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  • International Union of Crystallography (IUCr)  (6,726)
  • 1975-1979  (6,726)
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Year
  • 101
    Electronic Resource
    Electronic Resource
    A Gauss–Seidel least-squares refinement procedure with rigid-group and parameter restraint capabilities (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1010-1015 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A fast Gauss-Seidel least-squares procedure (FGLS) has been developed for crystallographic refinement of atom parameters in large asymmetric units. The procedure combines the minimum matrix requirements and the rapid convergence of the Gauss-Seidel algorithm with rigid-group constraint and subsidiary parameter elastic restraint capabilities. As the procedure is essentially block diagonal, considerable computational efficiency is achieved by determining contributions from one atom to all reflections at a time, instead of the usual practice of determining the contributions from all atoms to each reflection in turn. Application of the method to a 1104-atom protein at 2.5 Å resolution is described.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479002229
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  • 102
    Electronic Resource
    Electronic Resource
    International Tables for X-ray Crystallography, Vol. IV. Error in table of equivalent reflections in the presence of dispersion effects (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1018-1018 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Table 2.3.2 of Reciprocal Lattice Points Equivalent under the Operations of a Given Noncentrosymmetric Point Group, on p. 151 of International Tables for X-ray Crystallography [Vol. IV (1974), Birmingham: Kynoch Press] contains an error for the point group 422. The entry h{\bar k}l should be replaced by h{\bar k}{\bar l}.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479002254
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  • 103
    Electronic Resource
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    Prices of Acta Crystallographica and Journal of Applied Crystallography (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1084-1085 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479002291
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  • 104
    Electronic Resource
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    William Henry Bragg 1862–1942: man and scientist by G. M. Caroe (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1086-1087 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479002333
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  • 105
    Electronic Resource
    Electronic Resource
    Correction of single-crystal intensities for average values of multiple reflection (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 73-78 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: For short wavelengths, Umweganregung cannot be experimentally eliminated. Its average unscaled intensity for a random orientation of the crystal is U(2θ1) = ∫θ, φ (sin 2 θ/sin 2θ1)P(θ1, θ, φ) exp [-2K(sin2 θ + sin2 θ12)] dθ dφ, where (2θ, φ) are the spherical coordinates of a point on the Ewald sphere when the intensity measurement is made at (2θ1, 0), the angle between the two directions being 2θ12, while K is the slope of a plot of In (F(obs)) as a function of sin2 θ, and P(θ1, θ, φ) is the polarization correction appropriate to the double reflection. Diffraction intensities were measured for epidote and quartz. A plot of (|F(calc)|)/(F(obs)) as a function of F(obs) for epidote was considerably improved by the subtraction from each intensity measurement of a number of counts SU(2θ1), where S is an experimental scale factor while U(2θ1) was calculated for epidote by numerical integration over the Ewald sphere. The refinement residuals for both compounds improved after correction of the intensities and the weights.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000139
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  • 106
    Electronic Resource
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    Problems of resolution and bias in the experimental determination of the distribution of electron density and other densities in crystals (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 122-130 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The limited 'window' in reciprocal space through which it is possible to observe diffraction phenomena sets a reciprocal limit to the resolution of detail in density distributions. The limit of resolution depends on whether the detail is one-, two-, or three-dimensional, and to some extent it is possible to choose between (i) lack of resolution associated with a large central maximum representing a point-object and (ii) false details associated with a smaller central maximum and more pronounced diffraction troughs. In any case, however, the limit of resolution is about one-quarter to one-half of the wavelength of the radiation used. Intensities measured by photon or particle counting are unbiased estimates of the true intensities, but their square roots are not unbiased estimates of the structure factors, and this bias may carry over into parameters based on structure factors rather than intensities. A satisfactory correction can be made for the strong reflexions, but weak reflexions (which are required if the theoretical limit of resolution is to be reached) remain a problem.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000218
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  • 107
    Electronic Resource
    Electronic Resource
    New layered structure of Bi2W2O9 determined by 1 MV high-resolution electron microscopy (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 142-145 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The crystal structure of bismuth tungstate, Bi2W2O9, has been examined by 1 MV high-resolution electron microscopy. The symmetry is orthorhombic with lattice parameters a = 5.43, b = 5.41 and c = 23.7 Å and space group Pna21. The point group is determined directly from the images in which each cation site is resolved. The structure is similar to that of conventional layered Bi compounds with respect to the Bi2O2 sheets normal to the c axis; however, the layer between the sheets is of a deformed ReO3 type. The strong chemical bonds between the Bi ions and the apex O atoms cause the rotation of the WO6 octahedron about the c axis. W ions are displaced antiparallel in the WO3 layer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000255
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  • 108
    Electronic Resource
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    Diffraction profiles from small crystallites (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 171-176 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The diffraction profile for a small crystallite has been obtained as the orientational average of the diffraction intensity given by Ino & Minami [Acta Cryst. (1979), A35, 163-170]. The formula obtained is a type of Debye interference function modified by a function {cal V}(r) (the self-convolution of a crystal shape function) and is expressed as a sum over all the atomic distance vectors in the crystal structure. Since the set of the vectors has Laue symmetry (the order of the group: L), the summation can be simplified to a sum over a reduced range corresponding to 1/L of the original range, while the {cal V}(r) is changed to {\bar v}(r) = ΣLp = 1= {cal V}(Rpr)/(LVt) (Vt: volume of the crystal; R1, . . . , RL: element of Laue symmetry group). Once the {\bar v} function is determined, the profile for a complicated crystal of any size and any crystal system can be systematically and efficiently calculated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000292
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  • 109
    Electronic Resource
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    X-ray interferometry with divergent polychromatic beams (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 201-213 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The theory of a strongly absorbing Laue interferometer for an arbitrary composition of the X-radiation has been developed on the basis of the Takagi equations. The coherent properties of interfering beams of polychromatic radiation are discussed. Formulas have been derived showing the dependence on the moiré pattern and the contrast of interference fringes of the type of deviation of the interferometer geometry from the perfectly aligned case. The results of an experiment for obtaining the moiré pattern of a symmetric Laue case (LLL) interferometer with Bremsstrahlung radiation are presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000358
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  • 110
    Electronic Resource
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    Lindemann's rule and the X-ray Debye temperature of anisotropic Zn and Cd single crystals (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 243-245 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Lindemann's rule was applied to Zn and Cd single crystals and found to be valid for these solids, xz being in the range 0.2 to 0.25.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000425
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  • 111
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    Copying Fees and Copyright Law (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 251-252 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479000474
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  • 112
    Electronic Resource
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    Anharmonic contributions to dynamic electron densities caused by large librations of molecules (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 838-844 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The effect of large librations of molecules on electron density distributions is investigated with the already published structures of β-NaN3, p-nitropyridine N-oxide and anthracene, and with some model calculations for C, N, O atoms. For this purpose, the temperature factor of the atom concerned is expanded with cumulants, and the smearing function of the nuclear positions and the dynamic electron density is calculated by an Edgeworth series. The results are represented as difference densities p(anharmonic) - p(harmonic) ≡ p(A - H) maps. For the end N atom in β-NaN3 at 295 K and for the C(7) atom in p-nitropyridine N-oxide at 30 K, these maps contain minima of 0.53 e Å-3 (N) and 0.22 e Å-3 [C(7)] in front of the nuclei, and maxima of 0.44 e Å-3 (N) and 0.21 e Å-3 [C(7)] behind the nuclei. Thus, due to large librations, the density is shifted from in front of the nuclei towards the back of the nuclei, as viewed from the centre of libration. The magnitude of the region of non-vanishing p(A - H) density largely depends on the magnitude of the atomic motions (temperature). With the end N (295 K) mentioned above the radius of this region around the nucleus is about 0.5 Å, and with C (7) about 0.2 Å. Apart from the temperature, the p(A - H) densities are essentially determined by the distance of the atom from the centre of libration, and by the ratio of the librational/translational contributions to the atomic motions. A procedure is described to estimate the extrema in the p(A - H) densities in an actual structure.
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    URL: http://dx.doi.org/10.1107/S056773947900190X
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  • 113
    Electronic Resource
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    Secondary extinction in rotating single-crystal slabs (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 861-870 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Variation of the diffraction conditions for neutrons in a rotating crystal frame is discussed. It is shown that the angle of deviation from the Bragg position varies with a rate equal to double the angular velocity of the crystal. This makes it possible to calculate the diffraction power for type-I Zachariasen mosaic crystals and then to obtain a solution of the Darwin equation for a rotating crystal slab with zero absorption. Further considerations refer to the space distribution of diffracted beam intensity. It is shown that, if a suitable experimental configuration is adopted, the structure factor modules can be determined from the intensity dependence on the angular velocity, and the result is independent of the mosaic-spread distribution in the sample. Results of experiments are presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479001972
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  • 114
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    The direct determination of centrosymmetric electron density distributions by maximizing the integrated cube of the electron density (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 966-970 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The two characteristics of all fully refined electron density distributions are that they do not go negative and the electron density is concentrated into a small number of peaks. The correct set of structure factor signs is defined as a set which gives rise to an electron density distribution which has these characteristics. A simple function, sensitive to these characteristics, is the integrated cube of the electron density; it can distinguish between positive and negative peaks and between a 'peaky' and a dispersed distribution. An electron density map is calculated using only the known signs. Then each structure factor's contribution to the map is added with both alternative signs, in order of magnitude. The correct sign is that giving rise to the larger value of the integrated cube of the electron density. The term is added with the correct sign and the next structure factor similarly dealt with. A recycling procedure is also described. The method of refinement has been tried on several known two- and three- dimensional structures with great success enabling the electron density to be fully refined without recourse to any model. The same criterion can be used to determine the signs of the initial set of structure factors. Electron density distributions are calculated for all permutations of signs of the initial set. The particular combination chosen as correct is that combination giving the largest value of the integrated cube of the electron density. In all the cases tried the method gave the correct signs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479002151
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  • 115
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    Short-range ordering in PbMg1/3Nb2/3O3 (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1015-1017 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: High-resolution electron-microscope images in conjunction with selected-area electron diffraction were used to investigate the short-range order in PbMg1/3Nb2/3O3. Intensity fluctuations of the [110] image indicated ordered domains extending over about 20 to 50 Å. Decomposition phases were observed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479002230
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  • 116
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    An empirical method for correcting rotation-camera data for absorption and decay effects (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 925-933 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: When the same set of structure factors is measured twice from separate crystals, some of the differences between the two sets of data will be due to differences in absorption and crystal disorder. This will particularly affect the isomorphous-replacement method. By carefully choosing crystals of the same shape and size, and by recording the reflections in the same order, these sources of error may sometimes be minimized. However, there will be cases where serious errors of this kind will prove unavoidable. A method of deriving a correction for the absorption and decay differences between two sets of structure factors after the measurements have been made is presented. The method is designed to correct protein data collected on a rotation camera. Where an absorption-corrected subset of the observed data is available, the rotation-camera data may be corrected to this. Where this cannot be done, the method allows a relative correction between native and isomorphous-derivative data sets. The method has been implemented on a mini-computer. Results are presented which show that in some cases a substantial improvement in the quality of the data is obtained, as judged by the agreement of symmetry-related amplitudes. Furthermore, the method is capable of improving the reliability of measurements of anomalous scattering from heavy-atom-substituted isomorphous derivatives. The potential usefulness of this method is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479002072
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  • 117
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    Ein einfacher Weg, Strukturinformation aus den diffusen Reflexen von Kristallen mit eindimensionaler Lagenfehlordnung zu erhalten (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 957-961 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: One-dimensionally disordered crystals consisting of layers that are equal with one another but stacked with displacement faults show sharp reflexions and diffuse streaks on their diffraction patterns. From the sharp reflexions alone an averaged structure (with fractional site-occupation factors) can be deduced. To obtain the real layer structure, the diffuse streaks have to be of considered. A simple experimental and mathematical procedure is described which enables their use without the need of measuring integrated streak intensities or streaks analysing the diffuse intensity profiles. Essentially, one has to measure the intensities of equal portions Δζ at equal positions ζ on different diffuse streaks; the values can be used directly, as they are proportional to the squared value of the layer structure factor. Additionally, a procedure is described for the measurement of the diffuse streak intensity profile on a four-circle diffractometer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479002138
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  • 118
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    The defect structure of VOx. II. Local ionic arrangements in the disordered phase (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 975-989 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In VOx (0.8 〈 x 〈 1.3), there are large numbers of cation and anion vacancies and interstitial vanadium ions. To determine the local arrangements of these defects, X-ray diffuse scattering was measured in absolute units with single crystals of VOx (x = 0.89, 1.17 and 1.28). For x 〉 1, the interstitials are present near vacancy clusters, similar to the defect arrays found in the semiconductor FexO. For x 〈 1, there are few interstitials, but an increased concentration of anion vacancies. The anion and cation vacancies are arranged (so as to minimize the electrostatic energy) in rows along (110) directions with alternating vacancy-rich and vacancy-deficient { 111 } layers. Some portions of this vacancy arrangement resemble that found in metallic TiOx. VOx behaves like a semimetal for x 〈 1, but a semiconductor for x 〉 1, and the present results, when compared to the defect structures in TiOx and FexO, show that the nature of the bonding between cations and anions controlling the conduction mechanism is reflected in the defect structure.
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    URL: http://dx.doi.org/10.1107/S0567739479002175
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  • 119
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    Direct phasing of electron diffraction data from organic crystals: the effect of n-beam dynamical scattering (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1001-1009 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The conditions under which direct phasing methods might be used to determine the crystallographic phases from electron diffraction intensity data are investigated through numerical calculations for two organic crystals. The Cowley-Moodie multislice formulation of dynamical diffraction theory is used to calculate the diffraction intensity data for various crystal thicknesses, at electron energies of 100 keV and 1.0 MeV. The direct phasing method is found to give generally correct phases up to a crystal thickness of about 75 Å at 100 keV. The use of higher electron energy (i.e. 1.0 MeV) produces a significant improvement in the success of the direct phasing approach at crystal thicknesses greater than 75 Å.
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    URL: http://dx.doi.org/10.1107/S0567739479002217
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  • 120
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    Early History of the Union (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1085-1085 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479002308
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  • 121
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    An introduction to microscopy by means of light, electrons, X-rays or ultrasound by T. H. Rochow and E. G Rochow (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1087-1087 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479002345
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  • 122
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    Physics of semiconductors (Topics in current physics, Vol. 15) edited by M. B. Salamon (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1089-1090 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739479002382
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  • 123
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    Use of the neutron time-of-flight technique for structure investigations (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 131-136 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The application of a time-of-flight neutron diffractometer for crystallographic investigations, designed on the basis of a pulsed neutron source, is discussed. The procedure for obtaining integrated intensities and introducing corrections is described. The results of a test experiment with perdeuteronaphthalene are given. The deuterium atom positions in the structure of La2Mg3(NO3)12.24D2O are also refined and compared with those obtained using X-ray diffraction techniques. The observed systematic differences in bond lengths and valence angles are explained by the differences between the 'X-ray' and 'neutron' coordinates of the light atoms.
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    URL: http://dx.doi.org/10.1107/S056773947900022X
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  • 124
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    Transformations to optimize the superposition of similar structures (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 158-163 
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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, efficient and general method is described for finding the linear orthogonal transformation to superpose two similar structures given by sets of equivalent points, usually atomic position vectors. Formulae are also given for extracting the independent variables of rotation from the resulting transformation matrix. In addition, general transformations are derived, both in the case of proper rotation and in the case of rotatory inversion, to convert to a molecular frame of reference based on the superposition axis of symmetry.
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    URL: http://dx.doi.org/10.1107/S0567739479000279
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  • 125
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    The structure of ordered Ag3Mg: an ordinary long-period alloy as opposed to periodically antiphased alloys (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 181-188 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A necessary distinction is established between two kinds of long-period ordered structures: (1) periodic antiphases (PAP), whose principal characteristic feature is a continuous variation of antiphase half-period M as the composition changes; (2) 'ordinary' long-period structures, which show, as the composition changes, a series of phases stable in small composition ranges, every phase displaying a different, but well-defined, crystallographic structure. It is shown, through electron diffraction studies, that the ordered Ag3Mg alloy belongs to the second kind, unlike standard PAP alloys such as AuCu, AuCu3, Au3Cu, Cu3Pt and Cu3Pd. It is stressed that standard PAP structures can always be obtained by heating, above some transition temperature, a L 12 (or L 10) phase which always exists at lower temperature. This is not the case for ordered Ag3Mg. In this connection, the theory about stabilization of long-period structures is discussed [Sato & Toth (1965). Alloying Behaviour and Effects in Concentrated Solid Solutions, pp. 295-419. New York: Gordon & Breach].
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    Basic theorems of vector symmetry in crystallography (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 115-122 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An attempt has been made to deduce the condition necessary for diffraction enhancement of symmetry to occur in the diffraction pattern of a structure X, and because the symmetry of the diffraction pattern of X coincides with that of its vector set V, the symmetric feature of X derived from the symmetry of V was studied. The symmetry with the point group GV or GV/GI according as X is inversion-symmetric or not, is defined as the vector symmetry of X, where GV is the point group of V and GI, is the inversion group, and when the vector symmetry of X is Cn, for example, X is specified as Cn-vector-symmetric. When X is homometric with itself by a rotation of 2π/n, it is specified as n-fold self-homometric. X being n-fold self-homometric is the necessary and sufficient condition for X to be n-fold vector-symmetric. Also, X exhibits an enhanced vector (diffraction) symmetry if it is a space-groupoid structure with the kernel whose point-group symmetry is, other than by addition of an inversion, higher than the point-group symmetry of X. Four examples of enhanced vector symmetry are examined.
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    Experimental electronic valence charge density of NaCl and SrCl2 along the nearest-neighbour axis (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 140-142 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Valence-electron charge distributions have been calculated by difference Fourier inversion of experimental structure factors. The contribution of core electrons has been subtracted by using a one-term Gaussian representation. The results are in agreement with theoretical charge distributions obtained by the local-orbitals Hartree-Fock method.
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    X-ray diffraction by small crystals (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 163-170 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A general method of calculating the intensity of X-ray diffraction from small crystalline particles whose boundary is defined by a shape function is discussed. The intensity formula which is generally given by a double sum over the reciprocal-lattice points is simplified into the form of a single sum, using 'the random-shift treatment' which assumes that the position of the boundary relative to the crystal lattice varies at random from crystal to crystal. By the use of Fourier theorems, the intensity formulas are also converted into a single sum over the direct lattice. Although the electron distribution in the particle has been defined in various ways by the shape function, a more reasonable expression of the electron density appropriate to small crystals is introduced. The intensity formulas derived on the basis of the new form of the electron density are compared with other intensity formulas which have so far been proposed.
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    Mögliche Kristallstrukturen für oktaedrische Moleküle MX6 bei dichtester Packung der X-Atome (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 188-193 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: As long as there is no thermally induced disorder, octahedral MX6 molecules crystallize with a close-packed arrangement of X atoms in which one sixth of the octahedral holes are occupied by M atoms. On consideration of the symmetry restrictions imposed by the partial occupation of the octahedral holes, the possible space groups and structures are deduced with the aid of group--subgroup relationships. Centrosymmetric space groups that can be achieved include: (1) all trigonal, rhombohedral and monoclinic space groups, Pnmn, Pnma, Pnab and P{\bar 1} for hexagonal close packing of the X atoms; (2) Pcmn and Pcab for the double-hexagonal close packing; (3) Fddd, Bbmb, all monoclinic space groups and P{\bar 1} for cubic close packing. The most important molecular arrangements are illustrated and their expected cell dimensions are given. The known structures of WCl6, UCl6 and numerous hexafluorides correspond to some of the predicted possibilities.
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    Derivation of three-phase invariants from the Patterson function (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 213-220 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The idea of Anzenhofer & Hoppe [Phys. Verh. (1962), 13, 119] that all the Fourier coefficients of the function ρ(r).ρ(r + u) are zero if the Patterson function is zero at vector point u is first developed; it is shown that the three-phase cosine and sine invariants may be derived jointly by solving two sets of linear equations. A least-squares method exploiting the entire Patterson function is then presented; this may allow the three-phase cosine and sine invariants to be determined and/or refined. As expected, the low-valued Patterson regions contribute most to the least-squares procedure.
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    Derivation of selenium–selenium nonbonded potential parameters for molecular crystals (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 236-240 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Potential-function parameters for Se-Se non-bonded atom-atom pair interactions have been derived from the room-temperature heat of sublimation of a mixture of monoclinic α- and β-Se8 and from the crystal structures of these two modifications. The first procedure applied was that of Nauchitel' & Mirskaya [Sov. Phys. Crystallogr. (1972), 16, 891-892]; the second was that of Williams [J. Chem. Phys. (1967), 47, 4680-4684]. In the latter procedure, the exponential parameter for the Se-Se repulsive term from the previous procedure was used and the coefficients of the attractive and repulsive terms were fitted to 22 observational equations by a weighted least-squares calculation. The second procedure proved to be more reliable than the first. Unlike the first procedure it yielded parameters with which a heat of sublimation of α-Se8 could be calculated that was 0.8 kcal mol-1 higher than that of β-Se8. At the same time, the calculated heats of sublimation of both modifications deviate by less than 2% from the experimental value of the monoclinic mixture. A low average value of 2.7% for the standard deviation of the parameters derived was obtained.
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    Absorption corrections for neutron diffraction (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 248-248 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The transmission factors tabulated by Rouse, Cooper, York & Chakera [Acta Cryst. (1970). A26, 682-691] for μr 〈 1 can be expressed as the product of two factors: a scale factor and a Debye-Waller factor. In the case of a sphere or a cylinder, the absorption corrections then reduce to simple adjustments of these parameters at the end of the refinement, instead of tedious corrections to the data itself. The results are of particular use for powder data. A printing error in the original paper is also corrected.
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    Prices of Acta Crystallographica and Journal of Applied Crystallography (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 252-252 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    On the choice of elastic constants in evaluating thermal diffuse scattering corrections (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 314-316 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The elastic stiffness constants for the calculation of the thermal diffuse scattering correction of X-ray reflexions should be the isothermal ones. However, adiabatic ones are mostly used. It is shown that, despite a relatively large difference between both types of elastic stiffness constants for pyrazine, the resulting thermal diffuse scattering correction is hardly affected.
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    X-ray attenuation coefficients of graphite in the range 0.40 to 1.54 Å (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 316-318 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The mass attenuation coefficients of graphite have been measured at 23 different wavelengths in the range 0.40 to 1.54 Å. Above 1 Å, the coefficient is proportional to λ2.88.
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    On antisymmetric atomic vibrations in zinc (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 325-327 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Recent elastic neutron scattering data [Merisalo & Larsen (1977). Acta Cryst. A33, 351-354] have been reconsidered. A significant value for the cubic anharmonic force constant α33 = -1.80 (30) x 10-19 J Å-3 of the one-particle potential was obtained, and is compared with other recent determinations.
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    A modified Patterson superposition technique using multiple vectors (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 339-340 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A modified Patterson superposition procedure is presented in which multiple vectors instead of single vectors are utilized. It is demonstrated that by using certain guidelines a map which results from a superposition using a multiple vector can be shifted and superimposed on itself to yield an image of the structure containing many fewer extraneous peaks.
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    Crystallographic pedigree of organic compounds: erratum (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 342-342 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In Haisa [Acta Cryst. (1978). A34, 753-759], to correct a printer's error, the first ten lines in the left-hand column on page 758 should be moved to the bottom of that column.
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    Magnetism and metallurgy of soft magnetic materials by C.-W. Chen (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 345-345 
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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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    Structure reports. Sixty year index, supplement for 1974-1975, A. Metals and inorganic compounds edited by J. Trotter (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 346-346 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    Structure reports. Sixty year index 1913-1973, B. Organic and organometallic compounds edited by J. Trotter (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 346-346 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    Physics and chemistry of materials with layered structures. Vol. 4. Optical and electrical properties edited by P. A. Lee (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 347-348 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    X-ray Bragg scattering in the Born–Oppenheimer approximation (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 340-342 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is shown that for any type of motion of atomic nuclei the following hold in the Born-Oppenheimer approximation of the wave functions: (1) the structure factors for the Bragg intensities are given by the Fourier transform of the average density in the unit cell, (2) for the Bragg intensities, the Boltzmann weight factor of a thermal state appears as a factor of the amplitude (and not of the intensity).
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    Eleventh International Congress of Crystallography. Communicated Abstracts: Supplement to Acta Crystallographica (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 344-344 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    Topics in applied physics. Vol. 22. X-ray optics. Applications to solids edited by H. J. Queisser (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 344-345 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    Structure reports. Sixty year index 1913-1973, A. Metals and inorganic compounds edited by J. Trotter (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 346-346 
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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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    Structure reports. Strukturbericht, cumulative index for volumes 1-7 (1913-1939) edited by J. Trotter (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 346-346 
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    Neutron inelastic scattering, 1977 edited by International Atomic Energy Agency (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 348-349 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Electron diffraction 1927-1977 edited by P. J. Dobson, J. B. Pendry and C. J. Humphreys (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 349-350 
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    Elements of X-ray diffraction by B. D. Cullity (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 350-350 
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    The probabilistic theory of the three-phase structure seminvariant in P21, with applications (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 371-381 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: By embedding the three-phase structure seminvariant T0 and its symmetry-related variants T1, T2 and T3 in suitable quintets Q0, Q1, Q2 and Q3, respectively, one obtains the extensions Qj of the seminvariants Tj. Owing to the space-group dependent relationships among the phases, the value of T0 is simply related to the values of the Qj's. Thus the probabilistic theory of the seminvariant T0 is reduced to that of quintets, which is well developed. In particular the discriminant Δ of T0 is defined in terms of the discriminant of the quintet, and extreme values of Δ are correlated with extreme values of T0 in the sense that T0 ∼ 0 or π according as Δ 〉〉 0 or Δ 〈〈 0, respectively. Applications to two real structures in P21 are described which show the usefulness of these results in the solution of crystal structures.
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    Electron microscopy studies of Mo3CoSi (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 387-390 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Crystals of Mo3CoSi were studied with a high resolution electron microscope. Planar defects of different kinds were easily and frequently observed. The structure of one kind of defect can be derived with crystallographic shear operations.
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    A probabilistic theory of two-phase seminvariants of first rank via the method of representations. IV. Practical aspects and applications (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 401-412 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An algorithm is devised to calculate the expected values of two-phase seminvariants of first rank in space groups up to orthorhombic. This algorithm is based on the probabilistic theory [Giacovazzo (1979). Acta Cryst. A35, 296-305]. The method has been tested on several known structures using the probabilistic formulae obtained via the Gram-Charlier expansion of the characteristic function. We report here the results of the calculations which show how the method can secure a good estimate of a limited number of two-phase seminvariants. These estimates can certainly be of great help in the initial stages of phase determination. In addition, the estimated values of the two-phase seminvariants can be used as a figure of merit to discriminate among the several sets of phases generated in a multisolution process. The use of two-phase seminvariants for enantiomorph discrimination in space groups of class 222 is also illustrated. A detailed comparison with the coincidence method shows the noticeable improvement attained by the present algorithm.
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    Determination of crystal point group by a high-resolution electron microscope image (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 429-430 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Crystal symmetry is exposed in a high-resolution electron microscope image more clearly at the thick part than at the thin part of the crystal, through the dynamical scattering. The crystal point group can be determined by observing how the symmetry in the image changes with thickness. A proposed method is demonstrated with an example of a bismuth titanate, Bi7Ti4NbO21.
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    X–N and X–X (1s2 core) maps for cyanuric acid (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 458-462 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: X–N maps and X–X (1s2 core) maps for cyanuric acid, C3H3N3O3, at 100 K are presented. The differences in scale and in the atomic parameters have large effects (≤ 0.4 e Å-3) on the difference density at the nuclei but smaller effects (≤ 0.15 e Å-3) on the bond density distributions. In particular, the density distribution around the O atoms, where polarization is likely to occur, is different in the two types of maps. It is concluded that, in these regions, the X-N maps yield the more reliable results.
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    Determination of secondary-extinction-free values for the flipping ratio in polarized neutron diffraction (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 468-475 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In the analysis of data obtained by polarized neutron scattering, corrections for extinction are of vital interest. It is demonstrated that, in some materials like metal alloys, secondary extinction in samples obtained from ingots by standard methods is very unlikely to be described by a simple mosaic model. It is shown that cold working of the samples improves the sample properties in this respect. Examples of such treatment are given and a method is described to analyse R-on- rocking-curve data in order to obtain data corrected for secondary extinction.
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    Le groupe P1 et ses sous-groupes. I. Outillage mathématique: automorphisme et factorisation matricielle (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 485-496 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Nowadays there is a good knowledge of space subgroups but, curiously, the subgroups of the simplest space group P1 are poorly known. This is due to the fact that P1 is the sole space group possessing only isosymbolic subgroups while more complicated space groups possess extra translationengleich, klassengleich and other subgroups whose derivation is easier than that of isosymbolic subgroups. Thus the examination of the subgroups of P1 has been undertaken. This problem is connected to a large extent with matrix transformations and automorphisms which constitute the mathematical tools developed in this first paper. The subgroups of P1 are connected with changes of the unit cell, i.e. with matrices having integer coefficients. Such matrices can be decomposed into the products of simpler matrices and reduced to diagonal matrices having integer coefficients. The results of these theoretical considerations will be applied, in a following paper, to subgroups of P1.
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    The number of permitted modes of propagation in N-beam dynamical diffraction of X-rays (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 543-547 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is shown that the real part of the root of the dispersion equation for the permitted modes of propagation is always positive for two-beam Laue and Bragg reflections at the exact diffraction position. Based on this, a general rule is proposed to determine the number, Np, of permitted modes of propagation for N-beam dynamical diffraction, where no extremely asymmetric reflections are involved. In other words, for both σ- and π-polarized wavefields, Np= 2(N- NBragg), where NBragg is the number of Bragg reflections involved. This conclusion is supported by calculations for three-, four-, six- and eight-beam cases.
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    The crystal structure of deuteroammonia between 2 and 180 K by neutron powder profile refinement (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 569-571 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The structure of ND3 has been refined by neutron powder profile analysis at 2, 77 and 180 K to a resolution of sin θ/λ = 0.85 Å-1. No orientational phase transition was found in this range. However, even at 2 K, a strong librational motion exists. The N--D bond length, after correction for libration, is constant with temperature (1.06 Å), as are the D-N-D angles (107.5 ± 0.2°). This bond length is 5% longer than that found in the free molecule, because of hydrogen bonding, but the bond angles are virtually identical.
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    Optimisation de la correction d'absorption (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 587-590 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is shown that the bias in absorption corrections can be reduced by a method based on a least-squares fit of an anisotropic linear absorption coefficient from Ψ-scan observations. This method is carded out by a Fortran program and has been used with success on many data collections, especially for electron density studies.
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    An improved transformation procedure for radial distribution function analysis (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 598-603 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Various methods of reducing the effect of termination error on radial distribution functions are briefly reviewed. A new approach is introduced in which the RDF is calculated by generating the Fourier transform at predetermined points. The resulting sampled transform is relatively free of termination ripple and the spacing of the points reflects the true resolution of the method for the particular, experimentally limited, Smax. The optimum choice of sampling points is determined in relation to special data terminations such as at a zero value of the interference functions or at a peak or trough position. The effectiveness of the sampled transform routine in reducing termination error is demonstrated by applying it to prematurely truncated interference functions derived from scattering data from atactic polystyrene. The advantage of the sampled transform approach to RDF analysis is that it prevents a termination discontinuity in the interference function, such as is often unavoidable even when special care is taken to apply precise corrections to the data, from causing an obscuring ripple on the RDF. In fact, in the extreme, it enables generation of useful RDF's of glassy polymers from data which have been neither corrected nor normalized.
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    On the contributions of the internal modes of molecules to the Debye–Waller factors. II. Theoretical considerations (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 610-613 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is shown that the expressions for the contributions of the internal modes of molecules to the Debye-Waller factors as derived, on the one hand, by means of lattice dynamics and, on the other hand, by means of the FG method common in spectroscopy, are identical after some approximations have been made in the lattice dynamical formulation. It is pointed out that for those methods of establishing the force constant matrix of the internal modes, where the eigenvalues of the FG matrix are constrained to be the squares of the observed frequencies, only standard eigenvector routines need be applied in determining the mean-square-amplitude matrix. Hence, in this case, series expansions which were suggested to circumvent the determination of eigenvalues are superfluous.
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    On the relative stability of ordered, F\overline{4}3m, and disordered, Fm3m, adamantane crystals (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 627-629 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Atom-atom calculations show that Fm3m, F\bar 43m, and I\bar 43m adamantane crystals are close in free energy at room temperature. While Fm3m is stablest for an unstrained crystal, a small ( ̃3%) uniform tetragonal distortion stabilizes an orientationally ordered, strained I\bar 43m or F\bar 43m structure. The structures may well also be stabilized in a crystal of small mosaic block size in which there is a wide distribution of local tetragonal strain about a mean unstrained value.
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    Gyrotropy: an implicit form of ferroicity (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 629-633 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Some optically active ferroic crystals (a few ferroelectrics and one ferroelastic) are known to exhibit reversal of the sign of spontaneous optical rotation in certain directions when a suitable electric field or uniaxial stress is applied. This phenomenon, to be called gyrotropy, is examined in the framework of Aizu's formalism of ferroicity. The case of dicalcium strontium propionate, Ca2Sr(C3H5O2)6, a ferroelastic-ferroelectric, is analysed to illustrate a simple procedure for enumerating gyrotropic orientation states. The 18 'pure' gyrotropic species of crystals are listed: these are those species for which gyrotropy is not accompanied by primary ferroicity. Ni-Cl boracite, BaMnF4, RbNO3, and some related crystals, are suggested as new potential gyrotropics. The first two of these are ferromagnetic gyrotropics, of which no examples are known so far.
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    The use of electric field gradient calculations in charge density refinements. II. Charge density refinement of the low-quartz structure of aluminum phosphate (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 658-664 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The experimental determination of the electron distribution in quartz (SiO2) is problematic since the structure is pronouncedly non-centrosymmetric. The structure of AlPO4 is derived from the quartz structure by replacing half the Si atoms by Al and the other half by P. The physical properties of the two substances are similar. The electric field gradient tensor ∇E at the site of Al (site symmetry 2) has already been measured. This electrostatic information, which is not available for quartz itself, is used in the determination of the charge density. Deformation parameters describing multipolar deformation functions are refined by least-squares methods with respect to X-ray data and the elements of the field gradient tensor simultaneously. A κ refinement yielded the atomic charges + 1.4 for Al, + 1.0 for P and -0.6 for O. The electron distribution obtained with a standard charge density refinement does not reproduce the correct ∇E. Inclusion of ∇E in the refinement leads to a different least-squares minimum. The minimum is ill-defined by the X-ray data alone, whereas ∇E stabilizes the phases of the superstructure reflections (l odd). ∇E is extremely sensitive to the deformation density distribution. The resulting deformation maps show the maxima and minima always in the same places but the inclusion of ∇E changes the heights drastically and indicates the P-O bond to be more covalent than the Al-O bond. The trigonal arrangement of the lone-pair and bonding densities around the O atoms indicates sp2 hybridization. This leads to bent covalent bonds, the angle Al-O-P being 142.3° Double bonding appears to extend over nearly planar structural fragments defined by five atoms; metal-oxygen--metal-oxygen-metal.
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    Rotation-function study of flavocytochrome b2 (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 412-416 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A rotation-function study of flavocytochrome b2 has been carried out using X-ray data from crystals which contain one tetramer of 230 000 Dalton per asymmetric unit. The function computed from 10-5.5 Å resolution data clearly shows the orientation of the molecular 222 symmetry axes. The presence of these symmetry elements is consistent with previous structural and biochemical studies of the molecule.
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    Thermal diffuse scattering for molecular crystals: errors in X-ray diffraction intensities and atomic parameters (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 675-684 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: First-order TDS calculations have been made for naphthalene at 100 K to judge the suitability of different models for calculating TDS corrections used in practice. It appears that a long-wave model which takes account of the non-linearity of the frequency dispersion, gives about the same corrections for structural parameters as the lattice dynamical model for rigid body molecules and harmonic vibrations. Longwave models with linear frequency dispersion give an overcorrection of the errors. For second-order TDS no explicit comparison of the models has been possible, but rough estimates show that the comparison made for first-order TDS is also valid in good approximation for the total TDS.
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    X-ray linear absorption coefficients for silicon and germanium in the energy range 5 to 50 keV (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 696-697 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Experimental absorption data from different authors are commented on and, critically evaluated, presented in a table.
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    A weighting scheme for least-squares structure refinement (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 698-699 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The truncated Chebyshev polynomial provides a reliable scheme for the automatic determination of empirical weights for least-squares structure refinement when the errors are a function of |Fo|.
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    Direct methods in crystallography edited by H. A. Hauptman (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 701-702 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Theoretical consideration of the thermal-vibration effect on X-ray integrated intensities from a noncentrosymmetric crystal near an absorption edge (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 714-717 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The intensities of Bragg reflexions generally show a large change in the energy region near an absorption edge of the constituent atoms. The integrated reflecting powers, Rh(ω) and R-h(ω), measured in the energy-dispersive mode, have been calculated for a Friedel pair of reflexions from a noncentrosymmetric crystal. In order to investigate the effect of thermal vibrations, both perfect and mosaic cases are considered. It has been found that the values of the isotropic temperature factors strongly affect the shapes of the curves for Rh(ω) and R-h(ω). It is shown, for CuI, that the intensity ratio Rh(ω) between the Friedel pair tends to the limit value, unity, when the relevant Bj factors become large: i.e. large thermal vibrations screen the effects due to the polarity.
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    A proposed rigorous definition of coordination number (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 772-775 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is proposed that a weighted coordination number, Z*, be used as a measure of the number of neighbours of an atom in a crystal. Coordinating atoms contribute faces to the Voronoi polyhedron around a central atom and their contributions are weighted in proportion to the solid angle subtended by that face at the centre. The advantages of this definition over other proposals are pointed out.
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    Structures in nature as a strategy for design by P. Pearce (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 510-510 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    The calculation and interpretation of high-resolution electron microscope images of lattice defects (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 511-524 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Limitations on the calculation of high-resolution detail in electron microscope images of defects in crystals are discussed and it is shown how images of crystals with arbitrary strain can be calculated by means of a reinterpretation of the normal dynamical equations. An alternative approach, which in some cases is preferable for numerical calculations, is the method of periodic continuation. These methods, and various approximations to them, are applied to the study of high resolution images of edge dislocations. It is shown that at the 5 Å level of resolution, the column approximation is adequate for calculating the important features of weak-beam images of an edge dislocation with an extended core. However, when applied to the calculation of lattice fringes near a core, it may lead to serious error. Calculations are presented which show that, because of spherical aberration, care is required in the interpretation of lattice-fringe images near a dislocation core even for thin crystals.
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    Stacking variations in cancrinite minerals (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 825-828 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: TEM reveals (0001) stacking faults and periodic superstructures in cancrinite-related minerals, afghanite and franzinite, which are interpreted as aperiodic or periodic displacements of the hexagonal [(Si,Al)6O12] ring units. Franzinite also shows a superstructure when the structure is projected onto the xy plane; satellites occur in the hk0 diffraction patterns.
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    X-ray anomalous scattering factors for silicon and germanium (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 852-857 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Accurate measurements of X-ray attenuation coefficients for Si and Ge have been used to determine those dispersion corrections which are related to the photoeffect cross section. The results are compared with calculations based on relativistic as well as non-relativistic quantum mechanics. A new notation is proposed to facilitate the comparison between experimental and theoretical results.
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    Analysis of orientationally disordered structures. IV. Correlations between orientation and position of a molecule (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 881-885 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The concept of a 'rotational form factor' analysis of molecular crystalline structures is generalized by allowing correlations between the orientation of a molecule and its centre-of-mass position. This coupling results in correction terms to the well known rotational form factor. The method is illustrated with the example of (i) tetrahedral molecules and (ii) dumbells at a site of cubic symmetry (m3m) and the rô1e of the site symmetry as well as the magnitude of the mean-squared amplitude (u2) of the centre-of-mass motion are investigated.
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    Determination of crystal structure factors of Si by the intersecting-Kikuchi-line method (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 895-900 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The intersecting-Kikuchi-line method, which is one of the electron diffraction methods of determining crystal structure factors, has been applied to Si at 1000 kV, with the object of improving the accuracy. It is shown that, with a favourable choice of line intersections, the accuracy is improved by at least a factor of two at 1000 kV compared to the case of 100 kV. From the intersections of the weak Kikuchi lines 1004, 9{\bar 1}3 and 0106 with the {\bar 1} {\bar 1} {\bar 1} line and from those of the 9{\bar 1}3, 8{\bar 2}2 and {\bar 1}95 lines with the {\bar 2}{\bar 2}{\bar 2} line, both the 111 and 222 structure factors are determined simultaneously, including their sign. The 220 structure factor, which cannot be determined by the critical-voltage method unless an accelerating voltage higher than 1000 kV is applied, is also determined from the intersection of the 95{\bar 3} line with the {\bar 2}{\bar 2}0 line. The results obtained agree with those determined by the X-ray Pendellösung method.
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    The stella quadrangula as a structure building unit (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 934-937 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In order to get a simple and accurate description of complex crystal structures, the stella quadrangula is used as a building unit to describe the following structures: SiF4, scheelite, NaZn13, Ru7B3 and Ba3Fe3Se7.
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    Directional Compton profiles, X-ray scattering factors and the one-electron density matrix for diamond, silicon and germanium (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 953-957 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Experimentally determined directional Compton profiles and the X-ray scattering factors f(2,2,2) of crystalline diamond, silicon and germanium are fitted by using a model for the one-electron density matrix containing only interactions between nearest neighbours. It is concluded that such a model is only capable of reproducing the anisotropic properties of the electronic ground state, whereas isotropic ground-state properties require the consideration of intraatomic hybrid-hybrid interactions as well as interatomic interactions between higher-order neighbours.
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    URL: http://dx.doi.org/10.1107/S0567739479002126
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    X-ray diffuse scattering from wurtzite-type compounds (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 971-974 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The diffuse scattering of the wurtzite-type compounds β-AgI, CdS, CdSe, AlN, SiC, ZnO and ZnS has been investigated by X-ray diffraction using film methods, β-AgI, CdS and CdSe show strong non-radial diffuse streak patterns at room temperature, whereas the remaining compounds show only weak or no diffuse scattering. The pattern of the diffuse scattering consists of planes and rods running through the Bragg reflections of the reciprocal lattice of the wurtzite-type compounds. None of these planes and rods runs through the origin of the reciprocal lattice. Most of the diffuse intensity is concentrated in planes parallel to { 10{\bar 1}0 }* and in the rods parallel to a* and c* axes. High- and low-temperature experiments indicate a thermal origin of the diffuse scattering. The observed patterns can be explained by the uncorrelated thermal vibrations of structural units as chains and layers. The intensity shows a correlation to the ionicity and bond length of the investigated wurtzite-type compounds.
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    Syntactic coalescences between the monoclinic and orthorhombic polymorphs of the long-chain even n-alkane, n-C34H70 (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 992-996 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Crystals of pure n-C34H70 grown from heptane solutions at low temperature (278-283 K) exhibit syntactic coalescences between monoclinic and orthorhombic polymorphs. The stackings of the vertical paraffin chains on the tilted ones are revealed by growth spirals which develop on the (001) common faces of the aggregates. From the interaction energy calculations performed with a Lennard-Jones 6-12 potential function, it is shown that coalescences are possible from both structural and energetical points of view.
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    Temperature effects on the X-ray scattering factors of crystals in the LCAO approximation (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 996-1000 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A new convolution method is presented for a rigorous LCAO calculation of the lattice-vibration effects on the X-ray scattering factors of crystals. The theory is applied to a lithium hydride crystal and a good agreement with experiment is obtained.
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    Report of Executive Committee for 1978 (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1068-1083 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S056773947900228X
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    Radiation Leakage around X-ray Tube Shields (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1085-1085 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S056773947900231X
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    The influence of profile analysis on structural parameters and deformation density distributions (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 685-688 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Nett X-ray diffraction intensities have been determined from measured step-scan profiles of deuterated 2,5- dimethyl-3-hexyne-2,5-diol by four different profile analysis procedures. Least-squares refinements based on the different sets of F values showed that variations in profile analysis method may change the coordinates by 1.5 times the σ of 0.8 x 10-4 Å and the thermal parameters by twice the σ of 1.4 x 10-5 Å2. The change in scale factor is 1.2% or 4σ. A correct choice of refinement procedure, in which full-angle data are used for the determination of the scale, turned out to be more important for the calculation of reliable deformation densities than the choice of profile-analysis method.
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    Analysis of orientationally disordered structures. III. Positive definiteness of orientational distribution functions (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 876-880 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In previous publications [Press & Hüller (1973). Acta Cryst. A29, 252-256; Press (1973). Acta Cryst. A29, 257-263], the analysis of molecular crystalline structures by symmetry-adapted surface harmonics has been discussed. Implications of both molecular and site symmetry have been emphasized. Here the question is raised whether the magnitude of symmetry-allowed expansion coefficients is subject to constraints or not. Obvious restrictions result from the requirement that the density distribution at a molecular site has to be positive definite. Even stronger conditions are imposed by the requirement that the orientational distribution function g(τ) has to be positive definite, too. The area in parameter space for parameter values which are in agreement with the strong conditions is shown. Physically meaningful expansion coefficients are guaranteed by starting from a symmetry-allowed rotational potential V(τ).
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    The oscillation method for crystals with very large unit cells (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 901-911 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The processing of data collected by oscillation photography from crystals with very large unit cells (average cell constant ∼ 250 Å) requires important modifications of standard methods. In particular, it is often necessary to correct partially recorded intensities to their fully recorded equivalents. A procedure is described for accurate determination of relevant parameters (crystal setting, cell constants). It relies on the redundancy present in most data-collection strategies, which yields fully recorded, symmetry-related counterparts of a number of partially recorded reflections. A detailed description of one example (tomato bushy stunt virus) is presented, with complete intensity statistics.
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    The relation between the structures proposed for parawollastonite, β-CaO.SiO2 (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 938-941 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The two structures proposed for monoclinic parawollastonite [Tolliday (1958). Nature (London), 182, 1012-1013; Trojer (1968). Z. Kristallogr. 80, 291- 308] are in fact identical within the limits of experimental error. The nature of the relationship is discussed and it is shown that the equivalence is due to the specialized values of the y coordinates of all atoms in the structure. A review of the data sets used in the structure determinations, which allows a decision to be made on the true space group, is also given.
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    The line profile for a random assemblage of identical parallelepiped crystals. A correction (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 1018-1020 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Allegra & Ronca [Acta Cryst. (1978), A34, 1006-1013] proposed an incorrect analytical expression for the line profile of identical parallelepiped crystals. The correct general expression is now given; in the special case of cubic crystals with a cubic unit cell it reduces to the expression proposed long ago by A. J. C. Wilson [X-ray Optics (1949), equation 26, p. 43. Methuen]. The implications of the new result upon the line profile of a polycrystalline sample obeying a Gaussian distribution of crystal sizes are discussed.
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    Contribution of diffraction to the contrast of dislocations in X-ray topography (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 38-44 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The influence of both diffraction and geometrical optics in the formation of the contrast of dislocations in X-ray topography has been studied; it is shown that newly created wavefields, i.e. diffraction, are mainly responsible for the contrast of such a defect except in the case of a minimum of contrast which may be explained by geometrical optics only. It is concluded that it is not possible in general to determine the sense of the Burgers vector of a dislocation with geometrical theories alone and that diffraction must be taken into account.
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    A comparison of the least-squares and maximum-likelihood estimators for counts of radiation quanta which follow a Poisson distribution (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 57-60 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The weighted least-squares method cannot correctly be used when measurements have errors given by counting statistics. The usual procedure results in bias in the values and errors in the calculated variances of the parameters. The maximum-likelihood method requires only a minor change in the least-squares equations and is generally thought to have more desirable properties for its estimates.
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    The role of Debye–Waller-factor measurement in the analysis of diffuse-scattering data (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 61-62 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The cumulant expression for the diffuse intensity scattered from disordered crystals shows that the often observed anomalies in total diffuse scattering also extend to the total displacement scattering. Comparison of experimental data on displacement scattering with Debye-Waller factors for Cu3Au shows the anomaly to be due primarily to inaccurate extrapolation in the vicinity of fundamental reflections. It is suggested that Debye-Waller-factor measurements are a necessary adjunct to diffuse-scattering measurements if the errors incurred in extrapolation are to be reduced.
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    On extinction. III. An improvement of the secondary-extinction theory (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 9-16 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The previous theory [Kato (1976). Acta Cryst. A32, 458-466] is improved by taking into account the higher-order correlations of lattice phase factors. The previous coupling constants in the energy-transfer equations have, in general, to be reduced. In the simplest case of non-absorbing crystals the reduction factor can be given as R = 1 + (τ2)-1\sum_{j=1}^{\infy} V(J + 1)2(-κg κ-g)j, where κ±g is the kinematical diffraction amplitude per unit length for ±g reflection, and V(j)2 is the correlation volume, which is defined for the 2jth-order correlation function. The quantity V(j)2 is an extended concept of the correlation length τ2 for the second-order correlation function in the sense that V(1)2 = τ2. The set of {V(j)2} characterizes the statistical nature of the crystalline medium. The present theory is conjectured to be applicable up to τ2 ≲ Λ, in which Λ is the extinction distance.
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    The absolute measured value of f(220) for Cu –the importance of extrapolation to zero extinction (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 45-50 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A method of deriving an extinction-free estimate of an X-ray structure-factor value is outlined. The method is based on the availability of experimental estimates of the level of extinction from the mosaic distribution of the crystal. A plot of the diffracted intensity or its corresponding structure factor (uncorrected or nominally corrected for extinction) against the percentage extinction effect yields an extinction-free value in the limit on extrapolation to zero level. Such a procedure can also reveal any inadequacy in the correction procedures by lack of internal consistency. An illustration of the potential of this procedure is given using the set of 11 experimental values of f for the 220 reflexion of Cu derived by Schneider [Acta Cryst. (1977), A33, 235- 243] from γ-ray diffractometry. Use of the experimental estimates of the level of extinction together with the nominally corrected f data indicates the presence of systematic residual error. Linear extrapolation of these f values to zero extinction yields an estimate of f (220) in the region of 16.77. A value in this region is more in accord with a Hartree-Fock value than with the band-structure value, which was favoured by Schneider's original estimate of 16.46.
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    Net atomic charges and molecular dipole moments from spherical-atom X-ray refinements, and the relation between atomic charge and shape (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 63-72 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Refinement of the population and radial dependence of the spherical atomic valence shell is introduced in a general crystallographic least-squares program. The radial dependence is described by an expansion-contraction parameter κ, which, in the nine data sets tested, indicates contraction of positively and expansion of negatively charged atoms in agreement with theoretical concepts such as those incorporated in Slater's analytical rules for atomic orbitals. H atoms appear more contracted than concluded previously on the basis of a comparison of X-ray and neutron thermal parameters of sucrose. An average value of 1.40 for the radial contraction of H is used in structures for which no neutron thermal parameters are available. The resulting net charges are used to calculate X-ray molecular dipole moments whose magnitude and direction are in good agreement with theoretical and other experimental results, though some differences may be expected because of matrix effects. Net molecular charges in the one-dimensional conductor TTF-TCNQ agree with results obtained earlier by direct integration of the charge density over the molecular volume. A charge transfer from Si to V in the superconducting alloy V3Si is also in agreement with earlier results.
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    The joint probability distribution of the structure factors in a Karle–Hauptman matrix (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 101-105 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The joint probability distribution of all structure factors Ehi - hj (i, j = 0, ..., m) in an (m + 1) × (m + 1) Karle-Hauptman matrix is derived for structures in the space group P1.
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    Structural changes in β-FeOOH caused by radiation damage (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 197-200 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A high-resolution electron-microscope study of β-FeOOH (akaganéite) has been carried out. Examination of intact crystals and ultrathin sections provided no evidence for the hollow-rod structure proposed by earlier workers. It was found also that features, such as mottling and striations, previously thought to be indicative of a porous structure, were due to radiation damage. It is concluded that the β-FeOOH crystals are not porous, but are, in fact, crystallographically homogeneous.
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    Optical data processing. Topics in applied physics. Vol. 23 edited by D. Casasent (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 253-254 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Computer-simulation methods in X-ray topography (1979)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 35 (1979), S. 21-28 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Complex diffraction contrast features on X-ray topographs are often difficult to interpret by simple arguments. In such cases computer-simulation methods have frequently proved useful in understanding the observed contrast features and in elucidating the nature of the defect involved. The application of computer-simulation methods for interpreting X-ray diffraction contrast at planar and line defects in crystals is outlined.
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