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  • Wiley  (35,632)
  • International Union of Crystallography (IUCr)  (4,538)
  • 1970-1974  (40,170)
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Year
  • 1
    Electronic Resource
    Electronic Resource
    Structural studies by high-resolution electron microscopy: tetragonal tungsten bronze-type structures in the system Nb2O5–WO3 (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 22-29 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The contrast in many-beam lattice images from very thin crystals of 4Nb2O5. 9WO3 is shown to be directly related to its known structure. On the basis of this correlation, the structure of 2Nb2O5.7WO3 is derived from the observed image contrast; it is an ordered intergrowth of the ReO3 and tetragonal tungsten bronze structural types. The structures of typical fault boundaries and disordered intergrowths are described, and those of the reported compounds 6Nb2O3. 11WO3 and 3Nb2O5.8WO3 are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739474000039
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  • 2
    Electronic Resource
    Electronic Resource
    X-ray determination of the mean-square atomic displacements and associated Debye temperature of Mg2Sn (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 39-43 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The mean-square atomic displacements and associated Debye temperature of Mg2Sn have been determined from a least-squares analysis of 115 single-crystal X-ray scattering measurements. The mean-square displacements of Mg and Sn are 0.0138 (3) and 0.0097 (1) Å respectively, and the corresponding Debye temperature is 270 ± 3 °K. The data have been analyzed in terms of possible anharmonic contributions to the thermal motion of the Mg atom. An analysis of published specific-heat data was performed to obtain the minus-second moment of the frequency spectrum. The Debye temperature associated with this moment, 281 ± 9°K, is compared with the aforementioned X-ray value.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739474000064
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  • 3
    Electronic Resource
    Electronic Resource
    Anisotropic corrections of measured integrated Bragg intensities for thermal diffuse scattering – general formula (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 77-82 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The correction of measured integrated intensities for the first-order thermal diffuse scattering (TDS) is considered on the basis of the existing theory of X-ray thermal diffuse scattering for an elastic wave of long wave length. Generalized formula for the TDS correction α is found to be represented by a quadratic form in the Miller indices h, k, l and a tensor Δβ, as α = Δβ11h2 + Δβ22k2 + Δβ33l2 + 2Δβ12hk + 2Δβ23kl + 2Δβ31lh. Δβ is a tensor introduced in this paper which characterizes the anisotropy of the TDS correction. The form of the tensor Δβ is shown to depend only on the crystallographic system. The relation between Δβ and the temperature-parameter tensor is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S056773947400012X
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  • 4
    Electronic Resource
    Electronic Resource
    A determination of the Debye–Waller temperature factor and the X-ray Debye temperature for Ni, Cr, Fe, Mo and W (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 83-86 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The integrated intensities of high-order reflexions for Ni, Cr, Fe, Mo, and W powder samples were measured at 296 and 77°K with Mo Kα radiation. The values 397 ± 6, 510 ± 6, 435 ± 6, 407 ± 6, and 333 ± 6°K respectively were obtained for the Debye temperatures after correction for thermal diffuse scattering. These values are in good agreement with those calculated from force-constant models determined by inelastic neutron scattering.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739474000131
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  • 5
    Electronic Resource
    Electronic Resource
    Determination of the interstitial concentration of V in disordered VO1.23 by an intersecting Kikuchi-line method (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 93-95 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The concentration of V atoms at interstitial tetrahedral sites in VO1.23 above the ordering temperature has been determined through structure-factor measurements. Using an electron diffraction method which utilizes the splitting of weak Kikuchi lines near their crossing with strong bands, the 002 structure factor has been determined to within 3% accuracy. The value obtained is explained by the existence of a fraction of interstitial atoms close to that of the ordered phase. This supports the assumption that the defect cluster typical of the ordered phase persists above the ordering temperature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739474000155
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  • 6
    Electronic Resource
    Electronic Resource
    Some definitions and units in electron diffraction (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 297-298 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Changes are advocated in some of the definitions, units and notation current in electron diffraction. The purpose of those changes is to separate descriptions of properties of materials from the experiments intended to measure them, and to exhibit as clearly as possible the relationship between the differing properties of a material accessible in X-ray and electron work.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739474000635
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  • 7
    Electronic Resource
    Electronic Resource
    The root-mean-square vibrational displacements of sodium and fluorine ions in sodium fluoride (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 299-300 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: From the X-ray diffraction data from a single crystal of sodium fluoride at room temperature, the root-mean-square vibrational displacements of sodium and fluorine ions were determined to be 0.181 ± 0.001 Å and 0.183 ± 0.005 Å respectively. These values are compared with the values of 0.202 ± 0.007 Å or the two ions obtained by Meisalo & Merisalo [Ann. Acad. Sci. Fenn. AVI (1966). 211, 3-9] from powder data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739474000659
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  • 8
    Electronic Resource
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    n-Beam lattice images. III. Upper limits of ionicity in W4Nb26O77 (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 302-302 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Corrections are given to Anstis, Lynch, Moodie & O'Keefe [Acta Cryst. (1973). A29, 138-147].
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739474000684
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  • 9
    Electronic Resource
    Electronic Resource
    An alternative method for the calculation of a phase-grating function for use in dynamic electron-diffraction calculations (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 101-102 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A method is described which permits multislice computation of n-beam diffraction intensities directly from the structure amplitudes. This is achieved by successive convolution, a numerical technique compatible with the multislice procedure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739474000179
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  • 10
    Electronic Resource
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    The crystal-structural transformation NaCl-type → CsCl-type: analysis by martensite theory (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 13-22 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Orientation relations, interface (habit plane) orientations, and shape changes have been computed by martensite theory for the crystal-structural transformation NaCl-type (f.c.c.) → CsCl-type (primitive cubic), especially in alkali halides. The lattice correspondence used involves contraction along the three-fold axis of a primitive rhombohedron of f.c.c. For the matrix analysis a computer program was written in FORTRAN. Mathematical solutions were obtained using two lattice-invariant shears based on slip and one based on transformation twinning. There are three types of solution. Within each of the three types multiplicity due to symmetry leads to 24 variants. Possible habit planes are near to {\bar 111}I, {210}I and {310}I. Shape changes are large. Predictions agree with observations within experimental error. As the principles applied need not exclude some changes of stoichiometry, they may be relevant to topotaxy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739474000027
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  • 11
    Electronic Resource
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    Structural studies by electron microscopy: coherent intergrowth of the ReO3 and tetragonal tungsten bronze structure types in the system Nb2O5–WO3 (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 29-36 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: High-resolution lattice images from WO3-rich compositions in the system Nb2O5-WO3 contain direct evidence for the coherent intergrowth of extensive domains of several orientations of the ReO3-type structure with the more complex tetragonal tungsten bronze-type structure. A model for the transformation between the two structure types is proposed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739474000040
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  • 12
    Electronic Resource
    Electronic Resource
    Répartition et déplacements locaux des ions dans le spinelle MgAl2O4 non stoechiométrique (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 47-54 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Diffuse X-ray scattering measurements have been performed on a non-stoichiometric spinel single crystal, MgO-3Al2O3, with 10% octahedral vacancies, and the following conclusions have been reached. The probability of a vacancy occupying a site adjacent to another vacancy is □□ = 0.20, compared to 0.10 for a completely disordered model. This probability is consistent with the Jagodzinski-Haefner model. The distance between neighbouring A1 atoms in octahedral sites is 1% larger than the mean value. The distance between the octahedral Al and its neighbouring oxygen atoms is 0.5% shorter than the mean, and the vacancy-oxygen distance is 5% larger. By comparison between stoichiometric and non-stoichiometric spinels, a large Debye-Waller factor for static displacement is derived. This factor leads to a qualitative explanation of the observed anisotropy of the thermal and pseudo-thermal diffuse scattering. The large local displacement permits one to observe the second-order term, (kδri)(kδrj), of the displacement correlation between the oxygen atoms of the occupied octahedral sites.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739474000088
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  • 13
    Electronic Resource
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    Une transition métal–isolant dans V0,90Nb0,10O2 due à un appariement local des atomes de vanadium (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 55-60 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: X-ray diffuse scattering from V0.90Nb0.10O2 shows that the metal-to-insulator phase transition which occurs around - 25°C without an overall structural change can be explained by local pairing of the vanadium atoms. This local order, which recalls the low-temperature insulating structure of pure VO2, is two dimensional and occurs in the (110) planes of the average rutile structure with little correlation between successive parallel (110) atomic planes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S056773947400009X
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  • 14
    Electronic Resource
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    Core deformation studies by coherent X-ray scattering (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 60-67 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: 'Core' deformation scattering factors have been determined for a series of diatomic molecules. These are defined as the difference between scattering factors calculated from corresponding molecular and atomic core orbital products. The core deformation scattering factors are too small to be measured by current X-ray diffraction methods. The core deformation scattering factors have been expanded in multipolar series about the nuclei. The major component of core deformation is a dipolar term, which is transferable for the same atom in different diatomic molecules. The multipole-expansion terms can be well represented by the Fourier-Bessel transforms of simple density-deformation multipole functions. The dipole density function makes a sizeable contribution to certain molecular physical properties (e.g. electric fields at the nuclei). Even though features of the core density deformation function are also present in the valence molecular orbital products, it is unlikely that total core deformation will be determined by X-ray diffraction measurements. The generalization of these findings to polyatomic systems is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739474000106
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  • 15
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    The effect of disorientation on the intensity distribution of non-crystalline fibres. II. Applications (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 639-645 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Starting with the integral form of the correction factor derived in the previous paper [Holmes & Barrington-Leigh (1974). Acta Cryst. A30, 635-638], the explicit form of the function describing intensity in reciprocal space for slightly disoriented non-crystalline fibres is described, and simplifying approximations are made. Correction factors for real data are calculated, and the validity of the theory tested with data from tobacco mosaic virus gels. A correction factor is also given for crystalline fibres.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739474001549
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  • 16
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    The effect of disorientation on the intensity distribution of non-crystalline fibres. I. Theory (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 635-638 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The intensity function in reciprocal space is derived for the case of non-crystalline fibres with a Gaussian distribution of orientations. An expression is found relating the observed intensity function with the weighted integral along a circle of the intensity derived from the molecular structure factor. From this result in the case where the layer-line width is very small a correction factor may be derived which, if applied to the observed intensity, yields the intensity which would be obtained from a perfectly orientated fibre. The general problem of correcting for disorientation can only be treated by a numerical deconvolution procedure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739474001537
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  • 17
    Electronic Resource
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    Debye–Waller coefficient of KCl by the powder neutron diffraction method (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 662-667 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The neutron diffraction pattern of powdered KCl has been measured using the triple-axis spectrometer both in the double-axis and the triple-axis modes. The thermal diffuse scattering (TDS) correction, which is very important in the double-axis pattern, is essentially eliminated in the triple-axis mode. Theoretical calculations are presented which give the relative TDS correction for the two cases by calculating the energy distribution of TDS. The Debye-Waller coefficient B using the triple-axis data is found to be (2.17 ± 0.16 Å2), which is in agreement with the TDS-corrected values of other workers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739474001586
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  • 18
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    Some neutron diffraction experiments on curved silicon crystals (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 798-805 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The diffraction properties of curved silicon crystals, with curvature radii between ∞ and 25 m obtained by microscopic techniques, were investigated by means of a neutron diffractometer. The ratio of neutron reflectivity between plane and curved silicon crystals was measured as a function of the neutron wavelength using different reflecting planes in both Bragg and Laue cases in symmetrical and asymmetrical conditions. The experimental results were interpreted with the results of theoretical investigations on the dynamical theory of diffraction applied to the curved-crystal case. The implications of this work on neutron monochromator design are briefly discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739474001896
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  • 19
    Electronic Resource
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    On the concept of reciprocal basis vectors (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 846-847 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Ewald's concept of reciprocal basis vectors can be defined in a more general way. It is suggested that the definition of these basis vectors should be presented within the framework of the matrix notation, and not lean so heavily on the concept of a skew product of vectors. The metric matrix, which bears significance in any vector space is the crucial matrix in such a definition.
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    URL: http://dx.doi.org/10.1107/S0567739474001963
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  • 20
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    Technique for Berg–Barrett stereo topography (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 852-852 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A stereo pair of Berg-Barrett topographs of an (0001) surface of zinc is obtained with a single exposure. One topograph is the 0{\bar 1}13 reflection while the other is the 10{\bar 1}3 and an 84° stereo separation angle results. Errors due to dislocation displacements which might occur between two successive topographs are eliminated, and correction for geometric image distortions is not required.
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    URL: http://dx.doi.org/10.1107/S0567739474002002
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  • 21
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    Debye–Waller factors and extinction in sodium fluoride (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 278-280 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: From consideration of the normal crystal strain in real crystals it is deduced that the value of mean domain size of a small sphere of sodium fluoride obtained by the application of Zachariasen's theory of extinction is physically meaningless and that no physical significance can be given to domain size so obtained. The values of Debye-Waller factors for the sodium and fluorine ions [B(Sodium) = 0.860 + 0.010 Å2; B(fluorine) = 0.880 ± 0.050 Å2] agree well with the theoretical values of 0.8685 Å2 and 0.8671 Å2 respectively for the two ions, recently reported by Reid & Smith [J. Phys. Chem. Solids (1970). 31, 2689-2697].
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    URL: http://dx.doi.org/10.1107/S0567739474000568
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  • 22
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    Numerical evaluations of N-beam wave functions in electron scattering by the multi-slice method (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 280-290 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A method for the numerical evaluation of N-beam diffraction amplitudes and intensities which has been successfully employed over the last few years is described. This derives from the multi-slice formulation of Cowley and Moodie. The physical basis of the method and practical approaches to calculation are described.
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    URL: http://dx.doi.org/10.1107/S056773947400057X
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  • 23
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    On the mass independence of the mean-square atomic displacement in the classical limit (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 301-302 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A recent suggestion that the mean-square atomic displacement in the classical limit is mass dependent in the case of a polyatomic crystal is shown to be incorrect.
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    URL: http://dx.doi.org/10.1107/S0567739474000672
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  • 24
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    Magnetic symmetry groups and their representation by stereographic projections (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 353-358 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Using the theory of representation analysis [Bertaut, E. F. (1968). Acta Cryst. A24, 217-231] and with the aid of some newly introduced symmetry symbols we present the stereographic projections for all the magnetic symmetry groups. These groups are useful in studying the properties of magnetically ordered crystals.
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    URL: http://dx.doi.org/10.1107/S0567739474000817
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  • 25
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    Low-energy electron diffraction from the cleavage face (001) of magnesium oxide (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 374-380 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: With a triple-axis low-energy electron diffractometer, the intensity curves of the specular reflexion 00 from the cleaved face (001) of a MgO crystal were measured against the glancing angle, for the electrons in the energy range from 180 eV to 500 eV and for three crystal azimuths in a range near [010]. By the use of diagrams drawn on the basis of the Ewald construction to indicate the surface wave resonance conditions for various reciprocal rods, a number of fine structures as well as gross features of the intensity curves were interpreted fairly well in terms of the resonance effect.
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    URL: http://dx.doi.org/10.1107/S0567739474000854
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  • 26
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    Debye characteristic temperature as a measure of the ordering parameter. II. Application to β-brass (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 388-390 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The usual method of X-ray diffraction for determining the ordering parameter cannot be applied to α-brass. This problem has been solved by considering the Debye characteristic temperature, rather than the intensity of a diffraction line, as a measure of the parameter. By using the expression obtained in part I of this series of papers [Mitra & Chaudhuri (1974). Acta Cryst. A30, 385-387], the Debye characteristic temperatures of α-brass at different values of the ordering parameter have been calculated. From the X-ray diffraction data of α-brass after different heat treatments the Debye characteristic temperatures were determined. The ordering parameters were obtained by comparing the experimental values of the Debye characteristic temperatures with the theoretical values.
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    Etude des phases haute température de NaNb3 et des correlations qui les caracterisent (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 423-430 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: From the study of X-ray diffuse scattering diagrams, it has been shown that the diffuse scattering located along three equivalent reciprocal rods (100) in the cubic phase disappears successively at 641, 575 and 520°C. The interpretation is given in terms of planar local order.
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    Multiple diffraction of X-rays and the phase problem. Computational procedures and comparison with experiment (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 413-423 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The general theory of n-beam X-ray diffraction (n 〉 2) has been developed in the framework of classical dynamical theory and applied to the Bragg case. It is shown that the crystal wave-vectors are the eigen-values of a 4n × 4n dispersion matrix. The boundary conditions are applied to a parallel-sided crystal slab and show that for an infinite thickness only 2n wave fields survive. The Umweganregung plot of Ge(222) with Cu Kα radiation has been considered in detail. The integrated intensity of an Umweganregung peak is defined here as a double integral with respect to θ (angle of incidence) and φ (azimuthal angle). The 222-113 and 222-{\bar 11}3 absolute integrated intensities were measured on a dislocation-free Ge crystal. Excellent agreement is obtained between experimental and calculated values. The ratio between the two integrated intensities (of the order of 7) did not change appreciably for a Ge mosaic crystal, although both reflections exhibited increases with respect to the perfect-crystal values. Since the two Umweganregung peaks considered in this experiment involve crystallographically equivalent reflections with different phases, it is suggested that the present technique can in principle be used for phase determination in crystal structure analysis.
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    Crystallographic groups by T. Janssen (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 462-462 
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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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    Space groups and lattice complexes by W. Fischer, H. Burzlaff, E. Hellner and J. D. H. Donnay (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 462-462 
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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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    Relation entre la symétrie des groupements CuCl42− tétraédriques et les propriétés physiques des cupritétrachlorures. I. Moment magnétique moyen (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 484-486 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: For some crystals, the structures of which have already been determined, the flattening, D, of the CuCl42- tetrahedra has been estimated. The paramagnetic susceptibilities of many tetrachlorocuprates have been measured by the Faraday method. The mean magnetic moment decreases linearly as D increases, if D is greater than 0.2. From the known mean magnetic moment, it is therefore possible to estimate the D value of the CuCl42- tetrahedron.
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    Theoretical calculation of the time-averaged electron density distribution for vibrating ethyne molecules in a model crystal structure (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 497-502 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Calculations have been made on an ethyne model structure to investigate the influence of internal and external thermal motions of the molecules in a crystal on the time-averaged electron density distribution as obtained by X-ray diffraction. The density distributions in the chemical bonds have been studied from D(r) = Q(mol, r)-Q(at, r) difference maps, where Q(at, r) is the density of the free-atom structure. Vibrations expected to occur at low temperatures reduce the difference densities at the centres of the C-C and C-H bonds by 12 and 23% respectively. Reliable time-averaged difference density distributions can be calculated from the static distributions obtained by theory by use of the root-mean-square deviations (u2)1/2 of the atoms which can be determined by X-ray diffraction. Only negligibly small errors occur in the theoretical difference density maps if librations are accounted for by linear vibrations. In the experimental [Fo-Fc] difference densities errors due to treating librations as linear vibrations are largely, but not completely, compensated by shifts of the atoms.
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    Energy calculations for the lattice dynamics of NH4IO3 (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 518-521 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The lattice dynamics of ammonium iodate have been studied experimentally and theoretically for centre-zone phonons. The frequencies of normal modes have been measured by Raman spectroscopy. The experimental results have been used to obtain the parameters of a simplified rigid-ion model. It is shown that the strongest bonds are the linear I-O and N-H covalent interactions, while the low-frequency external NH4-IO6 mode is mainly ionic in character. The effective ionic charge of the NH4 group is - 0.82 e.
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    A method of determining the distortion of coordination polyhedra (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 513-517 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The distortion of an observed coordination polyhedron can be evaluated from a comparison of this polyhedron with the least-squares best-fit polyhedron with optimum location, orientation, size parameters and prescribed symmetry. A set of atoms at positions, x(1) .... x(n), may be fitted to the set y(1) .... y(n) by rearranging the matrix equations: y(i) = t + Rλ(i)x(i) (i = 1,n) and solving for the unknown parameters of the translation vector, t, the rotation matrix, R, and the (diagonal) dilation matrices, λ(i), which optimize the fit between the two sets. The elements of the (one or more) dilation matrices may be constrained to fix the fitted set to the desired symmetry. The solution is effected by means of a two-stage iterative least-squares technique employing the so-called 'small- angle' rotation matrix. The average distance between corresponding atoms of the two sets, which is a minimum at the point of optimum fit, provides a unique one-parameter characterization of the degree of distortion between the two configurations. The magnitudes of the operations needed to produce the best fit are also recoverable from the least-squares solution.
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    A contribution to the determination of a correct system of signs of structure factors for centrosymmetric crystals (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 576-579 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: To find a solution of the phase problem of centrosymmetric crystals, it is as a rule necessary to find among many possible solutions a correct system of signs of the structure factors. This paper describes the procedure used for finding a correct solution based on a comparison of the theoretical and empirical distribution functions of the positive signs of structure products.
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    The dynamics of atoms in crystals by W. Cochran (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 604-605 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    The dynamical scattering amplitude of an imperfect crystal. III. A dynamical diffraction equation for topography in the spatial coordinate representation (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 525-535 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Based on a general dynamical theory of diffraction, an integral equation for dynamical diffraction in imperfect crystals is obtained in the spatial coordinate representation. This equation is derived for diffraction topography in the symmetrical Laue geometry from the basic dynamical equation of diffraction previously derived in the momentum representation. Discussion is presented on an interpretation of this integral equation beyond the given diffraction geometry. This equation can be considered as a basic equation for diffraction topography.
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    Determining heavy-atom positions using non-crystallographic symmetry (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 672-677 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: If there are a number of chemically identical molecular subunits in the asymmetric unit of the crystal cell, related by previously established non-crystallographic symmetry, it is probable that heavy atoms attached specifically to the subunits of the native molecule will show the same symmetry. The Patterson vectors between non-crystallographically equivalent heavy atoms can then be generated for arbitrary trial positions and compared with the actual Patterson synthesis. A search of all positions within the molecular subunit can thus establish the sites of larger heavy-atom substitutions. Once these have been determined, vectors between molecules can be computed and compared with the actual Patterson synthesis in order to establish the position of the molecular center if it is unknown. These methods have been demonstrated in the determination of the major heavy-atom sites, in the presence of non-crystallographic 222 symmetry, for the glyceraldehyde-3-phosphate dehydrogenase molecule.
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    Notes and News (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 682-682 
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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 state chemistry and physics, Vol. 2 edited by P. F. Weller (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 683-684 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    A comment on Prasad & Srivastava's paper, On the occurrence of Zhdanov numbers 1, 2 and 3 in the zigzag sequence of cadmium iodide polytypes (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 603-603 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: As a comment on the paper by Prasad & Srivastava [Acta Cryst. (1972). A28, 494-497] it is shown that the possibility of occurrence of CdI2 polytypes having Zhdanov numbers higher than 3 in zigzag sequence cannot be ruled out theoretically for their structure determination.
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    Computed electron micrographs and defect identification by A. K. Head, P. Humble, L. M. Clarebrough, A. J. Morton and C. T. Forwood (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 605-605 
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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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    Neutron diffraction study of asymmetric anharmonic vibration of the copper atom in cuprous chloride (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 655-661 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Neutron diffraction measurements of CuCl with the zincblende structure were carried out at room temperature and the results examined on the basis of three possible structure models: (1) harmonic thermal vibration of atoms, (2) asymmetric anharmonic vibrations and (3) statistical disorder arrangement of the copper atom. It was found that the anharmonic model was the most plausible for the structure of CuCl. The final R value in the least-squares analysis was 0.037. The parameters in the effective one-atomic-potential fields for the zincblende structure Vj(r) = V0j + ½αj(u21 + u22 + u23) + βju1u2u3 + . . . were obtained as αCu = 0.74 ± 0.01, αC1 = 1.35 ± 0.02 × 10-12erg Å-2, and βCu = 1.15 ± 0.66, βC1 = 0.0 ± 1.6 × 10-12erg Å-3. It is also shown that the temperature dependence of the Bragg reflexion observed from room temperature to 523°K can be explained very well with the use of the above parameters.
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    The thermal transformations in solid ammonium nitrate containing potassium and caesium ions (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 678-679 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The four solid-solid phase transformations in NH4NO3 and its solid solutions, Kx(NH4)1 - xNO3 are studied by differential thermal analysis and X-ray diffraction. The formation of the cubic phase (nearest to the melting point) is attributed to the onset of rotational disorder of the NO-3 ions.
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    The computation of radial distribution functions for glassy materials. Errata (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 679-679 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Corrections are given to equations (5), (6), (8) and (15) in the paper by Konnert & Karle [Acta Cryst. (1973). A29, 702-710].
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    X-ray diffraction from close-packed structures with stacking faults. III. hhcc crystals (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 693-698 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The kinematical theory of X-ray diffraction by hhcc crystals with stacking faults is developed. The intensity distribution in reciprocal space is derived as a function of seven parameters which represent four growth and three deformation fault probabilities. Only reflexions with H - K ≠ 3N, N an integer, are affected by faulting and exhibit generally changes in integrated intensity, profile peak shift, broadening and asymmetry. It is shown that eleven independent combinations of the seven fault probabilities can be evaluated from the measured profile characteristics.
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    X-ray diffraction from close-packed structures with stacking faults. II. hhc crystals (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 689-693 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The kinematical theory of X-ray diffraction by hhc (samarium-type) crystals with growth and deformation faults is developed. The intensity distribution in reciprocal space is derived as a function of five parameters which represent three growth and two deformation fault probabilities. Only reflexions with H - K ≠ 3N, N an integer, are affected by faulting and exhibit generally changes in integrated intensity, profile peak shift, broadening and asymmetry. It is shown that nine independent combinations of the five fault probabilities can be evaluated from the measured profile characteristics.
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    Data collection in protein crystallography: capillary effects and background corrections (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 740-748 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In protein crystallography, observed diffraction intensities must be corrected for background radiation due to scatter from air and scatter and absorption by capillary., crystal and mother liquor. A systematic study shows that a major contribution to background intensity is air scatter arising from the air intercepted by the direct X-ray beam as 'seen' by the receiving-counter aperture. As a result there is a first-order dependence of background on the 20 angle. The second-order variations in this function are principally due to absorption of the direct beam or air-scattered radiation by the capillary and to diffraction by the glass in the direct beam. To reduce data collection time and crystal exposure, individual background measurements may be approximated by interpolation from empirical background curves or, alternatively, by collecting background intensities for short times and fitting these data with a multidimensional function. If isotropic interpolation is used, i.e., if background is considered to be a function of 2θ alone, systematic errors of up to about 30% can be introduced into the interpolated backgrounds. Methods of accounting for the anisotropy in the background are derived and shown to reduce this error to I-2%.
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    On the libration of 9,10-anthraquinone at five temperatures (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 768-771 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Potential energy profiles corresponding to libration of 9,10-anthraquinone about its molecular axes were constructed for the five structures derived from data which were collected at - 170, - 112, -72, - 12.5 and 20.5°C [Lonsdale, Milledge & El Sayed (1966). Acta Cryst. 20, 1-13]. These profiles were represented by fourth-degree least-squares polynomials, whereafter r.m.s. libration amplitudes and rigid rotator frequencies of 9,10-anthraquinone were evaluated in the quadratic approximation. The temperature dependence of the calculated quantities is in most cases close to that of the observed ones thus reproducing, by comparison with observed Raman frequencies, the pseudoharmonic behaviour of 9,10-anthraquinone. Calculated r.m.s. libration amplitudes are only qualitatively comparable to the experimental ones and appear to be somewhat too low. The present representation of energy profiles makes it possible to estimate conveniently the contribution of anharmonicity to the profile shape.
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    Correlative microdomain model for short-range ordered alloy structures. I. Diffraction theory (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 792-798 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A diffraction theory is developed for diffuse scattering from disordered binary alloys with short-range order. It is based on a model of ordered microdomains embedded in a disordered matrix and interference effects between the domains are considered. There is a possibility that the fine structures of diffuse scattering as observed in the cases of Cu3Au, CuAu and Cu3Pd alloys result from the introduction of interdomain correlations. From experimental diffuse intensity values one can calculate the statistical distribution of microdomains in antiphase with one another.
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    On the theory and estimation of the cosine invariants cos(φl + φm + φn + φp) (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 822-829 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: For fixed h1 + h2 + h3 = 0, and uniformly distributed k, the conditional joint probability distribution of the pair of phases φk, φh1 + k, given |E-h3 + k|, |Ek|, |Eh1 + k| is found. If l + m + n + p = 0, this distribution leads, via a suitable sampling technique, to estimates having probabilistic validity for the cosine invariant cos (φl + φm + φn + φp) in terms of the seven magnitudes |El|, |Em|, |En|, |Ep|, |El + m|, |El + n|, |El + p|.
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    Interstitial space in hardsphere clusters (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 850-851 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The interstitial space in a cluster of spheres is examined to determine the largest sphere that can be placed in its voids. A method is given for obtaining the interstitial sphere belonging to a group of arbitrarily arranged spheres by examining tetrahedral configurations. Given the coordinates of the centers and the radii of the spheres of a tetrahedral group, the coordinates and radius of the tetrahedral interstitial sphere can be found. The method can be applied to interstices of any coordination number. It is applicable to sphere packings with or without crystallographic symmetry.
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    Notes and News (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 459-459 
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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 simple direct method for solving centrosymmetric projections: errata (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 458-458 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Corrections are given to Acta Cryst. (1973). A29, 717-720.
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    X-ray diffraction by polymers by M. Kakudo and N. Kasai (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 460-460 
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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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    Magnetically ordered crystals containing impurities by Yu. A. Izyumov and M. V. Medvedev (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 461-462 
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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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    Interpretation of the 10 Å rotation function of the satellite tobacco necrosis virus (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 552-559 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The rotation function calculated with 10 Å three-dimensional data from monoclinic crystals of the satellite tobacco necrosis virus was fitted numerically to an icosahedral axis set. The r.m.s. angular deviation of the observed peak maxima from the calculated model axis set was 0.67° and the largest deviation was 1.4°. Thus, there is no significant deviation from icosahedral symmetry at 10 Å resolution. An investigation of the effects of the data inclusion limits and the radius of integration on the resolution of neighboring peaks in the rotation function showed that the best resolution was obtained by using only a thin shell of the highest-resolution data available and a radius of integration no larger than the estimated diameter of the virus protein subunit.
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    Crystal structure determination by simultaneous use of cosine invariant computation and the multisolution method (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 564-568 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An algorithm is given for the rapid computation of the cosine invariants, cos (φ - H1 + φH2 + φH1 - H2), and the results are compared with the actual values for three structures. A weighting scheme is derived which enables this information to be incorporated directly into the multisolution tangent method of phase determination. Details are given of the determination of four unknown structures by this method.
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    A note on the probability distribution of phases in a non-centrosymmetric crystal with a degree of centrosymmetry (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 574-576 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The probability distribution of the magnitude of the phase angles in a non-centrosymmetric crystal with a degree of centrosymmetry is worked out for a structure with similar atoms. Curves showing the probable fractional number of reflexions for which the magnitude of the phase angles would deviate from the centrosymmetric values of 0 or π by any given amount α0 are also obtained for crystals with different degrees of centrosymmetry.
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    Refinement of merohedrally twinned crystals (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 603-604 
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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 procedure is outlined whereby merohedrally twinned crystals may be refined with existing least-squares programs. It is applicable to crystals containing any number of twin individuals and can handle different twin laws simultaneously.
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    Crystal structures – a working approach by H. D. Megaw (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 605-606 
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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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    Etude experiméntale des susceptibilités diamagnétiques moléculaires. I. Méthode générale (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 616-625 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Lonsdale and Krishnan used a method for relating crystal and molecular susceptibilities in which they calculated the molecular tensor from the principal crystal susceptibilities assuming that the directions of the principal axes of the molecular tensor are known. Their method is only applicable when the molecular symmetry permits. To avoid this inconvenience and to allow an interpretation of molecular anisotropies in terms of chemical groups, we assume that each molecule consists of a skeleton on which different substitutions are made. The molecular tensor is thus the sum of the different elementary tensors associated with the molecule and related to an orthogonal and common set of axes. Several molecules with common skeletons and substitutions were studied to obtain the different elementary tensors; the set of equations which relate elementary tensor components to crystal susceptibilities in the approximation of weak intermolecular interactions was solved by a least-squares method. Our method was applied to benzene, hexachlorobenzene and hexamethylbenzene separately at first and then all together. The results are given for the substitution of a hydrogen atom by a chlorine atom or a methyl on a benzene ring. Several possible applications of this method are discussed.
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    Probability distribution of Bijvoet differences when the group of normal scatterers is partly centrosymmetric (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 649-654 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Cumulative functions of the normalized Bijvoet differences x and Δ and their expectation values for a non-centrosymmetric crystal in which the group of normal scatterers is partly centrosymmetric are worked out for the cases when the number (P) of anomalous scatterers in the unit cell is one and many (MN and MC cases) respectively. The results are used to obtain the percentage of reflexions for which Δ ≥ 0.05. It is found that even when 50% of the normal scatterers form a single centrosymmetric group, the measurability of the Bijvoet difference is not affected significantly by the partial centrosymmetry of the group of normal scatterers.
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    Morphologie et propriétés optiques des cristaux de lysozyme de poule de type quadratique et orthorhombique (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 645-648 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Large crystals of hen egg-white (HEW) lysozyme, both tetragonal and orthorhombic, have been studied with a goniometer and optical methods. As the crystals are particularly stable, some of their optical properties have been measured, mainly the refractive indices determined from reflectance measurements under a photometric microscope. Orthorhombic HEW lysozyme: crystals show the faces (010), (011) and (110). Refractive indices n[001] = ng = 1-562 to 1.547 (λ = 480-640 nm); n[100] = nm = l.560 to 1.544; n[010] = np = 1.550 to 1.532. Maximum birefringence 0.013 (λ) = 589 nm). Optic axial angle 2V =48 to 51°. Tetragonal HEW lysozyme: crystals formed by the association of a tetragonal {110} prism and a {101} tetragonal bipyramid. Refractive indices n[001] = nc and n[100] = no, with an aging phenomenon in these crystals, ne varying between 1-580 and 1.545, no between 1.575 and 1.538 (λ = 589 nm). Maximum birefringence 0.007. HEW lysozyme crystals are examples of good-quality protein crystals. Their refractive indices are surprisingly high for organic substances.
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    Coherent-scattering amplitude of 243Am and 244Cm (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 667-671 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Neutron diffraction experiments have been completed on 243AmO2 and 244Cm2O3. The coherent-scattering amplitude of 243Am, relative to a value of 0.58 × 10-12 for oxygen, is 0.76 (1) × 10-12 cm. The value for curium is ̃0.7 × 10-12 cm, which cannot be determined accurately in this experiment because both C-type (cubic, a = 11.0 Å) and A-type (hexagonal, isostructural with La2O3) Cm2O3 are present in the sample. Some of the difficulties in performing neutron diffraction experiments on these radioactive isotopes, which exhibit self-heating and spontaneous fission, are discussed briefly.
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    A method of determining the coincidence site lattices for cubic crystals (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 680-680 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A quick method is presented of determining the coincidence site lattice when the original lattice is primitive cubic.
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    A neutron diffraction study of anharmonic temperature factors in SrF2 (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 806-813 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Accurate integrated intensities for the Bragg reflexion of neutrons from two single crystals of SrF2 have been measured at temperatures between 295 and 1043 °K. Corrections for extinction and thermal diffuse scattering were applied and the experimental data were analysed in terms of a model which includes both harmonic and anharmonic components in the thermal vibrations of the atoms. Significant contributions from the third-order anharmonic term, βF, in the one-particle potential of the fluorine atoms, consistent with their tetrahedral site symmetry, were observed at all temperatures. At 773 °K significant anisotropic fourth-order components, δSr and δF, became apparent in the thermal vibration of both the strontium and the fluorine atoms. At this temperature βF was refined to - 3.45 ( ± 0.11) × 10-12 erg Å-3, with δSr = 1.30 ( ± 0.75) × 10-12 erg Å-4 and δF = 0.84 (± 0.29) × 10-12 erg Å-4. The positive signs of the parameters δSr and δF result from the influence of Coulombic forces on the atomic vibrations.
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    Notes and News (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 872-872 
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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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    Fourier transforms of distributions and their inverses – a collection of tables by F. Oberhettinger (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 874-874 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Compton downscattering corrections to the crystal monochromator efficiency (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 37-38 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is shown that when both a characteristic line of the X-ray spectrum and its nearby continuum are incident on the sample, the crystal monochromator efficiency derived by Ruland is decreased.
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    Physical processes in lasers. Vol 56 edited by D. V. Skobel'tsyn (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 303-304 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    The molecular replacement method. A collection of papers on the use of non-crystallographic symmetry edited by M. G. Rossmann (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 304-304 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Phase determination from the Karle–Hauptman determinant. II. Connexion between inequalities and probabilities (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 342-353 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The probabilistic properties of a Karle-Hauptman determinant are investigated, with particular reference to the case where all elements are assumed to be known except one. In previous papers it has been shown that the matrix associated with a Karle-Hauptman determinant can be interpreted as a covariance matrix, and also that the probability law associated with one unknown element is a complex Gaussian law centred at the expected value given by the regression-plane equation. These results are now extended to the case where several structure factors are unknown. Furthermore, the connexion between inequalities, the Sayre-Hughes equation and probability relations is discussed. It appears that the Karle-Hauptman inequality defines the allowed domain as a hyperellipsoid, the centre of which corresponds to the most probable set of structure-factor phases. Factors concerned in the selection of a determinant suitable for efficient phase determination are given.
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    Ionic size effects on the thermal expansion coefficient in NaCl-type alkali halides (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 105-106 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Relations are observed in NaCl-type crystals between the coefficient of thermal expansion, α, and (1) the radius ratio r+/r-, and (2) the difference, Δx, between the observed and calculated bond length when the calculated bond length is obtained by adding the average effective ionic radii.
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    Elastic and thermoelastic properties of 1,3,5-trphenylbenzene derived from ultrasonic wave velocities (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 106-107 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: All elastic and thermoelastic constants of orthorhombic triphenylbenzene single crystals have been determined from diffraction of light by ultrasonic waves (Schaefer-Bergmann method). These measurements do not agree with the values previously reported by Suresh Chandra & Hemkar [Acta Cryst. (1973). A29, 25-28] which were derived from thermal diffuse scattering of X-rays and which are not consistent with elastic-stability criteria. This is another example of the great difficulties which still have to be visualized in the quantitative interpretation of thermal-diffuse-scattering intensities by complicated crystal structures.
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    Report of the Executive Committee for 1972 (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 109-124 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Mesure de la température de Debye de CsI par diffraction de rayons X (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 409-413 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The Debye temperature of powdered CsI has been measured at room temperature from continuous diffractometric recording of β-filtered Cu K reflexions, using a dead-time correction formula [Kouchkovsky, R. de (1974). Acta Cryst. A 30, 406-408] and taking into account the variation with Bragg angle of the Kα2/Kα1 intensity ratio. The value determined, 112 ± 3°K at 23 °C, is slightly below the one that can be deduced from elastic moduli (115°K), and in good agreement with the general theoretical predictions. Unfortunately, there is some discrepancy with the value calculated from a frequency distribution curve recently published.
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    Ni(NO3)2.6NH3, another example of KCN-type anomalous thermoelastic behaviour (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 455-457 
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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 dependence of all elastic constants of cubic Ni(NO3)2.6NH3, measured from the transition point at - 34.2 to 150°C by ultrasonic methods, exhibits KCN-type anomalous behaviour. Within a wide temperature range all elastic wave velocities increase with higher temperature. The shear resistance c” = (c11 - c12)/2 corresponds to the constant c44 in KCN. The change of c” with absolute temperature obeys a logarithmic law like c44 in KCN, namely c” = a log T/T0 with a = 0.05975 . 1011 dyn cm-2 and T0 = 156.22°K. The anomalous elastic behaviour should be assigned mainly to the thermally activated motions of the NO3 ions. The phase transition is directly related to the low c” value.
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    On the identity and estimation of those cosine invariants, cos(φl + φm + φn + φp), which are probably negative (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 472-476 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: If h1, h2, h3 are fixed reciprocal vectors which satisfy h1 + h2 + h3 = 0, and if k is the primitive, uniformly distributed random variable, then, under the assumption that each of |Eh1|, |E-h3 + k| is sufficiently small, the conditional probability distribution of the cosine invariant cos (φk + φ- h1 - k + φ- h2 + φ - h3), given |E - h3 + k|, |Ek|, Eh1 + k|, is found. The distribution leads to the surprising result that the conditional expected value of this cosine invariant is always negative and approaches - 1 with increasing |EkEh1 + kEh2Eh3|. If m, n, p, q are fixed reciprocal vectors satisfying m + n + p + q = 0, suitable sampling of reciprocal space then leads to a formula for the cosine invariant cos (φm + φn + φ p + φq) having probabilistic validity in the case that |Em + n|, |Em + p| and |Em + q| are sufficiently small. It follows, in particular, that under the stated conditions the value of the cosine is probably negative and the larger the value of |EmEnEpEq| the more negative the cosine is likely to be.
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    Relation entre la symétrie des groupements CuCl4− tétraédriques et les propriétés physiques des cupritétrachlorures. II. Absorption electronique dans l'infrarouge proche (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 487-489 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The maximum wavelength λd - d of the transition between d orbitals of tetrachlorocuprates depends on the deformation D of the CuCl42- tetrahedron in these crystals. On the basis of their experimental absorption spectra, the relation between λd - d and D is given for many tetrachlorocuprates so that the symmetry of the CuCl42- ion can be predicted in some other crystals on the basis of the position of the d-d transition.
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    Secondary reflexions of neutrons diffracted by a single-crystal bar vibrating at high frequency (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 560-564 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Diffraction of neutrons by a longitudinally vibrating quartz single-crystal bar was investigated for thicknesses of 3 and 13 mm. The 1st, 3rd and 5th harmonic frequencies were excited in the bar. The observed increase of the integrated intensity of diffracted neutrons as a function of the vibration amplitude of longitudinal vibrations of the quartz bar is compared with that calculated from an approximate theory considering the possibility of secondary reflexions of neutrons during their flight across the sample.
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    Tchebycheff extraction of the periodic vector set from the Patterson function (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 153-160 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The image-seeking method of Buerger and the procedure of Tokonami and Hosoya appear to be capable of dealing with complex crystal structures via the Patterson function, provided that the periodic vector set is accurately determined. Unfortunately, a general and powerful method for the location of peaks in the Patterson function has not yet been developed, and it is the lack of such a method which now prevents the formulation of a general Patterson method of structure analysis. This paper presents further results in the author's attempts to formulate a general method of vector-set extraction by representing the Patterson function as a linear generalized polynomial in a system of independent interatomic functions. This approach has the advantage in that the essentially non-linear problem of vector-set extraction is reduced to an apparently simple linear problem, namely that of determining the coefficients of the approximating polynomial. In the present paper, the Tchebycheff approximation norm is employed with coefficient determination by linear-programming procedures. Since linear-programming methods are flexible and extremely powerful, this Tchebycheff vector-set-extraction procedure is much more promising than the author's earlier published methods, which were based on interpolatory approximations.
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    Extinction within the limit of validity of the Darwin transfer equations. I. General formalism for primary and secondary extinction and their applications to spherical crystals (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 129-147 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A theory of extinction is derived which is valid within the limit of the Darwin intensity transfer equations. An expression describing the effect of n-fold rescattering within an ideal crystallite is derived, which differs from the equation given by Zachariasen because independent coordinates x1 and x2 based on an external coordinate system have been used, rather than the coordinates t1 and t2 which are only mutually independent if the crystal is a parallelepiped with faces parallel to the incident and diffracted beams. Furthermore, the derivation of the earlier expressions is based on a generally unjustifiable reversal of the direction of the diffracted ray (interchange of t2 and t'2). An exact expression is derived for the diffraction cross section σ(ε1 in the perfect crystallite, which contains a factor sin 2θ neglected in the earlier work. As a result, the previously used classification of crystals into type I and type II becomes less well defined because at very small Bragg angles particle size always becomes the dominant effect. It is shown that the extinction factor yp (p = primary), for a perfect spherical crystallite, calculated with the present theory, is in good agreement with calculations based on the dynamical theory. Furthermore, the limiting behavior of the expressions at 2θ = 0 and π justifies some of the mathematical approximations made. For a mosaic crystal the extinction coefficient y is written as yp . ys (s = secondary), yp is evaluated numerically from the expressions derived. An analytical expression for yp is obtained by least-squares fit to the numerical values. A similar procedure is followed for ys, in the case of a Gaussian, Lorentzian and Fresnellian distributions of the crystallites and a spherical mosaic crystal. Analysis of the results shows that the Zachariasen expression can be used for small extinction (y 〉 0.8), provided the θ dependent factor is properly introduced for particle-size-affected extinction. Allowance for polarization effects in the X-ray case is discussed. Absorption effects cannot be treated separately from extinction for all but small values of 1 -y. Coefficients of the analytical extinction expressions are given for absorbing spherical crystals with μR values ≤ 4. Application of the expressions and extension to non-spherical geometries will be treated in following publications.
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    Incoherent X-ray scattering in the free-Fermigas approach (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 161-167 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A simple equation is derived from one- and two-particle density matrices in the free-Fermi-gas approximation for obtaining the incoherent intensity of X-rays scattered by an atomic N-electron system. The accuracy of the equation is tested on the Thomas-Fermi-Dirac statistical atomic model. The numerical results are compared with those obtained from the known Heisenberg-Bewilogua function and with the atomic incoherent intensities calculated by D. T. Cromer [J. Chem. Phys. (1967). 47, 1892-1893; (1969). 50, 4857-4859] using complete Waller-Hartree theory and the HF SCF model. The results obtained by the present method from the TFD model are in good agreement with Cromer's data.
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    Order–disorder in sapphire, aenigmatite and aenigmatite-like minerals (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 168-173 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The chemically very different minerals aenigmatite and sapphirine are crystallographically closely related, being ordered members of isomorphous families of OD structures consisting of equivalent layers. The symbol of the OD-groupoid family characterizing their symmetry is P1(2/n)l {1(22/c1/2)1 }. Formulae for the structure factor are deduced for the general member of the family and then specialized to explain the diffraction pattern of some members of interest. The twinned structures of aenigmatite, rhönite and krinovite are explained as OD twins; the constituting twin individuals are structures of maximum degree of order corresponding to the OD-groupoid family quoted above.
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    Systematic errors in polarization corrections for crystal-monochromatized radiation (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 176-179 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The polarization expressions for crystal-monochromatized radiation have been evaluated for both molybdenum and copper radiations under a variety of conditions in order to show the magnitude of the systematic error introduced by the use of an inappropriate expression. The effect of the polarization expression on the extinction correction is also discussed.
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    On the non-centrosymmetric structures which produce centrosymmetric diffraction patterns even with anomalous dispersion (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 173-176 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The geometrical conditions that a structure gives centrosymmetric diffraction patterns have been investigated. It is shown that Friedel's law may hold for certain non-centrosymmetric structures containing two or more kinds of anomalous scatterer. Therefore, their absolute configurations can never be established by ordinary absorption-edge techniques. The geometrical characteristics of such a structure are discussed in terms of a vector set.
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    Thermal diffuse scattering corrections for single-crystal integrated intensity measurement (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 184-189 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A computer program has been developed for calculating one- and two-phonon thermal diffuse scattering corrections for integrated X-ray intensity measurements. The correction includes the anisotropy in the diffuse-scattering intensity distribution and the geometry of the scan for crystals of any symmetry type. The calculated two-phonon correction is not negligible and may be as large as the one-phonon correction for high-order reflections. The effects of slit size, scan range, crystal orientation, crystal misalignment, and neglect of phonon dispersion on the calculated corrections are investigated.
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    Least-squares phase refinement. II. High-resolution phasing of a small protein (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 180-184 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A recently described technique for direct least-squares refinement of phases has been successfully applied to the problem of phasing a 1.5 Å data set of observed structure-factor magnitudes for C. pasteurianum rubredoxin (M.W. ∼ 6100), given as a starting point a set of phases to 2.5 Å resolution as determined from heavy-atom derivatives. The result is a 1.5 Å X-ray structure for rubredoxin which generally confirms that recently obtained by a method involving the production and refinement of an approximate atomic model of the protein. In the present work the structure was obtained without chemical assumptions and with a considerable saving of effort compared with the previous determination. The technique also avoided certain small errors in the assignment of side-group structures made in the previous determination. Final high-resolution phasing would result from conventional refinement of the direct-method structure. An additional point of interest is that the structure of a protein at the atomic level, elucidated on purely physical principles, has been found to agree with the deductions from chemistry.
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    The exact dynamical wave fields for a crystal with a constant strain gradient on the basis of the Takagi–Taupin equations (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 830-836 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: From the Takagi-Taupin equations for two-beam cases, the exact wave fields for a spherical incident wave are obtained in the Laue case for a crystal having a constant strain gradient. Absorption is taken into account. Both direct and Bragg-reflected waves are essentially expressed in terms of confluent hypergeometric functions. Their characters depend on strain gradient, structure factor and crystal thickness. The wave fields tend to those obtained by Eikonal theory as the strain gradient decreases. For an extremely large strain gradient, the wave fields reduce to those predicted by the kinematical theory.
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    Largest likely values of residuals (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 836-838 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The value of the residual R2 = ((I1 - I2)2)/(I21), where I1 and I2 are the intensities of reflexion for two unrelated structures having the same symmetry and containing the same atoms, has the value 1 - Σ4/(2Σ2 - Σ4) for non-centrosymmetric structures and 4/3 - 2Σ4/3(Σ2 - Σ4) for centrosymmetric; the corresponding values for R1 = ((F1 - F2)2)/(F21) are approximately 2(1 - π/4) ∼ 0.43 and 2(1 - 2/π) ∼ 0.73. More complex expressions are derived for hyper- and sesquisymmetric structures. If a residual with a scaling factor, such as S2 = ((I1 - EI2)2)/(I21), is used, and the scaling factor E is refined by least-squares, the value of E obtained is about ½ or ½, instead of the true value unity.
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    (for The Neutron Diffraction Commission). Coherent neutron scattering amplitudes (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 847-847 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A list is given which summarizes additions and significant changes which have been reported since the publication of a full list of scattering amplitudes in 1972 [Acta Cryst. (1972). A28, 357-358].
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    Multivariate error analysis by A. A. Clifford (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 460-460 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
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    The crystal structures of solids by P. J. Brown and J. B. Forsyth (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 461-461 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Group-theoretical considerations concerning domain formation in ordered alloys (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 431-440 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Domain formation in ordered alloys has been studied from the group-theoretical point of view. A method is derived to determine the number of orientation variants as well as the number of translation variants from the point groups and lattices of the ordered and disordered structures. The number of orientation variants is found to be equal to the order of the point group of the disordered phase divided by that of the ordered phase. It has been shown that under certain conditions the set of operations that produce all variants from a given original variant can be chosen so as to form a group. The operations relating the orientation variants are the elements of this group called the variant generating group. The results of the theory derived in this paper are general and can be applied to any disorder-order transformation. A few examples are worked out explicitly to illustrate the different theorems.
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    Generalized structural geometry (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 440-447 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Arbitrary combinations of bond lengths, bond angles and torsion angles can be used as generalized coordinates for describing molecular models. It is shown how these and the conventional Cartesian or fractional unit-cell coordinates can be interconverted. Algorithms are also given for the geometrical analysis of rigid structures of links joined by flexible connectors (where only bond lengths are specified). Properties of the connectivity matrix, as an alternative complete description of a structure, are developed. Several applications of the above procedures are described.
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    Primary and secondary extinction (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 454-455 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The assumption commonly made in the application of Zachariasen's extinction correction, i.e. that primary extinction is negligible, is shown to be invalid in many cases.
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    Use of a two-axis neutron diffractometer on an angled reactor channel (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 448-453 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The resolution function is derived for a two-axis neutron diffractometer in which the monochromator scattering plane is inclined to the specimen scattering plane. The analysis shows that useful results can be obtained from powder diffractometers which operate in the horizontal plane and are installed on reactor holes inclined by up to 35° from the horizontal. It is also shown that single-crystal diffractometers may be installed on reactor holes with angles up to 50° from the horizontal.
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    Multiple scattering and dynamical effects in diffuse electron scattering: errata (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 457-457 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Corrections are given to Acta Cryst. (1973). A29, 663-672.
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    Tenth General Assembly and International Congress of Crystallography (1974)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 30 (1974), S. 458-458 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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