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  • International Union of Crystallography (IUCr)  (6,726)
  • 1975-1979  (6,726)
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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
    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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  • 3
    Electronic Resource
    Electronic Resource
    Ewald dynamical diffraction symposium (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1-1 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000012
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  • 4
    Electronic Resource
    Electronic Resource
    Permissible angles for coincidence-site-lattice rotations (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 255-259 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In addition to symmetry rotations (Σ = 1), lattices may admit coincidence-site-lattice (c.s.l.) rotations of various degrees of coincidence, Σ. The conditions for the occurrence of c.s.l, rotations are formulated in general terms by introducing the metric matrix of a lattice. When all lattices are considered, it is found that there are 4Σ possible values for the angle of rotation which give rise to a degree of coincidence Σ. These angles have cosines which are integral multiples of 1/2Σ. A particular lattice admits only some of these c.s.l, rotations. Cubic lattices are discussed in detail and it is shown that the number of permissible rotation angles for each odd value of Σ is approximately 3.414 × √Σ. Conversely, a particular rotation angle originates a c.s.l, of degree of coincidence Σ in those lattices which satisfy particular metric conditions. Finally, the effect of a uniform strain due to temperature or pressure changes is analysed, and it is shown that while symmetry rotations are invariant to this form of strain, only very few c.s.l, rotations are unaffected.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000553
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  • 5
    Electronic Resource
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    Some consequences of the presence of antisymmetric atomic features in noncentrosymmetric structures (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 260-265 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Noncentrosymmetric structures are considered in terms of a generalized substructure formulation. Inclusion of dispersion, bonding and anharmonicity leads to generalized expressions for Bijvoet ratios. Methods of numerically estimating the effects due to bonding and anharmonicity upon the Bijvoet ratios are suggested. Allowance for bonding and anharmonicity is shown to have wide implications for the breakdown of Friedel's law. Subsets of reflections that obey Friedel's law in the conventional approximation are now shown to violate it. These violations are of considerable importance for measuring antisymmetric contributions due to bonding and anharmonic thermal vibrations. The possibility that bonding effects will result in the appearance of Bijvoet differences in noncentrosymmetric structures of elements is explored.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000565
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  • 6
    Electronic Resource
    Electronic Resource
    A priori estimates of errors in intensities for imperfectly spherical crystals (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 78-82 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A priori estimates of errors in intensities for spheres arising from non-sphericity in crystal shape may now be readily determined from a recently published table of (1/A*)(∂A*/∂μR) values [Flack & Vincent (1978). Acta Cryst. A34, 489-491] using an expression given by Jeffery & Rose [Acta Cryst. (1964), 17, 343-350]. However, a new relationship between σ(R) and σ(r) is determined. The effects of crystal radius, non-sphericity and wavelength on intensity errors are discussed. The importance to the selection of crystals for electron density measurements is stressed. Universal curves of σ(r)/r, the error due to non-sphericity, against μr calculated for a range of intensity errors at θ = 0 and 45 ° are given.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000140
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  • 7
    Electronic Resource
    Electronic Resource
    A simplified calculation and tabulation of tensorial covariants for magnetic point groups belonging to the same Laue class (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 95-101 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Four types of static tensors can be distinguished according to their parity with respect to space inversion and to time reversal. However, all magnetic point groups belonging to the same (oriented) Laue class consist, apart from inversions, of the same proper rotations. Tensors differing only by parities transform identically under the same proper rotations; their transformation properties under different groups of the same Laue class may therefore differ only by an additional change of sign, which depends on the tensor parity and on the way in which inversions are combined with proper rotations in a given group. It is shown that, for a certain natural choice of typical representations of magnetic point groups of the same Laue class, it is sufficient to calculate tensorial covariants (symmetry-adapted tensorial bases) of even parity with respect to both space inversion and time reversal for the group of proper rotations. Tensorial covariants of other parities and for other magnetic point groups of the same Laue class can then be obtained by the use of a simple conversion table and of parity arguments. The scheme is illustrated by an example from the Laue class D4.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000164
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  • 8
    Electronic Resource
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    X-ray high-order refinements of hydrogen atoms: a theoretical approach (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 137-139 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Recent work in X-ray crystallography has demonstrated that reasonable nuclear H positional parameters can be obtained by refinements using high-order data only. A theoretical model for high-order refinements of H atoms has been developed which simulates the experimental results. For a suitable choice of the lower sin θ/λ cut-off, the H atoms are shown to refine to their actual nuclear positions. The calculations imply that the proper cut-off is indeed dependent on the type of atom to which the H is bonded, in agreement with experimental results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000231
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  • 9
    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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  • 10
    Electronic Resource
    Electronic Resource
    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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  • 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
    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 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    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+).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479001108
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  • 13
    Electronic Resource
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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 
    Details
    ISSN: 1600-5724
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479001133
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  • 14
    Electronic Resource
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    Errors in diffuse scattering measurements associated with Bragg diffraction (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 503-505 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Multiple-scattering processes are identified as a source of error in diffuse-scattering measurements on solid solutions. Of the various methods used to account for such effects, a multiple-regression analysis using reduced weights for measurements having maximum interference appears to be the most satisfactory.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479001169
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  • 15
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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 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479001194
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  • 16
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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 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479001236
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  • 17
    Electronic Resource
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    Independent fourth-order elastic coefficients for all crystal classes (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 525-533 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Numerical methods can be used to obtain the dependence relations between components of an nth-rank tensor for the particular symmetries of each crystal class. The program has been used successfully to calculate the fourth-order elastic coefficients, corresponding to a tensor of eighth rank for all crystal classes and the isotropic case.
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    URL: http://dx.doi.org/10.1107/S0567739479001261
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  • 18
    Electronic Resource
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    Magnetic structure of PrCo2Ge2, a neutron diffraction study (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 533-536 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The crystallographic structure of the compound PrCo2Ge2 is of the BaAl4 type. The space group is I4/mmm and the lattice constants are a = 4.048 and c = 10.178 Å. A neutron diffraction study revealed a transition to a magnetically ordered state at TN ̃27 K and six superlattice lines below TN. It is possible to index these lines according to a sinusoidal magnetic structure with a period τ-1 = 13.94 Å along c. Observed intensities are consistent with ordering of the Pr sublattice with the magnetic axis along c.
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    URL: http://dx.doi.org/10.1107/S0567739479001273
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  • 19
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    Semi-metals & narrow-bandgap semiconductors by D. R. Lovett (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
    URL: http://dx.doi.org/10.1107/S0567739479001248
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  • 20
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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 
    Details
    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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  • 21
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    Derivation of possible framework structures formed from parallel four- and eight-membered rings (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 547-553 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The number of possible combinations of parallel four-membered rings, of the same kind, forming an eight-membered ring is systematically derived by a different method from that of Smith & Rinaldi [Mineral. Mag. (1962). 33, 202-211]. 17 different configurations (including six enantiomorphic) in the UUDD ring (U and D represent upward- and downward-pointing tetrahedra respectively), four different ones (two enantiomorphic) in the UDUD ring, and sixteen different ones (seven enantiomorphie) in the UUDD ring are shown with their new pattern representations and symbol notations.
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    URL: http://dx.doi.org/10.1107/S0567739479001315
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  • 22
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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 
    Details
    ISSN: 1600-5724
    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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    URL: http://dx.doi.org/10.1107/S0567739479000620
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  • 23
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    The theoretical deformation density for oxalic acid (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 319-325 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Theoretical deformation densities for oxalic acid and oxalic acid dihydrate have been determined by ab initio Hartree-Fock calculations using extended atomic basis sets. The results are in good agreement with experimental X-N maps. The non-sphericity of the deformation density in certain bond cross sections has been established. Special emphasis is placed on the discussion of lone pairs and hydrogen bonds. The deformation density for the hydrate water has been analyzed.
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    URL: http://dx.doi.org/10.1107/S0567739479000668
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  • 24
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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 
    Details
    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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    URL: http://dx.doi.org/10.1107/S0567739479000693
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  • 25
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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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    URL: http://dx.doi.org/10.1107/S0567739479001431
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  • 26
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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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    `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 
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    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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    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 
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    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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    A matrix formulation for choosing starting sets in direct methods (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 695-696 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A method for finding the strongest starting set of reflections for multisolution direct methods is proposed and compared with the Convergence Method of Germain, Main & Woolfson [Acta Cryst. (1970), B26, 274-285].
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    Molecular vibrations in crystals by J. C. Decius and R. M. Hexter (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 701-701 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    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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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    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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    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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    The defect structure of VOx. I. The ordered phase, VO1.30 (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 745-756 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Integrated X-ray intensities were obtained from a single crystal of VO1.30 annealed below the ordering transition. The space group is I41/amd, and the unit-cell contents are V51.6O64. The atomic arrangement is similar to that proposed by Andersson & Gjönnes [Acta Chem. Scand. (1970), 24, 2250-2252], but there are more vanadium vacancies and interstitial ions. The latter are surrounded by four vacancies as in the defect structure of FexO. The oxygen ions around an interstitial vanadium ion are displaced away from it; oxygen and vanadium ions on the octahedrally coordinated sites exhibit strongly correlated displacements. There are anisotropic electron density distributions at vanadium ions near a vacancy. These effects indicate that the order--disorder transition is not due to a Jahn-Teller effect, but instead is a result of a long-range cooperative interaction, presumably due to the semi-metallic nature of this oxide.
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    A theoretical weighting scheme for tangent-formula development and refinement and Fourier synthesis (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 757-764 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A weighting scheme for use in tangent-formula phase development and refinement is derived by application of joint probability distribution functions. It may easily be incorporated in existing computer programs. A weighting scheme for Fourier synthesis is also described; it takes into account the uncertainty of the phases assigned by a direct procedure.
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    Thermal expansion of α-NH4HgCl3 (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 923-924 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: α-NH4HgCl3 belongs to the tetragonal system with the space group P4/mmm. Earlier X-ray studies indicate that this crystal can be described as a layer structure, the layers being perpendicular to the tetragonal axis. With a Rigaku camera, modified for work at high temperatures, the temperature variation of the lattice parameters and thermal-expansion coefficients of this crystal were determined. The coefficient of expansion, at room temperature, along the tetragonal axis is less than that in the perpendicular direction.
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    Phase extension and refinement (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 941-946 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Two procedures are described to extend and refine phases starting from a medium-sized set of known phases. From tests with 376 and 400 atom structures it was found that for extension purposes the tangent formula is suitable; for refinement purposes the tangent formula was adapted in order to maintain the enantiomorph.
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    Simultaneous diffraction: Umweganregung peaks in the case of a vibrating single crystal (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 962-965 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The positive diffraction intensity changes on forbidden (222) planes of a vibrating single crystal of Si, which are produced by simultaneous diffraction, are experimentally treated for the symmetric Laue case. The neutron diffraction experiment was performed for a flexurally vibrating as well as a non-vibrating perfect silicon bar. Several Umweganregung peaks were observed after exciting the bar into vibration while no Umweganregung peak was observed in the case of a non-vibrating crystal.
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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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    A solution of the X-ray `phase problem' (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 17-21 
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    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.
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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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    Tensorial covariants for the 32 crystal point groups (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 83-95 
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    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.
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    Handbook on the physics and chemistry of the rare earths, Vol. 1: Metals edited by K. A. Gschneider Jr and L. Eyring (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1089-1089 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Union office, 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
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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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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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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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 equivalent subgroups and supergroups of the space groups (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 733-745 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Symmetry operators (α\τα) of a space group G and (β\τβ) of a subgroup g are conjugated with respect to a similarity operator S = (S|T) where S is a 3 x 3 matrix and T a column matrix. The matrix S relates the lattice vectors of G to those of g whilst T describes the origin of the lattice of g in the coordinate system of G. We determine here the coefficients of the matrix S when G and g are equivalent, i.e. isosymbolic or enantiomorphic. Here the index of g in G is equal to |det S|. Its lowest values are tabulated for all space groups.
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    Statistical properties of normalized difference-structure factors for non-centrosymmetric structures (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 765-772 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The probability density of phase angles for structures with one or more atoms on known positions [Sim (1959). Acta Cryst. 12, 813-818] is expressed in terms of normalized difference-structure factors, and used to calculate standard deviations of phases and expectation values for amplitudes (|Er|) of the normalized difference-structure factors. Numerical results are tabulated for various values of |E1| and |E2|, i.e. the minimum and maximum |Er| value a given reflection can have. Applications to the DIRDIF procedures [van den Hark, Prick & Beurskens (1976). Acta Cryst. A32, 816-821] are described. New applications are the calculation of the a priori probability density function for |Er| values, and a statistical rriethod for the detection of a centre of symmetry in the remaining part of the structure. For the statistical method, a centricity parameter, Xc, which is unity for centric and zero for acentric distributions, is defined; for the difference structure, Xc is calculated by an iterative procedure, extrapolating towards zero contribution of the known part of the structure. Numerical results for 13 test structures are given.
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    Intensity errors due to beam inhomogeneity and imperfectly spherical crystals (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 795-802 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An analysis of the intensity errors from imperfectly spherical crystals with an inhomogeneous incident X- ray beam is presented. The crystals are considered to have either no absorption or strong absorption. The inhomogeneity of the incident beam profile is represented by three possible models: (a) a Lorentzian, (b) a Gaussian and (c) a second-order Taylor series expansion. The results are compared with intensities for electron density studies from five crystals.
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    Temperature dependence of X-ray reflection intensity from an absorbing perfect crystal near an absorption edge (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 831-835 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The temperature effect of X-ray integrated reflection intensities is discussed theoretically in the energy region near the absorption edge of Ga in GaAs perfect crystal. From the calculation based on the dynamical theory of diffraction, it is concluded that the integrated intensity of {\bar 5}{\bar 5}{\bar 5} in the Bragg case shows a complicated temperature effect depending on the change of the anomalous scattering factor across the absorption edge. Calculated results are compared with a preliminary measurement which shows a qualitative agreement with the theory.
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    X-ray intensity measurements on large crystals by energy-dispersive diffractometry. IV. Determination of anomalous scattering factors near the absorption edges of GaAs by the one-intensity-ratio method (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 828-831 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The imaginary part f" (ω) of each scattering factor for Ga and As has been measured through the absorption energy regions near the K absorption edges of both Ga and As atoms. The intensity ratio r555 between 555 Friedel-pair reflexions has been measured in the same energy regions. From these data, the real part f'(ω) has been determined in the same way as has already been published [Fukamachi & Hosoya (1975). Acta Cryst. A31, 215-220]. The f'(ω) values above the edges thus obtained show fairly good agreement in fine structure with those calculated from f" (ω) values with the dispersion relation, but some discrepancies are found in the regions below the edges.
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    A method for rapidly relocating misplaced molecules (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 860-860 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A recurrence relationship allows structure factors to be rapidly calculated for a trial structure translated to alternative sites in the unit cell.
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    Constant intensity curves in critical scattering and von Laue diagrams (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 859-859 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Neumann's principle is applied for drawing an analogy between the constant intensity curves of critical scattering and the von Laue diagrams of X-ray scattering in single crystals.
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    Eleventh International Congress of Crystallography, Warsaw, Poland. 3-12 August 1978 (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1021-1067 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Introduction to solid state theory (Springer series in solid state sciences. Vol. 2) by O. Madelung (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1087-1088 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Solid Surface Physics. Springer Tracts in Modern Physics. Vol. 85 edited by G. Höhler (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1088-1089 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Landolt–Börnstein. Numerical data and functional relationships in sciences and technology. Group III. Crystal and solid state physics. Vol. 11. Elastic, piezoetectric, pyroelectric, piezooptic, electrooptic constants and nonlinear dielectric susceptibilities of crystals by M. M. Choy, W. R. Cook, R. F. S. Hearmon, H. Jaffe, J. Jerphagnon, S. K. Kurtz, S. T. Liu and D. F. Nelson (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1090-1090 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    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 
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    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.
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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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    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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    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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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    On freedom from extinction and the (universal) kinematical limit (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 50-57 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Examination of the limiting relation of extinction and diffraction makes it clear that extinction is only zero, in an absolute sense, when diffracted power is identically zero. This latter condition is the proper operational identifier for the attainment of the kinematical limit and is valid irrespective of the state of perfection of the crystal medium. At the limit of zero diffracted power, the kinematical (single-scattering or first Born) approximation is asymptotically exact so that experiment and theory become strictly compatible. Experimental structure-factor values which are free from extinction effects can therefore be derived in this limit. In practice, the advantages of this approach have to be gained by greater attention to data collection. Typically, the method involves (i) determination of integrated reflectivity at a series of levels of interaction (attained by controlled variation of a suitable physical parameter) and (ii) extrapolation of an appropriate function of the measurements to zero level of interaction as identified by zero diffracted power. Various possible procedures for effecting this approach are discussed here in general terms. The approach proposed here has advantages over the earlier prescription of the kinematical limit [Bragg, Darwin & James (1926). Philos. Mag. 1, 897-922] based on the state of the crystal medium ('ideally imperfect'). It avoids any need for the necessarily approximate assumptions inherent in the Darwin-Zachariasen treatment of extinction. It also avoids dealing with the complications arising from idiosyncratic or anisotropic extinction effects since it refers all cases to zero level of interaction. The kinematical limit, as defined here, is a universal limit.
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    A probabilistic theory of two-phase seminvariants of first rank via the method of representations. III (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 296-305 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The estimation of two-phase seminvariants of first rank is carried out for all space groups. Representations theory [Giacovazzo (1977). Acta Cryst. A33, 933- 944] is suitably combined with the joint probability distribution method. Formulae are obtained which exploit the knowledge of the diffraction magnitudes belonging to the first phasing shell of two-phase seminvariants both via the exponential form the characteristic function and via its Gram-Charlier expansion.
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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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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    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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    Electron density mapping in molecules and crystals edited by F. L. Hirschfeld (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 254-254 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    A geometrical-approach to solving crystal structures (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 266-275 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The Karle & Hauptman and Goedkoop matrices are regarded as metric tensors in Hilbert space. This enables an E value to be expressed in terms of a finite number of other E's. It is shown that the knowledge of a finite number of E's is theoretically sufficient to determine all the atomic coordinates. This number is smaller than 8(N + 3) divided by the order of the point group, counting a complex E twice (N is the number of atoms in the unit cell). The phase problem is analyzed from a geometrical point of view and it is shown how probability becomes certainty with a finite number of data. The Σ2 relationship is obtained as a particular approximation of an exact relationship between E's. This theory enables a criterion to be established which is equivalent to Tsoucaris's maximum determinant rule but more restrictive. The theory is valid for structures with both equal and unequal atoms.
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    High-resolution diffuse X-ray scattering study from nearly perfect silicon single crystals (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 286-295 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: High-resolution measurements of diffuse X-ray scattering (DXS) have been made at and above room temperature around 111, 333, 444 and 555 reciprocal lattice points (relps) using highly collimated Mo Kα1 and Cu Kα1 radiations with the specimen set in (1, -1, 1) symmetrical Bragg geometry. The distribution of DXS intensity around different relps has shown that at temperatures up to at least 573 K the contribution of thermal DXS to the observed DXS is very small. This is apparently due to the high value of the Debye temperature (640 K) of silicon. A remarkable feature of these results is that for the same value of the scattering vector |K*| the DXS intensity is different for the parallel and antiparallel orientations of K* relative to R*. The amount of anisotropy varied from sample to sample and depended on the thermal history of the specimen. This and the other features show that the observed DXS is predominantly due to point defects and their aggregates. A typical size parameter for the aggregates is 3000 to 10 000 Å.
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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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    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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    The diffraction of X-rays by face-centred cubic crystals containing extrinsic stacking faults – a derivation from the general theory (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 337-338 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A solution is given to that problem of extrinsic faulting in face-centred cubic crystals in which additional layers may be inserted after layers of the original crystal or, generally with a different probability, after previously inserted layers. Two previous results are deduced as special cases.
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    Environment international. A journal of science, technology, health, monitoring and policy edited by A. A. Moghissi and B. D. Moghissi (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
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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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
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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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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    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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    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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    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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    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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    ISSN: 1600-5724
    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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    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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    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 new Monte Carlo method for phase determination (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 220-224 
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    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.
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    Electrogyration and piezogyration in NaClO3 (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 225-232 
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    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.
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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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    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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    Third-order elastic constants of cubic sodium cyanide and potassium cyanide (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 240-243 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The complete third-order elasticity tensors of cubic NaCN and KCN have been determined from stress-shifted resonance frequencies of thick plates at 293 K. The two cyanides exhibit quite similar nonlinear elastic properties which, however, differ strongly from those of comparable alkali halides such as NaCl and KCl. The 'longitudinal' constants c111 are ca five times smaller and the 'transverse' constants c112 and c113 ca eight times larger than those in NaCl and KCl. The Cauchy relations are strongly violated, as is already known for the second-order elastic constants. The critical shear resistance c44, which is connected with the phase transitions observed at 283 K (NaCN) and 168 K (KCN), exhibits a considerable dependence on hydrostatic pressure only in the case of NaCN: c44 drops rapidly with increasing pressure. The absolute change of c44 per hydrostatic pressure unit, however, is comparable to that in alkali halides. The nonlinear elastic behaviour of NaCN and KCN is qualitatively interpreted by interactions of volume-conserving type (as existing in fluids).
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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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    ISSN: 1600-5724
    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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    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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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Dynamical scattering of X-rays in crystals by Z. G. Pinsker (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 347-347 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    A 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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  • 84
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    Application of quartet and quintet invariants in phase determination (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 381-387 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Recent derivations of probability expressions for quartet and quintet structure invariants allow the reliable estimation of the phase sums of quartets and quintets in the range 0 to π. A new quartet figure of merit, ENQUAC and a new quintet figure of merit, ENQUIC, based on these estimates are described, which are particularly useful in non-centrosymmetric symmorphic and polar space groups. An adapted tangent-refinement procedure employing selected triplets with a phase sum of 0, together with the quartet and quintet phase estimates, enables enantiomorph-specific phase refinement. The way in which the figures of merit and the refinement technique can be used in various practical procedures is demonstrated and applications to two structures in space group P1 are presented.
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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 
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    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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    Molecular spectroscopy. Vol. 5 edited by R. F. Barrow, D. A. Long and J. Sheridan (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 702-702 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Electron diffraction study of diffuse scattering due to atomic displacements in disordered vanadium monoxide (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 718-727 
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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 intensity pattern observed by electron diffraction from VO1.23 annealed above the ordering temperature, has been interpreted in terms of short- range-order parameters and by model calculations. Emphasis was put on the ordering of displacements, which gives the major contribution at large scattering angles - a range where the intensity variations at 1 MV are well suited for recording. Through detailed dynamical calculations it is shown that a greatly simplified procedure for calculation of the diffuse intensity is justified when the number and strength of dynamical interactions is moderate. A few displacement parameters were derived through a linear, multiple-regression method. Models for the displacements were based on these results and from information about the ordering of defect tetrahedra. These results agree with previous proposals which indicate large static displacements associated with ordering of defects. Apart from the presence of (100) acoustic phones, the indications of an expansion of the nearest-neighbour distance with a large transverse component were most significant. Such rotation may result from movement of atoms towards selected defect clusters. Model calculations demonstrated that rotated tetrahedra or a zig- zag string of atoms is probable - both fitting well into the assumed surroundings of the clusters.
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  • 88
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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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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    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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    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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    ISSN: 1600-5724
    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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  • 90
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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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    ISSN: 1600-5724
    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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    Neutron diffraction edited by H. Dachs (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 509-510 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 92
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    X-ray scattering intensities from a shell model for silicon (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 785-788 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Accurate theoretical intensities are presented for the phonon scattering at 295 K for Si. The one-phonon and n-phonon cross sections are shown for the principal symmetry directions and the total phonon scattering contours for a section of the (10\bar 1) plane. Previous total phonon calculations have been confined to materials of simpler structure and to a coarser mesh of wavevectors. The Si cross sections show general features which are expected to be seen in a wide variety of materials. These include the behaviour of the scattering near forbidden reflexions, the behaviour in three essentially different kinds of zones and the behaviour close to Bragg peaks. Results are also presented for the temperature dependence of the harmonic Debye-Waller factor.
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    Application of the matrix method to the calculation of diffuse scattering in linearly disordered crystals (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 803-810 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The matrix method for the calculation of diffuse scattering has been applied to linearly disordered crystals. Although it is shown that the matrix method and the difference formula are equivalent for equal length cells, the algebraic and computational advantages of the matrix method are stressed and are illustrated by the example of short-range order in hollandite, K1.54Mg0.77Ti7.23O 16.
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    A discussion on constrained refinements (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 539-543 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Three crystal structures are discussed in order to emphasize the difference between statistical and chemical or physical grounds of the different models used to represent crystal structure. The constrained models give an evident improvement over the unconstrained models for disordered crystal structures; they can also explain anomalies resulting at the end of the conventional refinement.
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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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    High-resolution electron microscopy images of defects in Mg- and Li-stabilized β''-aluminas (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 572-580 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Crystals of the magnesium- and lithium-stabilized solid electrolyte β"-alumina have been investigated by high- resolution transmission electron microscopy (HRTEM) and a resolution of 2.7 Å was obtained. Crystals were found to contain two types of defect. Broad close-packed Al-O slabs cause a blockage of the ion-conducting layers in the solid electrolyte. It is shown how such slabs with different widths always have the same type of blocking junction. Growth of the broad Al-O slabs while the ion-conducting layers disappear under the influence of the electron beam was recorded at different stages. The growth always starts in the crystal and moves out to the edge. The broad Al-O slabs grow only to certain thicknesses given by the formula (4N + 1) × 2.2 Å. A mechanism for the growth of the broad Al-O slabs is suggested, involving an exodus of Na + O2 or Na2O followed by a collapse of the conducting layer. The importance of the defects for the understanding of material problems of constructing Na--S batteries with β"-alumina as the solid electrolyte is discussed.
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    Diffraction-broadening studies of textured materials (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 583-587 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Reciprocal-space calculations are carried out with the aim of finding out where on an electron or X-ray diffraction picture one has to measure diffraction line broadening, so as to obtain reliable information about the apparent domain size (DS) and the microstrain (MS) in a textured material. It has been shown that in both cases there are geometrical limitations. They are greater for X-ray diffraction broadening studies, and if the texture axis is arbitrarily inclined, a readjustment of the texture axis to positions parallel to the X-ray beam, or parallel to the camera axis, is required. Deviation of the texture axis from an ideal orientation increases the number of diffraction spots requiring correct correlation of their diffraction broadening with the DS and MS of the material.
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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 
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    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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    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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    The interpretation of neutron powder diffraction measurements on α-AgI (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 989-991 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Recent neutron powder diffraction measurements on α-AgI by Wright & Fender [J. Phys. C (1977), 10, 2261- 2267] have been reanalysed using conventional least-squares analysis of the integrated Bragg intensities and have been shown to be consistent with other recent neutron diffraction measurements and with recent extended X-ray absorption fine structure (EXAFS) measurements. It is concluded that, although the distribution of silver ions within the unit cell can be modelled by locating isotropically vibrating ions in a number of crystallographic sites with partial occupancy, the most appropriate model is one in which the silver ions are located entirely in a single site but have significant anisotropic harmonic and higher-order components in their thermal vibration.
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    URL: http://dx.doi.org/10.1107/S0567739479002187
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