp. 295-419. New York: Gordon & Breach].
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    URL: http://dx.doi.org/10.1107/S0567739479000310
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  • 11
    Electronic Resource
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    A hard-disc system: structures of a close-packed thin layer (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 194-196 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The possible structural transitions between n and (n - 1) thin layers of hard discs are considered. The apparently obvious homogeneous transition postulated on the basis of n = 2 is shown by an analogue experiment not to be the general solution. It is argued that the maximum density is retained by a 'Δ-form' transition which minimizes the density deficit at the boundary walls, and can be referred to different sections of a two-dimensional close packing.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000334
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  • 12
    Electronic Resource
    Electronic Resource
    Structural changes in β-FeOOH caused by radiation damage (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 197-200 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A high-resolution electron-microscope study of β-FeOOH (akaganéite) has been carried out. Examination of intact crystals and ultrathin sections provided no evidence for the hollow-rod structure proposed by earlier workers. It was found also that features, such as mottling and striations, previously thought to be indicative of a porous structure, were due to radiation damage. It is concluded that the β-FeOOH crystals are not porous, but are, in fact, crystallographically homogeneous.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000346
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  • 13
    Electronic Resource
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    A new Monte Carlo method for phase determination (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 220-224 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An extended version of the multi-solution method has been devised by introducing a Monte Carlo technique. This Monte Carlo direct method differs from the ordinary multi-solution procedure in two respects: (1) The starting set usually consists of as many as 10-50 phases; (2) Tentative phase values assigned to the members of the starting set are derived from successively generated random numbers. The application of the new method to several unknown structures has shown that it can be used as an effective means of phase determination.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000371
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  • 14
    Electronic Resource
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    Electrogyration and piezogyration in NaClO3 (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 225-232 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A method is developed for measuring electrogyration and piezogyration effects in polar crystals. The influence of linear electro-optic and piezo-optic effects is discussed in detail. By the use of different directions of measurement, the tensorial behaviour of the induced gyration is proved unequivocally. The magnitudes of the electrogyration and piezogyration coefficients are 10-16 m V-1 and 10-15 m2 N-1 respectively. The relations between constants measured under different thermodynamic conditions are considered. An analysis of results on the basis of a coupled oscillator model reveals a strong sensitivity of induced gyration to changes in the arrangement of the oscillators.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000383
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  • 15
    Electronic Resource
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    On the treatment of protein data measured on the oscillation camera (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 146-157 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The method used in Oxford for the collection and reduction of protein data from the Arndt-Wonacott oscillation camera [Arndt, Champness, Phizackerley & Wonacott (1973). J. Appl. Cryst. 6, 457-463] is described. The photographs are digitized with an Optronics Photoscan P-1000 drum scanner operated in an off-line mode, and the magnetic tapes are subsequently processed on an ICL 1906A computer. Results are presented for the protein phosphorylase b. The estimation of standard deviations for both fully and partially recorded observations is discussed, and the intensities for these two categories of term are compared.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000267
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  • 16
    Electronic Resource
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    Extinction in sodium fluoride (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 176-180 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: X-ray intensity measurements for a single crystal of sodium fluoride obtained at four wavelengths by Howard & Jones [Acta Cryst. (1977). A33, 776-783] have been reanalysed using both the Cooper-Rouse and Becker-Coppens extinction formalisms in order to study the wavelength dependence of the extinction in this crystal, since the original analysis was unable to account for the wavelength dependence of the intensities of the strong reflections. The results indicate that the crystal is intermediate between type I and type II in nature and are consistent with the wavelength dependence predicted by the theories, although the extinction is not large enough at the shorter wavelengths to provide a useful test of the validity of the theoretical wavelength dependence. The analysis also demonstrates some of the possible consequences of the use of unbalanced weighting schemes and of inappropriate models for the scattering factors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000309
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  • 17
    Electronic Resource
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    X-ray diffraction by small crystals (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 163-170 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A general method of calculating the intensity of X-ray diffraction from small crystalline particles whose boundary is defined by a shape function is discussed. The intensity formula which is generally given by a double sum over the reciprocal-lattice points is simplified into the form of a single sum, using 'the random-shift treatment' which assumes that the position of the boundary relative to the crystal lattice varies at random from crystal to crystal. By the use of Fourier theorems, the intensity formulas are also converted into a single sum over the direct lattice. Although the electron distribution in the particle has been defined in various ways by the shape function, a more reasonable expression of the electron density appropriate to small crystals is introduced. The intensity formulas derived on the basis of the new form of the electron density are compared with other intensity formulas which have so far been proposed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000280
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  • 18
    Electronic Resource
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    Defect structure of the ionic conductor lithium nitride (Li3N) (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 309-314 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The defect structure of Li3N (space group P6/mmm, a ∼ 3.65, c ∼ 3.88 Å) has been investigated with data sets measured at seven temperatures in the range 153 to 678 K. The data have been used for normal and high-order refinements and for the calculation of the corresponding difference electron densities. The final Rw ranged between 0.7 and 1.3%. The N atom position at z = 0 and the Li(1) positions at z = ½ are completely occupied and remain ordered within the whole temperature range. The Li(2) sites at z = 0 already show a vacancy concentration of 1-2% at low temperatures, which does not change significantly with increasing temperature. The Li(2) ions exhibit strong anharmonic thermal vibrations within the Li2N layers at z = 0 along the Li(2)-Li(2) connection lines. The high Li ionic conduction is caused by jumps of Li(2) ions from an occupied Li(2) site into an unoccupied one. The anharmonic thermal vibrations are the precursors of these jumps. Interstitial sites are not involved in this mechanism.
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    URL: http://dx.doi.org/10.1107/S0567739479000632
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  • 19
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    Derivation of three-phase invariants from the Patterson function (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 213-220 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The idea of Anzenhofer & Hoppe [Phys. Verh. (1962), 13, 119] that all the Fourier coefficients of the function ρ(r).ρ(r + u) are zero if the Patterson function is zero at vector point u is first developed; it is shown that the three-phase cosine and sine invariants may be derived jointly by solving two sets of linear equations. A least-squares method exploiting the entire Patterson function is then presented; this may allow the three-phase cosine and sine invariants to be determined and/or refined. As expected, the low-valued Patterson regions contribute most to the least-squares procedure.
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  • 20
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    X-ray interferometry with divergent polychromatic beams (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 201-213 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The theory of a strongly absorbing Laue interferometer for an arbitrary composition of the X-radiation has been developed on the basis of the Takagi equations. The coherent properties of interfering beams of polychromatic radiation are discussed. Formulas have been derived showing the dependence on the moiré pattern and the contrast of interference fringes of the type of deviation of the interferometer geometry from the perfectly aligned case. The results of an experiment for obtaining the moiré pattern of a symmetric Laue case (LLL) interferometer with Bremsstrahlung radiation are presented.
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    URL: http://dx.doi.org/10.1107/S0567739479000358
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  • 21
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    The crystal structure of hexabromobenzene: constrained refinement of neutron powder diffraction data (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 233-235 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The structure of hexabromobenzene, C6Br6, has been determined at 300 K by neutron powder diffraction. The crystal is monoclinic, a = 15.357 (4), b = 4.007 (1), c = 8.364 (2) Å, β = 92.65 (2)°, space group P21/n, Z = 2. The problem is well suited for a constrained refinement with the program EDINP. Within the accuracy of the analysis the orientation of a planar molecule is found and all intramolecular separations correspond to normal van der Waals interactions. No explanation of the multiple-crystal formation can be proposed and, in fact, packing considerations would support twinning on (101) which has not been observed.
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    URL: http://dx.doi.org/10.1107/S0567739479000395
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  • 22
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    Lindemann's rule and the X-ray Debye temperature of anisotropic Zn and Cd single crystals (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 243-245 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Lindemann's rule was applied to Zn and Cd single crystals and found to be valid for these solids, xz being in the range 0.2 to 0.25.
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    URL: http://dx.doi.org/10.1107/S0567739479000425
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  • 23
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    Absorption corrections for neutron diffraction (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 248-248 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The transmission factors tabulated by Rouse, Cooper, York & Chakera [Acta Cryst. (1970). A26, 682-691] for μr 〈 1 can be expressed as the product of two factors: a scale factor and a Debye-Waller factor. In the case of a sphere or a cylinder, the absorption corrections then reduce to simple adjustments of these parameters at the end of the refinement, instead of tedious corrections to the data itself. The results are of particular use for powder data. A printing error in the original paper is also corrected.
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    URL: http://dx.doi.org/10.1107/S0567739479000449
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  • 24
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    Refinement of metal d-orbital occupancies from X-ray diffraction data (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 536-539 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Expressions are derived relating the occupancies of transition-metal d orbitals in a trigonal distortion of an octahedral field to the populations of multipole density functions refined from X-ray diffraction data. The expressions are used to obtain the iron d-orbital configuration in cubic FeS2 by least-squares refinement of very high resolution single-crystal X-ray intensity measurements. The refined populations correspond to a large distortion of the iron electron density from spherical symmetry towards a low-spin configuration.
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    URL: http://dx.doi.org/10.1107/S0567739479001285
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  • 25
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    Structure, morphologie et macles des polymères (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 422-428 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Polymer crystal structures are tentatively grouped according to the relations between the symmetries of the macromolecular chain and the space group. Three possibilities occur: helical molecular symmetry (a) compatible with a crystalline lattice, or (b) incompatible, but with the chain axis parallel to the lattice principal axis; and low symmetry of the chain and lattice with random orientation of the chain. In the first two cases, the affinity between molecular and lattice symmetry allows us to describe the morphology and twinning of dilute-solution-grown crystals according to the classical laws. In the last case, these laws should be adjusted to the two-dimensional lattice which characterizes the chains packing orthogonally to their axis.
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    Intersecting defect structures in gallia-and magnesia-doped rutiles (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 449-458 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The possible { 210 } (0½0) defect intersection structures of rutile are analysed. There are 41 distinct intersection classes, 26 of which may exist as enantiomorphic forms. A classification scheme is introduced and used to analyse 386 observed intersections. Examples of 25 classes occur; eight of these account for 87% of the observations. The occurrence frequency may be understood by analysis of the stair-rod dislocation Burger's vector, required for continuity of the rutile structure around the intersection. Structural models for the commonly observed intersections and the nature of the 'dislocation-core' structures are presented. Exceptionally large tunnels are generated at orthogonal intersections and some high-resolution images are discussed. Statistical analysis shows that the distribution over the possible classes is not random, suggesting that an equilibrium network of intersecting defects may arise after long reaction times. The stability and mobility of intersections is expected to determine the kinetics of the reaction between rutile and dopants. Intersection structures may act as nuclei for low- to high-temperature phase transformations and they may aggregate and order to form new ordered structures.
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    A statistical method for selecting reflections for Bijvoet difference measurement in crystals with heavy atoms (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 463-467 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The conditional probability distribution of the Bijvoet ratio δ for given |F'N| and |F'P| is worked out for a noncentrosymmetric crystal with one species of anomalous scatterers (besides a large number of normal scatterers) in the unit cell. The expression for the conditional probability for |δ| to be greater than any given value δo is then deduced. The use of this expression for testing the suitability of a given Bijvoet pair of a crystal for Bijvoet difference measurement is pointed out. Numerical tables which aid easy implementation of the results to actual crystals are provided. Application of the result in a few actual crystal structures is also discussed.
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    X–N and X–X (1s2 core) maps for cyanuric acid (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 458-462 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: X–N maps and X–X (1s2 core) maps for cyanuric acid, C3H3N3O3, at 100 K are presented. The differences in scale and in the atomic parameters have large effects (≤ 0.4 e Å-3) on the difference density at the nuclei but smaller effects (≤ 0.15 e Å-3) on the bond density distributions. In particular, the density distribution around the O atoms, where polarization is likely to occur, is different in the two types of maps. It is concluded that, in these regions, the X-N maps yield the more reliable results.
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    Le groupe P1 et ses sous-groupes. I. Outillage mathématique: automorphisme et factorisation matricielle (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 485-496 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Nowadays there is a good knowledge of space subgroups but, curiously, the subgroups of the simplest space group P1 are poorly known. This is due to the fact that P1 is the sole space group possessing only isosymbolic subgroups while more complicated space groups possess extra translationengleich, klassengleich and other subgroups whose derivation is easier than that of isosymbolic subgroups. Thus the examination of the subgroups of P1 has been undertaken. This problem is connected to a large extent with matrix transformations and automorphisms which constitute the mathematical tools developed in this first paper. The subgroups of P1 are connected with changes of the unit cell, i.e. with matrices having integer coefficients. Such matrices can be decomposed into the products of simpler matrices and reduced to diagonal matrices having integer coefficients. The results of these theoretical considerations will be applied, in a following paper, to subgroups of P1.
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    The chemical and magnetic transformation of the series NixFe2O4 (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 501-503 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The gradual degradation of the cubic spinel phase in ferrimagnetic compounds of composition NixFe2O4 (x = 1.0 to 0.1) has been doubly confirmed by the X-ray diffraction and microwave initial magnetic-susceptibility measurements on the samples. The sharp ferrimagnetic-antiferromagnetic transition occurring at a composition corresponding to (x = 0.1) indicates a change in crystal symmetry.
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    On the application of phase relationships to complex structures. XVII. When MULTAN fails (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 622-626 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Two new multisolution direct-methods procedures are described: MAGIC, which employs the magic-integer concept and YZARC which refines initially random sets of phases by a least-squares approach. Each procedure produces several sets of phases for a number of reflexions, usually in the range 35-100. These are then extended by the tangent formula but with the constraint that the basis phases are not allowed to change until the final cycle. It is shown that for difficult structures these methods, which deal simultaneously with many phase relationships, may have intrinsic advantages over the MUL TAN procedure. Examples of their use are given.
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    On the contributions of the internal modes of molecules to the Debye–Waller factors. III. Urea (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 613-616 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The contributions of the internal modes of urea molecules, CH4N2O, to the vibration tensors of their atoms are calculated at 0, 100 and 300 K. The calculations are based on IR and Raman date from the literature, and the force constants determined are of the Urey-Bradley type. The temperature dependence of the vibration tensors of the internal modes (diagonal components) is represented graphically in the 0-300 K region (interpolation over nine calculated points); in this region it is weak. Since, for urea, there are vibration tensors available which were determined by neutron diffraction, the contributions of the internal modes could be estimated. On average over the C, N, O atoms it is 1.8% at 300 K, 4.4% at 98 K and 5.8% at 60 K; on average over the H atoms it is 22.6% at 300 K, 39.9% at 98 K and 46.3% at 60 K.
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    On the derivation of integrated reflected energy formulae (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 634-641 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A systematic procedure for deriving the equations of the integrated reflected X-ray energy is presented and conditions for the validity of the equations are formulated. Existing formulae are critically examined.
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    Dynamical theory for electron scattering from crystal defects and disorder (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 28-37 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Dynamical diffraction calculations have been made by use of the periodic-continuation assumption for the diffuse scattering in electron diffraction patterns and for electron microscope images of single split interstitials in gold crystals for thicknesses up to 200 Å in order to demonstrate the strong fluctuations of scattering with thickness. The diffuse scattering from distributions of defects in crystals, described in terms of correlation functions, can be written in terms of 'dynamical factors' for each type of individual defect. These dynamical factors multiply the same Fourier transforms of correlation functions as are used in kinematical theory to give the effect of dynamical scattering on the diffraction intensities. Calculations of dynamical factors have been made by multi-slice dynamical diffraction methods for unit changes in atomic scattering factors and for atom displacements in gold and aluminum crystals in [001] orientation for thicknesses up to 100 Å. With increasing thickness the dynamical factors show rapidly reducing fluctuations with crystal thickness and become more nearly isotropic except for the effects of Kikuchi bands which are seen to develop.
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    A comparison of the least-squares and maximum-likelihood estimators for counts of radiation quanta which follow a Poisson distribution (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 57-60 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The weighted least-squares method cannot correctly be used when measurements have errors given by counting statistics. The usual procedure results in bias in the values and errors in the calculated variances of the parameters. The maximum-likelihood method requires only a minor change in the least-squares equations and is generally thought to have more desirable properties for its estimates.
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    Net atomic charges and molecular dipole moments from spherical-atom X-ray refinements, and the relation between atomic charge and shape (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 63-72 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Refinement of the population and radial dependence of the spherical atomic valence shell is introduced in a general crystallographic least-squares program. The radial dependence is described by an expansion-contraction parameter κ, which, in the nine data sets tested, indicates contraction of positively and expansion of negatively charged atoms in agreement with theoretical concepts such as those incorporated in Slater's analytical rules for atomic orbitals. H atoms appear more contracted than concluded previously on the basis of a comparison of X-ray and neutron thermal parameters of sucrose. An average value of 1.40 for the radial contraction of H is used in structures for which no neutron thermal parameters are available. The resulting net charges are used to calculate X-ray molecular dipole moments whose magnitude and direction are in good agreement with theoretical and other experimental results, though some differences may be expected because of matrix effects. Net molecular charges in the one-dimensional conductor TTF-TCNQ agree with results obtained earlier by direct integration of the charge density over the molecular volume. A charge transfer from Si to V in the superconducting alloy V3Si is also in agreement with earlier results.
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    Correction of single-crystal intensities for average values of multiple reflection (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 73-78 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: For short wavelengths, Umweganregung cannot be experimentally eliminated. Its average unscaled intensity for a random orientation of the crystal is U(2θ1) = ∫θ, φ (sin 2 θ/sin 2θ1)P(θ1, θ, φ) exp [-2K(sin2 θ + sin2 θ12)] dθ dφ, where (2θ, φ) are the spherical coordinates of a point on the Ewald sphere when the intensity measurement is made at (2θ1, 0), the angle between the two directions being 2θ12, while K is the slope of a plot of In (F(obs)) as a function of sin2 θ, and P(θ1, θ, φ) is the polarization correction appropriate to the double reflection. Diffraction intensities were measured for epidote and quartz. A plot of (|F(calc)|)/(F(obs)) as a function of F(obs) for epidote was considerably improved by the subtraction from each intensity measurement of a number of counts SU(2θ1), where S is an experimental scale factor while U(2θ1) was calculated for epidote by numerical integration over the Ewald sphere. The refinement residuals for both compounds improved after correction of the intensities and the weights.
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    The joint probability distribution of the structure factors in a Karle–Hauptman matrix (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 101-105 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The joint probability distribution of all structure factors Ehi - hj (i, j = 0, ..., m) in an (m + 1) × (m + 1) Karle-Hauptman matrix is derived for structures in the space group P1.
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    Derivation of selenium–selenium nonbonded potential parameters for molecular crystals (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 236-240 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Potential-function parameters for Se-Se non-bonded atom-atom pair interactions have been derived from the room-temperature heat of sublimation of a mixture of monoclinic α- and β-Se8 and from the crystal structures of these two modifications. The first procedure applied was that of Nauchitel' & Mirskaya [Sov. Phys. Crystallogr. (1972), 16, 891-892]; the second was that of Williams [J. Chem. Phys. (1967), 47, 4680-4684]. In the latter procedure, the exponential parameter for the Se-Se repulsive term from the previous procedure was used and the coefficients of the attractive and repulsive terms were fitted to 22 observational equations by a weighted least-squares calculation. The second procedure proved to be more reliable than the first. Unlike the first procedure it yielded parameters with which a heat of sublimation of α-Se8 could be calculated that was 0.8 kcal mol-1 higher than that of β-Se8. At the same time, the calculated heats of sublimation of both modifications deviate by less than 2% from the experimental value of the monoclinic mixture. A low average value of 2.7% for the standard deviation of the parameters derived was obtained.
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    Phase information from anomalous-scattering measurements (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 245-247 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The fundamental basis for alternative expressions for the phase probability distributions related to anomalous-scattering measurements is examined. Exact, general expressions are derived and these are then simplified for the special situations that normally apply in practice.
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    Extinction in single crystals of UO2 (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 250-251 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: he measurements of nuclear neutron diffraction intensities for a single crystal of UO2 by Faber & Lander [Phys. Rev. B. (1976), 14, 1151-1164] have been reanalysed using both the Cooper-Rouse and Becker-Coppens extinction formalisms. The results indicate that this crystal is type I in nature, not type II as was suggested by Faber & Lander, and this conclusion is essentially the same as that obtained for a different single crystal of UO2 used in our earlier study of the wavelength dependence of extinction in this material [Sakata, Cooper, Rouse & Willis (1978). Acta Cryst. A34, 336-3411. The analysis of the Faber & Lander data gave a value for the scattering-length ratio bU/bO = 1.448 (2).
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    Errors in coordinates, bond lengths, and bond angles (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 248-250 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Standard deviations in bond lengths and bond angles are related to standard deviations in atomic coordinates according to published equations [Cruickshank (1959). International Tables for X-ray Crystallography. Vol. II, pp. 331- 332]. These equations were derived for an idealized model in which the distribution of coordinate errors is isotropic. Tests show that typical structures exhibit only moderate deviations from this model, and so the calculated standard deviations are accurate. Furthermore, the standard deviation in a bond angle φ (in degrees) can be well approximated by the expression σ(φ) ̃81 [σ(R)/R]r.m.s., where the quantity in square brackets is the root-mean-square value of σ(R)/R for the two bonds forming the angle. Tests on typical published results show that this equation usually estimates σ(φ) to within the round-off error in the reported value.
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    Prices of Acta Crystallographica and Journal of Applied Crystallography (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 252-252 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Optical data processing. Topics in applied physics. Vol. 23 edited by D. Casasent (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 253-254 
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    Images by C. A. Taylor (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 254-254 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Landolt–Börnstein. Numerical data and functional relationships in science and technology. Group III. Crystal and solid state physics. Vol. 7. Crystal structure data of inorganic compounds. Part c1 by W. Pies and A. Weiss (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 254-254 
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    New layered structure of Bi2W2O9 determined by 1 MV high-resolution electron microscopy (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 142-145 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The crystal structure of bismuth tungstate, Bi2W2O9, has been examined by 1 MV high-resolution electron microscopy. The symmetry is orthorhombic with lattice parameters a = 5.43, b = 5.41 and c = 23.7 Å and space group Pna21. The point group is determined directly from the images in which each cation site is resolved. The structure is similar to that of conventional layered Bi compounds with respect to the Bi2O2 sheets normal to the c axis; however, the layer between the sheets is of a deformed ReO3 type. The strong chemical bonds between the Bi ions and the apex O atoms cause the rotation of the WO6 octahedron about the c axis. W ions are displaced antiparallel in the WO3 layer.
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    Symmetry- and composition-dependent cumulative distributions of the normalized structure amplitude for use in intensity statistics (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 282-286 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Centric and acentric cumulative distribution functions of the normalized structure amplitude, which explicitly account for the presence of outstandingly heavy atoms in crystals of any symmetry, have been derived. These cumulative distributions can now be readily evaluated for all triclinic, monoclinic and orthorhombic space groups, with the exceptions of Fdd2 and Fddd, and thus constitute an extension of the commonly employed cumulative distributions based on the Wilson statistics. Expected discrepancies between the distributions derived in this work and the corresponding Wilson-type distributions are illustrated, and their symmetry and composition dependence is discussed in view of relevant applications to intensity statistics.
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    The elongated rhombic dodecahedron in alloy structures (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 305-308 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: With the space-filling elongated dodecahedron or its truncated form as a coordination polyhedron for larger atoms, structures like BaAl4, CeMg2Si2, BaHg11 and ThMn12 can be accurately described.
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    X-ray attenuation coefficients of graphite in the range 0.40 to 1.54 Å (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 316-318 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The mass attenuation coefficients of graphite have been measured at 23 different wavelengths in the range 0.40 to 1.54 Å. Above 1 Å, the coefficient is proportional to λ2.88.
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    Mögliche Kristallstrukturen für oktaedrische Moleküle MX6 bei dichtester Packung der X-Atome (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 188-193 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: As long as there is no thermally induced disorder, octahedral MX6 molecules crystallize with a close-packed arrangement of X atoms in which one sixth of the octahedral holes are occupied by M atoms. On consideration of the symmetry restrictions imposed by the partial occupation of the octahedral holes, the possible space groups and structures are deduced with the aid of group--subgroup relationships. Centrosymmetric space groups that can be achieved include: (1) all trigonal, rhombohedral and monoclinic space groups, Pnmn, Pnma, Pnab and P{\bar 1} for hexagonal close packing of the X atoms; (2) Pcmn and Pcab for the double-hexagonal close packing; (3) Fddd, Bbmb, all monoclinic space groups and P{\bar 1} for cubic close packing. The most important molecular arrangements are illustrated and their expected cell dimensions are given. The known structures of WCl6, UCl6 and numerous hexafluorides correspond to some of the predicted possibilities.
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    Molecular images of thin-film polymorphs and phase transformations in metal-free phthalocyanine (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 327-332 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Molecular imaging of metal-free phthalocyanine, prepared expitaxically on KCI, showed two polymorphs. The unit-cell constants, measured from the ac projection, differed from previously quoted values and this was attributed to the effect of the preparation substrate. The transformation of one phase into another was apparently martensitic with lamellae of the opposite phase being present in both polymorphs.
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    On the use of neutron spin-echo to separate inelastic scattering from elastic scattering at high energy resolution (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 333-336 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Generally the measured intensity of a Bragg peak or of elastic diffuse scattering contains an inelastic component (thermal diffuse scattering, low-lying phonon branches). Theoretical corrections for such effects are only possible in the limited number of cases where appropriate data are available. An experimental correction is difficult because the high energy resolution imposed reduces the beam intensity in a conventional experiment. In the case of neutron diffraction, the spin-echo technique proposed by Mezei [Z. Phys. (1972), 255, 146-160] for neutron spectroscopy can be used as a filter for inelastic scattering without significant loss in elastic intensity. The application of the technique to an elastic neutron-scattering experiment is described, and it is shown that for a neutron wavelength of 1.5 Å an energy resolution of better than 50 μeV can be obtained. With this energy band-pass, scattering even from very soft phonons or magnons can be avoided when studying critical scattering near phase transitions. Simple estimates also imply that more than 90% of the TDS contribution can be removed even for soft materials.
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    A modified Patterson superposition technique using multiple vectors (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 339-340 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A modified Patterson superposition procedure is presented in which multiple vectors instead of single vectors are utilized. It is demonstrated that by using certain guidelines a map which results from a superposition using a multiple vector can be shifted and superimposed on itself to yield an image of the structure containing many fewer extraneous peaks.
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    Fritz H. Laves, 1906-1978 (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 343-343 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Dimensions of Material Deposited under the Supplementary Publication Scheme (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 344-344 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    Topics in applied physics. Vol. 22. X-ray optics. Applications to solids edited by H. J. Queisser (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 344-345 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    Structure reports. For 1974, Vol. 40B edited by J. Trotter (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 346-346 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    Structure reports. For 1975, Vol. 41A edited by J. Trotter (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 346-346 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Structure reports. Sixty year index 1913-1973, B. Organic and organometallic compounds edited by J. Trotter (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 346-346 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    Copying Fees and Copyright Law (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 251-252 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    Fundamental crystallography by Yu. E. Sirotin and M. P. Shaskolskaya (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 252-253 
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    Union office, Change of Address (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 252-252 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    The harvest of a quite eye. A selection of scientific quotations by A. L. Mackay (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 350-350 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    A new statistical model for describing errors in isomorphous replacement data: the case of one derivative (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 351-359 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A probability method for phasing macromolecular isomorphous replacement data from the viewpoint of errors arising in the isomorphous replacement process is considered. The assumption of imperfect isomorphism between the atomic positions in the native crystal and the atomic positions in the native component of the derivative crystal leads to estimates of phase dependent on the value of sin θ/λ. The mathematical techniques used are similar to those employed in deriving probability distributions of structure seminvariants. Some of the formulas, which apply only to the case of one derivative, are compared with earlier results.
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    The electrostatic term in lattice-energy calculations: C2H2, CO2 and C2N2 (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 366-370 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The widely used atom-atom approximation for evaluating intermolecular energies is deficient in its treatment of the electrostatic interactions. Lattice-energy calculations have been performed for three crystal structures with explicit incorporation of the electrostatic energy, at three levels of approximation, based on Hartree-Fock molecular charge distributions. Although the molecules chosen are all non-polar, the electrostatic term in each case provides most of the calculated lattice energy and leads to an appreciable contraction of the predicted equilibrium cell dimensions. In cyanogen the electrostatic contribution appears necessary to account for the observed orthorhombic structure rather than an alternative cubic form. Treating each molecule as a point quadrupole severely overestimates the interaction energies of nearest-neighbor molecules but for more distant neighbors agrees fairly well with more detailed models of the molecular charge distribution. Assigning point charges to the several atoms is an adequate approximation for the three systems examined but greater flexibility is likely to be required for molecules of lower symmetry.
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    Electron microscopy studies of Mo3CoSi (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 387-390 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Crystals of Mo3CoSi were studied with a high resolution electron microscope. Planar defects of different kinds were easily and frequently observed. The structure of one kind of defect can be derived with crystallographic shear operations.
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    A three-dimensional model of crystal-growth disorder (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 391-400 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The relationship between nearest-neighbour growth-disorder models and Ising models is described in detail. The growth model with only two-body interactions is shown to be equivalent, in section perpendicular to the growth direction, to a simple Ising model. Two approximate solutions for the growth model are compared with exact solutions of equivalent Ising models, where possible, and also with computer realizations; the behaviour of these approximate solutions is discussed.
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    Rotation-function study of flavocytochrome b2 (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 412-416 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A rotation-function study of flavocytochrome b2 has been carried out using X-ray data from crystals which contain one tetramer of 230 000 Dalton per asymmetric unit. The function computed from 10-5.5 Å resolution data clearly shows the orientation of the molecular 222 symmetry axes. The presence of these symmetry elements is consistent with previous structural and biochemical studies of the molecule.
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    Topotactical dehydration of chloritoid (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 416-421 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The dehydration of the silicate mineral chloritoid in air and in vaeuo has been investigated by single-crystal X-ray methods and high-resolution electron microscopy. Vacuum dehydration yields an amorphous product, but the reaction in air produces a topotactical transformation to an anhydrous structure with an alteration in the stacking arrangement and symmetry. To be consistent with these observed facts, considerable modification to one component of the structure is necessary, a structural model being proposed for the anhydrous variant.
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    Determination of crystal point group by a high-resolution electron microscope image (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 429-430 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Crystal symmetry is exposed in a high-resolution electron microscope image more clearly at the thick part than at the thin part of the crystal, through the dynamical scattering. The crystal point group can be determined by observing how the symmetry in the image changes with thickness. A proposed method is demonstrated with an example of a bismuth titanate, Bi7Ti4NbO21.
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    X-ray and neutron diffraction from liquid alkali metals (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 431-445 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: This paper deals with the accurate determination of the structure factors of three liquid alkali metals: Na, K and Cs. Special attention has been paid to the experimental aspects: the design of the X-ray and neutron goniometers including the sample holders and the corrections to be applied to the rough experimental results. An assessment of the accuracy achieved has been made, mainly by comparison with existing physical data. In order to interpret the final results, theoretical calculations with hard-sphere models have been carried out and a comparison with the results of computer experiments, based on more realistic potentials, has been made. Some physical quantities related to the structure factor (pair distribution function, direct correlation function, pair potential and electrical resistivity) have been calculated.
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    Dynamic deformation effects on the Debye–Waller factor (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 445-448 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The effect on the Debye-Waller factors of having scattering factors which depend on atomic motion is developed through the reciprocal-space formulation. The resulting dependence of the Debye-Waller B on the scattering vector is of a simple form when a shell model adequately represents the deformation of the electron distribution. Numerical results are presented, without any serious approximations, for the governing constant appropriate to six alkali halides and explicit results are given for ΔBk/Bk for NaF, NaCl and KCl. A comparison with the recent real-space formulation of March & Wilkins [Acta Cryst. (1978), A34, 19-26] suggests that the reciprocal-space approach might be more fruitful.
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    Determination of secondary-extinction-free values for the flipping ratio in polarized neutron diffraction (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 468-475 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In the analysis of data obtained by polarized neutron scattering, corrections for extinction are of vital interest. It is demonstrated that, in some materials like metal alloys, secondary extinction in samples obtained from ingots by standard methods is very unlikely to be described by a simple mosaic model. It is shown that cold working of the samples improves the sample properties in this respect. Examples of such treatment are given and a method is described to analyse R-on- rocking-curve data in order to obtain data corrected for secondary extinction.
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    On the vibrational average of diatomic molecular form factors: a study of the convolution approximation (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 476-481 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The zero-point vibrational average of molecular form factors for diatomic molecules is treated as an observable from X-ray diffraction measurements. This model allows exploration of the influence of a single internal vibration on FBragg(S). It is found that when anharmonic terms in the atomic vibration are included, then deconvolution to the static form factor, F(S;Re) is achieved. Neglect of the anharmonic terms gives rise to spurious sharp dipole features in the heavy-pseudo-atom charge densities, but not to the H-pseudoatom charge density. Non-rigid terms in the pseudoatoms are virtually negligible except for H where the non-rigid effect is as large as 6% in the hydrogen quadrupole scattering factor. The diatomic molecules studied include N2(1Σg+), CO(1Σ+), BF(1Σ+) and FH(1Σ+).
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    Temperature dependence of the Debye–Waller factor for magnesium measured by energy-dispersive diffractometry (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 482-484 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The Debye-Waller factor for powdered magnesium was measured in the temperature range 293 to 700 K by the energy-dispersive X-ray method. The results are compared with published theoretical calculations based on the quasi-harmonic and the anharmonic approximations. Anisotropy was found in the behaviour of the temperature factor B(T), with excellent agreement in the a axis direction between the experimental and theoretical calculations within the quasi-harmonic approximation. In the c axis direction, however, the experimental values at increasing temperature exceed the theoretical values, even in the anharmonic approximation.
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    On the significance of 'non-significant' reflexions (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 497-499 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The information content of so-called 'unobserved', or 'non-significant' reflexions obtained by diffractometer measurements has been experimentally studied, using data and input parameters for three previously deter- mined crystal structures. It can be concluded that even those reflexions having the largest σ(I)/I values contain significant information about both thermal and positional atomic parameters. It has been possible to carry out an acceptable refinement of one structure on the basis of only the set of structure factors discarded by the original investigator. Inclusion of the weakest reflexions in the refinements lowers the standard deviations, and so should be standard practice.
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    A simple statistical treatment of unobserved reflexions. Application to two organic substances (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 500-501 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A simple statistical method for estimating the unobserved reflexions is described together with its application to the solution, by means of direct methods, of the crystal structure of two organic compounds. The method proved to be successful in solving both structures for which more than 60% of the reflexions had not been measured because they were too weak.
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    Specific heat and compression curves of erbium (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 505-507 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Simple models for the thermal expansion of erbium, and for deducing the magnetic energy from the specific heat, are shown to give an approximate account of the facts. Values are given for the energy of magnetic ordering and the magnetic entropy.
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    Quantum chemistry of solids: the chemical bond and energy bands in tetrahedral semiconductors by A. Levin (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 508-509 
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    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/S0567739479001194
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    Structures in nature as a strategy for design by P. Pearce (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 510-510 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
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    An introductory guide to information sources in physics by L. R. A. Melton (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 510-510 
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    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/S0567739479001236
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  • 83
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    The calculation and interpretation of high-resolution electron microscope images of lattice defects (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 511-524 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Limitations on the calculation of high-resolution detail in electron microscope images of defects in crystals are discussed and it is shown how images of crystals with arbitrary strain can be calculated by means of a reinterpretation of the normal dynamical equations. An alternative approach, which in some cases is preferable for numerical calculations, is the method of periodic continuation. These methods, and various approximations to them, are applied to the study of high resolution images of edge dislocations. It is shown that at the 5 Å level of resolution, the column approximation is adequate for calculating the important features of weak-beam images of an edge dislocation with an extended core. However, when applied to the calculation of lattice fringes near a core, it may lead to serious error. Calculations are presented which show that, because of spherical aberration, care is required in the interpretation of lattice-fringe images near a dislocation core even for thin crystals.
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  • 84
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    `Polar flattening': non-spherical effective shapes of atoms in crystals (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 641-645 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An examination of the nearest non-bonded interatomic distances found in crystals shows that where the two atoms are each covalently bound to only one other atom they exhibit non-spherical effective shapes. Such atoms behave as if flattened at their poles. Despite the large number of crystal structures now known in detail, various factors can militate against discovering effective atomic shapes. Possible causes of flattening are briefly discussed.
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  • 85
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    Short-range ordering of vacancies in TiO at 1323 K (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 646-652 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The diffuse X-ray scattering from a single crystal of disordered TiO was measured at 1323 K in a volume in reciprocal space. The short-range order parameters between vacancies and cations and anions were determined by Williams's [(1972). Report ORNL-5140. Oak Ridge National Laboratory, Tennessee] least-squares procedure for fitting the total intensity to contributions from this local order and up to quadratic terms in the atomic displacements. Computer simulations with the short-range order coefficients reveal that the local atomic arrangements are similar to those in the low temperature phase. The relationship between this local order and the electronic conductivity is discussed.
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  • 86
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    The number of permitted modes of propagation in N-beam dynamical diffraction of X-rays (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 543-547 
    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 real part of the root of the dispersion equation for the permitted modes of propagation is always positive for two-beam Laue and Bragg reflections at the exact diffraction position. Based on this, a general rule is proposed to determine the number, Np, of permitted modes of propagation for N-beam dynamical diffraction, where no extremely asymmetric reflections are involved. In other words, for both σ- and π-polarized wavefields, Np= 2(N- NBragg), where NBragg is the number of Bragg reflections involved. This conclusion is supported by calculations for three-, four-, six- and eight-beam cases.
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  • 87
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    The crystal structure of deuteroammonia between 2 and 180 K by neutron powder profile refinement (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 569-571 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The structure of ND3 has been refined by neutron powder profile analysis at 2, 77 and 180 K to a resolution of sin θ/λ = 0.85 Å-1. No orientational phase transition was found in this range. However, even at 2 K, a strong librational motion exists. The N--D bond length, after correction for libration, is constant with temperature (1.06 Å), as are the D-N-D angles (107.5 ± 0.2°). This bond length is 5% longer than that found in the free molecule, because of hydrogen bonding, but the bond angles are virtually identical.
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  • 88
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    (4H)2–4C Type superstructure of TiS1.46 as determined by high-resolution electron microscopy (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 564-569 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The superstructure of titanium sesquisulfide, TiS1.46, is derived on the basis of 1 MV and 100 kV high-resolution electron microscope images. The crystal has monoclinic symmetry (pseudo-hexagonal) with lattice parameters a = 5-92, b = 10.3, c = 22.8 Å and β = 90°. The space group is Cc. The metal vacancies are confined to every second metal layer and ordered within the layer. They are arranged in a stacking sequence of the 4C-type along the c axis. The sulfur atoms are in the 4H-type stacking sequence along the c axis.
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  • 89
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    On the structure of Mn5Si3, Th6Mn23 and γ-brass (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 580-583 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The structures of Mn5Si3, Th6Mn23 andγ-brass are accurately derived from octahedra (Mn5Si3), tetrahedra and triangle-capping. The relationship between pyrochlore, Th6Mn23 and γ-brass is also derived.
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  • 90
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    Optimisation de la correction d'absorption (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 587-590 
    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 bias in absorption corrections can be reduced by a method based on a least-squares fit of an anisotropic linear absorption coefficient from Ψ-scan observations. This method is carded out by a Fortran program and has been used with success on many data collections, especially for electron density studies.
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  • 91
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    Breakdown of Friedel's Law in the Kikuchi patterns of tellurium (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 604-610 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Evidence of the breakdown of Frieders law in the Kikuchi patterns of tellurium crystals has been observed with a standard transmission electron microscope (TEM). These observations give a direct method of identifying enantiomorphously related phases in the TEM. An interpretation of the observed deviations from Frieders law is proposed on the basis of the model of Thomas & Humphreys [Phys. Status Solidi A (1970). 3, 599-615] for Kikuchi patterns. The possible differences between the phases of the Fourier coefficients of the crystal potential and of the absorption potential have to be taken into account. Some light is thrown on the physical significance of the phenomenological parameters.
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  • 92
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    On the contributions of the internal modes of molecules to the Debye–Waller factors. II. Theoretical considerations (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 610-613 
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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 the expressions for the contributions of the internal modes of molecules to the Debye-Waller factors as derived, on the one hand, by means of lattice dynamics and, on the other hand, by means of the FG method common in spectroscopy, are identical after some approximations have been made in the lattice dynamical formulation. It is pointed out that for those methods of establishing the force constant matrix of the internal modes, where the eigenvalues of the FG matrix are constrained to be the squares of the observed frequencies, only standard eigenvector routines need be applied in determining the mean-square-amplitude matrix. Hence, in this case, series expansions which were suggested to circumvent the determination of eigenvalues are superfluous.
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    URL: http://dx.doi.org/10.1107/S056773947900142X
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  • 93
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    A limited-range step-scan method for collecting X-ray diffraction data (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 616-621 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A limited-range, step-scan method for rapid diffractometer data collection is described. The data are reduced in a two-pass procedure. First, the step data from the more intense peaks are fitted by a Gaussian function yi = C1 exp [-(\bar x - xi)2/C22], where C1 is the peak maximum, C2 is proportional to the peak width and C1C2 is proportional to the integrated intensity. Because the peak widths may vary appreciably over the range of data observed and cannot be determined very precisely for weak reflections, an empirical function is determined from the more intense reflections in the first pass and the calculated peak widths are used for all reflections in the second pass. Comparison of data obtained by this method and the usual full-scan technique shows that comparable precision can be obtained even when the limited scan range data are collected at twice the rate of the full-scan technique. The method has been used to collect data for five proteins and typical results are cited.
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    On the relative stability of ordered, F\overline{4}3m, and disordered, Fm3m, adamantane crystals (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 627-629 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Atom-atom calculations show that Fm3m, F\bar 43m, and I\bar 43m adamantane crystals are close in free energy at room temperature. While Fm3m is stablest for an unstrained crystal, a small ( ̃3%) uniform tetragonal distortion stabilizes an orientationally ordered, strained I\bar 43m or F\bar 43m structure. The structures may well also be stabilized in a crystal of small mosaic block size in which there is a wide distribution of local tetragonal strain about a mean unstrained value.
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    Gyrotropy: an implicit form of ferroicity (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 629-633 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Some optically active ferroic crystals (a few ferroelectrics and one ferroelastic) are known to exhibit reversal of the sign of spontaneous optical rotation in certain directions when a suitable electric field or uniaxial stress is applied. This phenomenon, to be called gyrotropy, is examined in the framework of Aizu's formalism of ferroicity. The case of dicalcium strontium propionate, Ca2Sr(C3H5O2)6, a ferroelastic-ferroelectric, is analysed to illustrate a simple procedure for enumerating gyrotropic orientation states. The 18 'pure' gyrotropic species of crystals are listed: these are those species for which gyrotropy is not accompanied by primary ferroicity. Ni-Cl boracite, BaMnF4, RbNO3, and some related crystals, are suggested as new potential gyrotropics. The first two of these are ferromagnetic gyrotropics, of which no examples are known so far.
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    The use of electric field gradient calculations in charge density refinements. I. Hirshfeld-type deformation functions and the calculation of electric field gradients by Fourier series (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 652-658 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Single-crystal X-ray diffraction data contain in principle complete information on the spatial distribution of the bonding electrons. However, this information is subject to a variety of errors. Nuclear quadrupole resonance spectroscopy (NQR) provides a very sensitive measurement of an electrostatic property of the deformation density; the electric field gradient tensor at the site of a nucleus. It is proposed that diffraction and NQR data are combined so that more reliable charge densities can be obtained. Well known multipole deformation functions are used to describe a quasistatic and parametrized deformation density. The electric field gradient is computed by a Fourier series and expressed as a function of the same deformation parameters. These can then be refined by least-squares calculations simultaneously with respect to diffraction intensities and NQR results. Overlap corrections of the neutral spherical atoms and the effect of temperature vibrations are discussed.
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    An X-ray diffraction investigation of chromium-substituted lithium ferrite (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 665-672 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The loss of order in the lithium spinel system Fe(Li0.5Fe1.5 x/8Crx/8)O4 with 0 ≤ x ≤ 2 is examined as a function of the Cr3+ ion concentration through an analysis of the X-ray superlattice line integrated intensities and line breadths. In these materials it is found that the growth of the antiphase domains is inhibited by the action of Cr3+ ions at the domain boundaries and it is shown that the observed variation of the long-range order parameter may be explained on the basis of a cation distribution on the octahedral sites which adheres to the so-called tetrahedral invariance condition. Furthermore, a correlation exists between the probability of an unstable Cr3+ ion configuration and the formation of domain boundaries. Some discussion is presented on the variation of the domain thickness distribution with Cr3+ ion content.
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    Overall values of conventional discrepancy indices in the presence of random positional errors: crystal with similar atoms (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 672-675 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The overall values of the conventional discrepancy indices based on structure amplitudes [i.e. R(F)] and intensities [i.e. R(I)] are evaluated as functions of the mean positional error (|Δr|) and the fractional contribution of the known atoms to the local mean intensity (i.e. σ21) for crystals containing a large number of similar atoms. The results are tabulated for both the centrosymmetric and non-centrosymmetric cases.
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    Grain boundary structures in f.c.c. and b.c.c. metals and sites for segregated impurities (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 689-693 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The key features leading to low-energy grain boundaries in metals are discussed by reference to computer-simulated structures and geometrical analysis in terms of atom packing. Low energy is found to be associated with boundary structures consisting of relatively dense packing, and this can in turn be expressed in terms of the space-filling packing of coordination polyhedra. The geometrical method of analysis is shown to be well suited to the identification of interfacial sites for segregated impurities.
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  • 100
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    Thermal diffuse scattering for molecular crystals: errors in X-ray diffraction intensities and atomic parameters (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 675-684 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: First-order TDS calculations have been made for naphthalene at 100 K to judge the suitability of different models for calculating TDS corrections used in practice. It appears that a long-wave model which takes account of the non-linearity of the frequency dispersion, gives about the same corrections for structural parameters as the lattice dynamical model for rigid body molecules and harmonic vibrations. Longwave models with linear frequency dispersion give an overcorrection of the errors. For second-order TDS no explicit comparison of the models has been possible, but rough estimates show that the comparison made for first-order TDS is also valid in good approximation for the total TDS.
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