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  • Articles  (4,792)
  • International Union of Crystallography (IUCr)  (2,751)
  • Cell Press  (2,041)
  • 1980-1984  (2,617)
  • 1975-1979  (2,175)
  • 1980  (2,617)
  • 1977  (2,175)
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  • Articles  (4,792)
Years
  • 1980-1984  (2,617)
  • 1975-1979  (2,175)
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  • 1
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 32-36 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The dielectric constants (εr) of several cadmium iodide polytypes have been measured and found to be higher than that of the parent 4H structure. The dielectric constants for several long-period polytypes have been found to lie between 60 and 190 whereas the 4H parent structure has a value in the range of about 15 to 30. In particular, both increasing polytype periodicity and increasing disorder increase the dielectric constant, e.g. from 16 for an ordered 4H structure to 114 for a 32H structure and 67 for a highly disordered 4H structure. In the absence of disorder a particular polytype has its own characteristic dielectric constant value, e.g. the two 14H polytypes which have a common εr value of 63.
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  • 2
    Electronic Resource
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 57-59 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The separation or overlapping of adjacent subgrain images in Berg–Barrett X-ray (reflection) micrographs is described for a reasonably general misorientation between the crystal subgrains. The five crystallographic parameters which describe the diffraction experiment also specify, effectively, the direction of the rotation axis for the misorientation, the amount of the angular misorientation, and the relative orientation of the subgrain boundary interface.
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  • 3
    Electronic Resource
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 62-63 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: An effective method for greatly reducing the electron exposure of radiation-sensitive organic or biological specimens while recording their diffraction patterns in a high-voltage (MeV range) electron microscope is described. It involves recording on double-coated screen-type medical X-ray film and backing it with a luminescent radiographic screen which intercepts the transmitted electrons and emits visible light that exposes the bottom emulsion of the film. Values of sensitivity, resolution, and enhancement ratio are tabulated: the latter values range up to 41. Typical patterns taken with l-valine are shown.
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  • 4
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 191-196 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A new procedure for solving the integral equation which connects the crystallite-diameter distribution function pν(D) to the pure diffraction profile is presented here. The representation of pν(D) is based on a polynomial expansion modulated by a generalized Cauchy function. The influence of the truncation of the diffraction profile and of the number of coefficients of the polynomial expansion is studied. This procedure has been applied with success to some catalysts, the granulometry of which is known by other experimental approaches.
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  • 5
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 201-202 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Methylammonium manganese(II) trichloride dihydrate crystallizes with a monoclinic unit cell, a = 7.795 (2), b = 9.154 (2), c = 11.462 (4) Å, β = 91.28 (3)°, space group P21/c. Its structure is related to that of α-RbMnCl3.2H2O.
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  • 6
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 132-132 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Pb3(VO4)2 is monoclinic at low temperatures with space group P21. The lattice constants are a = 15.042, b = 6.106, c = 18.586 Å, β = 111.88°. The indexed powder diagram is given.
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  • 7
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 141-146 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: An analysis of thickness variances from an electron-microscope cross-section of myelin sheath shows that the standard deviation varies proportionally to the square root of the number of intervening lamellae. This result implies that the lamellar irregularities are additive and that a paracrystalline stacking model is an appropriate representation for the myelin structure. The X-ray diffraction data from myelin exhibit the characteristics of a paracrystalline material. Both electron microscope and X-ray diffraction evidence suggest that myelin is paracrystalline rather than crystalline.
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  • 8
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 344-348 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The paper refers to a previously described two-dimensional model of crystal-growth disorder in which the type of molecule or atom at a given lattice site is dependent on the previous neighbour in the two crystal directions. It is shown that a special case of the model for which some mathematical and simulation results have been published corresponds to the situation when the statistical properties of the lattice are symmetric to reflection in a plane. This symmetry may be achieved by applying a constraint to the growth probability parameters and, moreover, enables a solution to be found for the statistical properties of the lattices.
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  • 9
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 172-176 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: It is shown that the neutron small-angle scattering of pure polycrystalline metals is strongly connected with surface imperfections in the samples (surface irregularities and oxide layers) rather then with the grain boundaries. It is also shown that the asymptotic form of the scattering cross section of the surface imperfections is q−4 like the Porod law.
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  • 10
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 180-183 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Precision strain-gauge measurements of the basal-plane lattice asymmetry (a−b)/[(a + b)/2] in gallium metal have been made in the temperature range 190 K 〈 T〈 300 K. When analyzed in conjunction with previously published data these results indicate that the asymmetry is positive from the melting point, TM = 303 K, down to at least 2 K. The asymmetry reaches a limiting value of 5750 × 10−6 at low temperatures and is decreasing rapidly toward zero near TM. The extrapolated `tetragonality' temperature is T* ̃354 K.
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  • 11
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 210-210 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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  • 12
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 211-211 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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  • 13
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 212-212 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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  • 14
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 405-411 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Data have been collected from a standard silicon sample with Cu Kβ radiation selected by means of a pyrolytic graphite monochromator mounted in the scattered beam. The peak shapes can be described reasonably well by Lorentz-type functions, but not by a Gaussian function. These results have been applied to the profile refinement of data for rhombohedral La0.75Sr0.25CrO3, a perovskite-type compound which is slightly distorted from the ideal cubic structure. The analysis, which employs a slightly modified version of a neutron profile program [Rietveld (1969), J. Appl. Cryst. 2, 65–71] indicates that the best compromise for the peak shape is a modified Lorentz function of the form A[1 + C(Δ2&thgr;)2]−1.5 fitted over a range of two halfwidths on each side of the Bragg peak position. The results are compared with a similar refinement of neutron data collected from the same sample.
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  • 15
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 415-421 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A new numerical method is presented for simultaneous smoothing, desmearing and Fourier transformation of X-ray and neutron small-angle scattering data. The method can only be applied to scattering curves from dilute particle systems, i.e. for scattering media whose distance distributions are zero beyond a certain value. The distance distribution of the scattering medium is approximated by a linear combination of about 20 to 30 cubic B-splines. These spline functions have a restricted extension in real space. Their coefficients are adjusted by a weighted least-squares operation so that the series, after being Fourier transformed and smeared according to the geometry and wavelength distribution, represents an optimum smoothed approximation of the experimental data. Tendencies towards oscillations in the least-squares operation are suppressed by a new stabilization routine. The method offers a new possibility for the estimation of the radius of gyration, which is generally superior to the Guinier approximation.
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  • 16
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 315-320 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The channel systems of (4-methylpyridine)4Ni(NCS)2 and (4-vinylpyridine)4Co(NCS)2 are described and compared. Much similarity is found and the minimum diameters of free passage are estimated in each case. It is concluded that both complexes are clathrates with notable zeolitic properties.
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  • 17
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 349-350 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Gadolinium molybdate was twinned by irradiating the crystals for a few seconds with a 10.6 μm CO2 laser. Slit-shaped regions were irradiated to produce localized heating, thereby setting up suitably oriented thermal stresses of sufficient size to induce mechanical twinning. Ferroelastic twin patterns are difficult to control by this method because the required slit orientations are not parallel to domain-wall directions.
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  • 18
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 356-356 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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  • 19
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 502-503 
    ISSN: 1600-5767
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  • 20
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 503-503 
    ISSN: 1600-5767
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  • 21
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 504-504 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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  • 22
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 507-508 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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  • 23
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 508-509 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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  • 24
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 509-510 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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  • 25
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 13 (1980), S. 307-310 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Transparent yellow-greenish single crystals of Pb6GeO8 were obtained by spontaneous crystallization from a melt with a molar ratio of PbO:GeO2 = 4.7:1. The compound is monoclinic, probable space group P21/a, a = 52.665 (12), b = 15.063 (4), c = 23.949 (6) Å and β = 111.03 (5)°; Dm(293 K) = 8.65 (3), Dx = 8.653 Mg m−3 for Z = 64 formula units. Pb11Ge3O17 was synthesized by sintering high-purity oxides PbO and GeO2 of molar ratio 3.69:1 at 990 K. The compound is also monoclinic, probable space groups C2/c or Cc, a = 25.117 (5), b = 15.440 (2), c = 45.529 (13) Å and β = 103.20 (4)°; Dm(293 K) = 8.55 (2), Dx = 8.560 Mg m−3 for Z = 32.
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  • 26
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 13 (1980), S. 311-315 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Series of chalcopyrite-type solid solutions AgGaxIn1−xSe2 and CuGaxIn1−xSe2, 0≤x≤1, have been prepared by direct synthesis from the melt. The space group is I\bar 42d (No. 122), Z = 4. The unit-cell parameters a and c and the ratio c/a change linearly with x. Powder data at 298 K are given for AgGaSe2, AgGa0.80In0.20Se2, AgGa0.70In0.30Se2, AgGa0.40In0.60Se2, AgGa0.20In0.80Se2 and AgInSe2, and for CuGaSe2, CuGa0.60In0.40Se2, CuGa0.30In0.70Se2 and CuInSe2.
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  • 27
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 13 (1980), S. 318-337 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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  • 28
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 13 (1980), S. 338-339 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: If the length of the focal spot is increased while the width remains constant, X-ray output can be increased without changing the specific loading on the anode.
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  • 29
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 13 (1980), S. 391-394 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: To increase the integrated reflexion intensity of a monochromating crystal, the strain distributed due to impurity introduction can be utilized and, compared with conventional mechanical processing, can be a more controllable method. From this viewpoint, numerical computations on the dynamical diffraction theory for distorted crystals have been performed for the 220 reflexion of Cu Kα X-rays by Si crystals with several different strain distributions; the one changing linearly with depth from −10−3 at the surface to zero at 28 μm depth gives the maximum integrated intensity within the restriction that the full width of the intensity profile is 100′′. The integrated intensity in this case is 5.3 and 4.5 times as strong as that of a perfect crystal plate for σ and π polarizations respectively. It is more enhanced for shorter wavelengths, e.g. nine times for Mo Kα. In addition, that intensity profiles for distributed strains are cut sharply at both ends is certainly favourable for the purpose of monochromating white X-rays. A preliminary experiment with an intentionally processed sample showed a fourfold intensity increase for a σ-polarized Cu Kα line with a full width of 80′′.
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  • 30
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 13 (1980), S. 401-401 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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  • 31
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 13 (1980), S. 88-92 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The device described here has been designed to perform microcreep experiments and, simultaneously, to get X-ray topographic images of the sample. Such dynamic experiments are now possible with white synchrotron radiation (LURE-DCI Orsay). The sample material, germanium, is hard to machine; so a simple shape was chosen (rectangular platelet 20 × 5 × 0–2 mm). The grip system is quite simple and the whole assembly is compact enough to be adapted to fit a standard goniometer head. Temperatures as high as 923 K in air and forces up to few tens of Newtons can be reached. The first experiments on Ge bicrystals have shown the device to perform as expected.
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  • 32
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 13 (1980), S. 78-87 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A polished plate of type Ia natural diamond, containing regions which had been bombarded by electrons of energy between 2.0 and 4.5 GeV to total doses of 1̃024 electrons m−2 in polarized-photon production experiments, was examined, both before and after annealing at 1133 K for 8 h, using birefringence, cathodoluminescence photography and spectroscopy, X-ray topography and a novel method for mapping differences of interplanar spacing; `dilatation topography'. The annealing effected some reduction of overall strain, but it also generated an enhancement of diffraction contrast and cathodoluminescence at certain grown-in defects that the specimen contained. By means of dilatation topographs, interplanar spacings in a region of maximum electron-beam damage were compared with those in least-damaged regions some millimetres away. In the case of (110) planes that had been grazed by the electron beam, the maximum relative expansion, Δd/d, in damaged regions so found was reduced from 1.2 × 10−3 to 0.9 × 10−3 by the annealing. Evidence was obtained that the magnitude of expansion in the region of maximum damage was dependent upon crystallographic direction. Criteria are suggested for selection of diamonds to be used in polarized-photon production experiments.
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  • 33
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    Applied crystallography online 13 (1980), S. 93-95 
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    Notes: Cell data for monoclinic sebacic acid COOH(CH2)8COOH, C10H18O4, and dodecanedioic acid COOH(CH2)10COOH, C12 H22O4, are accurately given: (C10) a = 15.064 (6), b = 4.987 (4), c = 10.142 (6) Å, β = 133.14 (5)°; (C12) a = 17.594 (8), b = 4.933 (3), c = 10.174 (5) Å, β = 132.35 (3)°. It is shown that crystallographic data for molecular alloys [C10]x[C12]1 −x are significantly different for those prepared by fusion (m.a.f.) and those by sublimation (m.a.s.). X-ray powder data are given for the two series of solid solutions.
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  • 34
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    Applied crystallography online 13 (1980), S. 95-96 
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    Notes: The cell constants for the title compound, HMTSFT–CNQ4, (C12F4N4).(C12H12Se4), M.W. 748.21, were determined from a single crystal exposed on a Syntex P{\bar 1} automated diffractometer and then refined from precision powder data with Cr Kα1 (2.28962 Å): a = 21.906 (5), b = 12.918 (3), c = 4.0184 (10) Å and β = 91.658 (10)°; U = 1136.7 Å3, Z = 2, space group C2/m; Dc = 2.186 Mg m−3.
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  • 35
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 13 (1980), S. 128-131 
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    Topics: Geosciences , Physics
    Notes: A &psgr;-φ goniometer with two axes of rotation, driven by stepping motors, was designed as an attachment for a horizontal &thgr;/2&thgr; goniometer. By means of this arrangement it is possible to measure strains and compute stresses in single crystals and in separate crystals of a bicrystal or a coarse-grained polycrystalline specimen. The available rotations also make it possible to investigate residual macrostresses and textures in polycrystalline specimens. Results of the test measurements on a tensile specimen of a molybdenum single-crystal are given.
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  • 36
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    Applied crystallography online 13 (1980), S. 132-134 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    Notes: A calculation of γ(r), the correlation function which gives the X-ray scattering of a cubic object of uniform density, is given for the full range of r. A new method, which can be applied to other systems, is used. The function γ(r) and its first derivative are everywhere continuous, but γ′′(r) is discontinuous at r = L and r = L□2 (L = cube edge length). This leads to an intersect distribution of unexpected shape. The scattering intensity as a function of angle is calculated by Fourier inversion of γ(r).
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  • 37
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    Applied crystallography online 13 (1980), S. 135-140 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    Notes: An experimental method is described which enables the inelastically scattered X-ray component to be removed from diffractometer data prior to radial density function analysis. At each scattering angle an energy spectrum is generated from a Si(Li) detector combined with a multi-channel analyser from which the coherently scattered component is separated. The data obtained from organic polymers has an improved signal/noise ratio at high values of scattering angle, and a commensurate enhancement of resolution of the RDF at low r is demonstrated for the case of PMMA (ICI `Perspex'). The method obviates the need for the complicated correction for multiple scattering.
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  • 38
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 13 (1980), S. 148-153 
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    Notes: Until now, the calculation of intensities diffracted by layer structures has only been treated in the two extreme cases, namely (i) the layers interfere completely and (ii) the layers do not interfere (turbostratic structures). The present paper develops the mathematical treatment which allows the calculation in the intermediate case, namely when layers are only partially interfering.
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    Applied crystallography online 13 (1980), S. 141-147 
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    Notes: A niobium tungsten bronze crystal with a nominal composition 3 Nb2O5.8WO3 gives circular diffuse scattering in the electron diffraction pattern. The diffuse rings change shape and size when the crystal is reduced at high temperature. The observations are explained by improving the cluster model proposed by De Ridder, Van Tendeloo, Van Dyck & Amelinckx [Phys. Status Solidi A (1977), 41, 555–560]; the present clusters are composed of nine pentagonal-tunnel sites. The 1 MV high-resolution electron microscope images reveal that the clusters have several different atomic arrangements and compositions. The relation between the clusters and the micro-domains, formed in the initial stage of long-range ordering, is clarified.
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  • 40
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    Applied crystallography online 13 (1980), S. 163-167 
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    Notes: Analysis of Bragg X-ray intensities collected on a single-crystal diffractometer with a pyrolitic graphite monochromator in the diffracted beam has revealed small systematic errors which are a consequence of the necessarily large illuminated area of monochromator crystal.
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  • 41
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    Applied crystallography online 13 (1980), S. 154-162 
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    Notes: The amount of information which can be obtained from a small-angle X-ray experiment can be considerably increased by orienting the solute particles with external forces during the scattering experiment. In this paper it is shown that orientation by flow, for instance through a capillary tube, gives additional information about size, shape, flexibility and rotational diffusion of the particles. The only requirement, in order to obtain flow-oriented samples, is that the solute particles must be relatively large and asymmetric. On the other hand, if the scattering curve is dependent on the flow rate through the capillary, it can immediately be concluded that the solute particles are asymmetric (or they are deformed in the hydrodynamic field). Equations describing the relationship between flow rate, molecular shape and scattered intensity are given, and theoretical intensity patterns for some representative cases are presented. It follows that there is a fundamental difference in scattering patterns for oblate and prolate ellipsoids of revolution. This difference can be used to differentiate between these two cases. Some experimental results obtained using inorganic model colloids are presented.
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  • 42
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    Applied crystallography online 13 (1980), S. 168-175 
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    Notes: An analysis of the small-angle scattering problem based on Shannon sampling is presented. It is shown that this approach leads to an accurate assessment of the information contained in a given data set and rigorous estimates of the errors inherent in the parameters derived therefrom.
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  • 43
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    Applied crystallography online 13 (1980), S. 401-402 
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  • 44
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    Applied crystallography online 13 (1980), S. 605-607 
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    Notes: The crystal structure of Ta5P3 has been refined from Guinier–Hägg X-ray powder film data [Malmros & Thomas (1977). J. Appl. Cryst. 10, 7–11] using positional parameters from its structural isomorph Nb5P3 as starting values. The space group is orthorhombic Pnma (No. 62), with refined cell parameters a = 25.3210 (4), b = 3.4013 (1), c = 11.4614 (2) Å, V = 987.12 (5) Å3 and Z = 4. Final R = 4.9% [comparable with the R(F) of single-crystal work]. The structure exhibits the triangular prismatic Ta coordination about the P atoms and the dense packing of the Ta atoms, both familiar in many metal-rich phases formed between transition metals and non-metals of type P, As, S and Se.
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  • 45
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    Applied crystallography online 13 (1980), S. 611-611 
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    Notes: In the paper Revised and new crystal data for indium selenides by Popovic, Tonejc, Grzeta-Plenkovic, Celustka & Trojko [J. Appl. Cryst. (1979), 12, 416–420] it was concluded that P6122 or P6522 was the probable space group of the second high-temperature, γ, phase of In2Se3. Also, in the paper Phases, lattice parameters and thermal expansion of (GaxIn1−x)2Se3, 1 ≥ x ≥ 0, between room temperature and melting point by Tonejc, Popovic & Grzeta-Plenkovic [J. Appl. Cryst. (1980), 13, 24– 30] the same space groups were suggested for the phase γ1, existing in the In-rich region. However, the space group in both cases should be P61 or P65. This is in agreement with the work of Likforman, Carré & Hillel [Acta Cryst. (1978), B34, 1 5] who solved the crystal structure of one of the forms of In2Se3, and with the work of Schulte-Kellinghaus & Krämer [Acta Cryst. (1979), B35, 3016 3017] who solved the crystal structure of AlInS3. Likforman, Carré & Hillel [Acta Cryst. (1978), B34, 1 5] call their phase the low-temperature form of In2Se3.
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  • 46
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    Applied crystallography online 10 (1977), S. 184-190 
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    Notes: A new method for unfolding X-ray diffraction profiles is reported. The procedure is based on the minimization of an economical function defined as the sum of the squared relative differences between the experimental function and a calculated profile. The latter is numerically computed from the convolution equation by fitting the pure diffraction profile with a damped polynomial. The influence of the truncation and the width of the instrumental function is studied; some results are given concerning the evaluation of a constant background for both the experimental and instrumental functions and its influence on the precision of the results. Finally, the perturbation produced by the simulation of statistical noise is discussed.
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  • 47
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    Applied crystallography online 10 (1977), S. 200-200 
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    Notes: The tetrapolyphosphate (NH4)2GeP4O13 is triclinic (P{\bar 1}), with a unit cell: a = 15.08 (1), b = 7.763 (5), c = 4.914 (5) Å, β = 98.18 (1), β = 96.53 (1), γ = 84.04 (1)° and Z = 2. This salt is isotypic with the corresponding salt of silicon: (NH4)2SiP4O13, previously described by the author.
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  • 48
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    Applied crystallography online 10 (1977), S. 203-205 
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    Notes: Cell data for cubic solid electrolytes RbAg4I5 (i) and KAg4I5 (ii) are accurately given: (i) a = 11.235±0.006 Å, (ii) a = 11.143±0.006 Å. Precise crystal data are also shown for orthorhombic Rb2AgI3 (a = 19.957 + 0.006, b = 10.227±0.003, c = 4.896±0.003 Å) and K2AgI3 (a = 19.522±0.009, b = 10.011± 0.003, c = 4.775±0.003 Å). Crystallographic behaviour with respect to the value of x for (RbxK1−x)Ag4I5 and (RbxK1−x)2AgI3 solid solutions is described.
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  • 49
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    Applied crystallography online 10 (1977), S. 209-210 
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    Applied crystallography online 10 (1977), S. 211-211 
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    Applied crystallography online 10 (1977), S. 213-219 
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    Notes: In directionally solidified Pb–Sn eutectic, prepared by the Bridgman technique, it has been found that it is possible to obtain grains of nearly perfect lamellar structure and stable at elevated temperatures. The interlamellar spacing λ is 1 μm and the mean distance \bar D between fault lines 20 λ whereas they are respectively 1.5 μm and 7 λ in neighbouring grains. The orientation relation is: growth direction || [2\bar 11]Pb and ⊥ (12\bar 1)Sn, lamellar interfaces || (47\bar 1)Pb and (523)Sn. The spacing between planes perpendicular to the growth direction is the same in both phases. By studing the orientation relation with Bollmann's theory, zero planes and interfaces are found to be parallel. The coherent interphase boundary can explain the particularities of these grains. Their origin is discussed and related to the nucleation conditions.
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    Applied crystallography online 10 (1977), S. 220-227 
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    Notes: The intensity distribution observed in neutron diffraction experiments in Zr–20% Nb alloys is analyzed in terms of the essential anharmonicity in the free energy, believed to be important in the Group IVb transition metals. It is found that a second-order approximation in the atomic displacements for calculating the diffuse intensity readily accounts for the main features of the intensity distribution. In particular, the observed ratio of intensities between certain superlattice reflections is shown to be related in a straightforward manner to the ratio of harmonic to anharmonic free-energy coefficients. Furthermore. in order to describe the observed curvature in the diffuse streaking in some Ti and Hf alloys, the displacement field of a localized ω-like particle is obtained by adding longitudinal displacement plane waves with wave vectors distributed on one octant of a spherical surface centered at the 〈111〉 octahedral site of the reciprocal lattice, and passing through point km ∼ 0.71 [ 111 ]. Computer-generated patterns of dots simulating such displacement fields were Fourier-transformed optically, yielding diffuse intensity' in excellent agreement with the circular streaking found experimentally in electron diffraction patterns of certain Ti and Hf alloys.
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    Applied crystallography online 10 (1977), S. 256-261 
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    Notes: An X-ray diffraction method has been used to study the influence of solute concentration on molecular associations in aqueous solutions of urea. Calculations, from experimental intensities, of unsharpened electronic radial distribution functions (RDF) are presented for the range of urea concentration from 0.83 to 16.8 mol Kg−1 (5 to 100% solutions). The changes in RDF with increasing urea concentration are complex. Unequivocal interpretation of the distribution functions is not possible, but the major effect of the relatively bulky urea molecules on the water structure appears to be caused by distortion of the water matrix. At urea concentrations of 50% and 100%, features of the RDF not visible in the RDF's at lower concentrations indicate the presence of long-range (over a distance of about 8 Å) structural relationships possibly due to urea–urea interactions: such an interpretation would make urea–urea hydrogen-bonded associations unlikely at lower urea concentrations.
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    Applied crystallography online 10 (1977), S. 277-280 
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    Notes: A technique for including all neutrons scattered by the specimen in the refinement procedure is described. This eliminates the sensitivity of atomic parameters to the choice of background level which occurs when only the profile is used in the refinement.
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    Applied crystallography online 10 (1977), S. 281-286 
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    Notes: The contrast of dislocations in X-ray topographs taken in the Guinier–Tennevin mode with synchrotron radiation has been studied. In reflections where high orders were insignificant, dislocation images appeared very similar to those in Lang topographs taken with characteristic radiation. At large specimen-to-plate distances orientation contrast is important in the direct image. The sense of the Burgers vector of dislocations showing double contrast can be deduced.
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    Applied crystallography online 10 (1977), S. 308-314 
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    Notes: Aluminium single crystals have been elongated at 78 K along an axis close to 〈110)〉. During the recrystallisation process new crystals appear in the vicinity of the deformation bands (deformation heterogeneities) and have only two orientations. The mechanism of the single and double Rowland transformations of the matrix may be responsible of these orientations. It is also found that the double Rowland transformation may account for the change between the initial and final orientations of the matrix (after an 80° elongation). The correspondence between the slip deformation of the matrix and a deformation, due to the double Rowland transformation which may be localized in the deformation bands, is established.
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    Applied crystallography online 10 (1977), S. 412-414 
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    Notes: Polystyrene latex spheres of 1.011 and 0.234 μm diameter were used to observe low-angle X-ray scattering. For particles of about 1 μm in diameter the central scattering has not been observed accurately with characteristic X-rays such as Cu Kα and Mo Kα radiations, because it is strongly affected by the primary beam and the parasitic scattering of the slits. The present results show that the central scattering of particles of this size can be measured within 22′ without such influences by the use of C Kα radiation (wavelength 44.7 Å).
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  • 58
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    Applied crystallography online 10 (1977), S. 422-425 
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    Notes: The average radius of platelet-like defects in crystals may be estimated by the non-destructive technique of X-ray topography of `spike' diffuse reflections, applicable to platelet radii several orders of magnitude smaller than the resolution limit of conventional X-ray topographic techniques. With the most favourable diffraction geometry, the diffuse topograph image may be regarded simply as the convolution of the platelet number density projected along the diffracted beam direction with the Airy diffraction intensity pattern appropriate to the radius of individual platelets. Different integrals of the Airy diffraction intensity profile are involved in photometric traverses of `spike' section topographs and of `spike' projection topographs. The required integrals are evaluated and discussed.
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    Applied crystallography online 10 (1977), S. 431-438 
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    Notes: The special features of X-ray energy-dispersive powder and single-crystal diffraction using synchrotron radiation are discussed on the basis of experiments performed at the Deutsches Elektronen-Synchrotron, DESY. The method is shown to be of particular value for fast structure identifications, experiments for which large scattering vectors are important, studies of phase transformations and chemical reactions at elevated temperatures and high-pressure studies. Studies of time-dependent phenomena using pulsed external fields are discussed.
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    Applied crystallography online 10 (1977), S. 458-464 
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    Notes: A new method of texture analysis is presented that possesses the main advantages of both the Bunge–Roe and Williams methods. The general relations between pole densities and orientation distribution are given as well as an example of the result that can be obtained when only a single incomplete pole figure is available.
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    Applied crystallography online 10 (1977), S. 450-457 
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    Notes: A computer-controlled diffractometer has been built which permits intensity measurements to be made in any direction in reciprocal space in the diffraction plane with step sizes down to 0.01′′ of arc. Three examples illustrate the performance and application of the instrument: (a) perfect silicon, (b) gadolinium gallium garnet with growth striations and (c) niobium with low-angle grain boundaries.
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    Applied crystallography online 13 (1980), S. 340-341 
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    Applied crystallography online 13 (1980), S. 342-342 
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    Applied crystallography online 13 (1980), S. 354-358 
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    Notes: Image formation in a dispersive system, consisting essentially of a perfect cylindrically bent crystal in association with a point source, has been studied both for transmission and reflection. The Laue and Bragg conditions have been calculated together with the dispersive power. The treatment has been extended to include two crystals. In the single-crystal case and also for most versions of two-crystal combinations a point or line focus is required to obtain highly resolved spectra. A few versions of the two-crystal arrangement can be made to be focusing, thereby enabling the use of an extended radiation source.
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    Applied crystallography online 13 (1980), S. 364-367 
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    Notes: Zone-axis pattern maps for graphite have been obtained using multiple-micrograph montages. Maps have been constructed with bend-contour micrographs and with Kossel–Möllenstedt patterns, both obtained at 100 kV. The two kinds of map are compared.
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    Applied crystallography online 13 (1980), S. 385-390 
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    Notes: Analytical explicit expressions have been derived for multiple-scattering effects under assumptions which are usually fulfilled in small-angle scattering. The expressions are used to evaluate quantitatively distortions of scattering patterns caused by multiple scattering. The distortions are discussed in terms of changes of the forward-scattering cross section, the radius of the gyration, the integrated intensity, the Porod region and Porod constant. In addition, the analytical expressions have been used to calculate the scattering cross section of the sample from scattering patterns which are strongly affected by multiple scattering.
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    Applied crystallography online 13 (1980), S. 401-401 
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    Applied crystallography online 13 (1980), S. 403-409 
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    Notes: The phases of ammonium nitrate, NH4NO3, doped with small amounts of NiO or CuO have been studied by X-ray and neutron diffraction at various temperatures from 80 to 423 K. NH4NO3 with metal-oxide additives, which are believed to form complexes, exists as a solid solution. Phase IV is stable down to 140 K in the NiO-doped samples and 210 K in the CuO-doped sample. The thermal expansion of this phase is extremely anisotropic, with the b axis expanding rapidly with increasing temperature while the a axis contracts slightly. At higher temperatures the doped samples transform directly from phase IV to phase II, bypassing phase III, starting at about 328 K. The structure of phase II, which is disordered, was refined in the space group P4/mbm. The structure of phase III contains hydrogen-bonded chains parallel to the b axis.
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    Applied crystallography online 13 (1980), S. 433-437 
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    Notes: A white-X-ray (Bremsstrahlung) four-circle diffractometer with a solid-state detector [Si(Li)] has been constructed. The system is operated by a small computer with magnetic disk memories, and enables energy-dispersive intensity measurements to be carried out with the X-rays selected from white X-rays. The present paper describes (1) how the present system can be used for energy-dispersive diffractometry even with the white radiation from a standard X-ray tube, and (2) how this system allows the measurement of the integrated intensities with almost the same accuracy as in angle-dispersive diffractometry with characteristic X-rays.
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    Applied crystallography online 13 (1980), S. 425-432 
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    Notes: A unique low-temperature, full-circle goniometer for single-crystal X-ray diffraction has been constructed with the novel adaptation of a closed-cycle refrigerator in such a manner that the cold finger remains stationary during the φ and χ rotations. This has been made possible through the development of a flexible heat-transfer link between the top of the φ shaft and the cold station. The new design concept incorporates several important features: (1) the vacuum shroud and radiation shields also remain stationary so as to reduce the X-ray windows to narrow slots; (2) the sample crystal is enclosed in a black, nearly isothermal cavity (Hohlraum); and (3) the φ shaft is mechanically decoupled from the thermal contraction of the cold finger and other parts. The new system has considerable advantages over presently existing low-temperature adaptations for X-ray diffraction: (1) the goniometer operates between 300 and 18 K without expenditure of liquid cryogen and with minimal attendance; (2) the system is capable of operating for long periods of time with minimal risk of interruption of the cooling cycle; (3) absorption of X-rays by the windows is minimal and independent of the Bragg angle; and (4) data collection at any temperature down to its present lowest limit of 18 K is virtually as easy as with any conventional room-temperature diffractometer. At the time of writing the instrument has been in use for over 10000 h and has measured over 100000 X-ray reflections from single crystals.
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    Applied crystallography online 13 (1980), S. 459-461 
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    Notes: A RAdial Distribution MOdeling Routine (RADMOR) is available which calculates directly the radial distribution function that would be obtained from electron, X-ray and neutron diffraction for spherical regions of known bonding topology. Crystalline as well as model fragment topologies can be evaluated in modeling the atomic arrangement in amorphous materials.
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    Applied crystallography online 13 (1980), S. 462-462 
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    Applied crystallography online 13 (1980), S. 465-471 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The performances of various optical elements are presented for ideally monochromatic X-rays in terms of position–angle space, which is used in the phase-space method. The optical elements discussed are a synchrotron radiation source, flat perfect crystals (reflection and transmission geometries), mosaic crystals, curved crystals of reflection (Johansson, Johann and logarithmic spiral types) and transmission geometries, and an elliptic total reflection mirror. The angular widths of acceptance and emergence for diffraction are properly taken into account in both the flat and curved crystals. It is shown that focusing optical elements such as a curved crystal and an elliptical mirror should not be treated simply as lenses.
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    Applied crystallography online 13 (1980), S. 486-493 
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    Topics: Geosciences , Physics
    Notes: Both superlattice and fundamental reflections have been used in an X-ray determination of the vanadium-atom displacements induced by selected ordering of interstitial deuterium in a multi-domain crystal of V2D. The magnitudes of these displacements along X and Z directions of the pseudo-tetragonal (b.c.t.) unit cell are designated DAX and DAZ respectively, where Z is along the tetragonal c axis. Measurements of the lattice parameters yield DAX ∼0.03 and DAZ ∼0.17 Å. From intensity measurements, the best values for the displacements are DAX = 0.03±0.01 and DAZ = 0.15±0.02 Å, and the purely thermal Debye–Waller factor was found to be 2B = 0.025 Å2. The temperature dependences of both the long-range-order parameter, S, and the lattice parameters of the b.c.t. unit cell were also investigated. Both verify that the phase transition from β (monoclinic) to α (b.c.c.) is first order in character with no intervening ε (tetragonal) phase as in V2H. There is additional evidence that the disordering process includes random occupancy of both Z octahedral and random tetrahedral sites.
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    Applied crystallography online 13 (1980), S. 505-512 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: One lamellar phase, observed in the mitochondrial lipids–water system at low temperature (ca 253 K) and at low water content (ca 15%), contains four lipid monolayers in its unit cell, two of type α and two of type β. Previous X-ray scattering studies of this phase led to an ambiguity: the phase could contain either two homogeneous bilayers, one α and one β, or two mixed bilayers, each formed by an α and a β monolayer. A solution to this problem was sought in a neutron scattering study as a function of the D2O/H2O ratio. Because of limited resolution, straightforward analysis of the neutron scattering data leads also to ambiguous results. Using a more sophisticated analysis based upon the zeroth- and second-order moments of the Patterson peaks relevant to the exchangeable components, it is shown that the weight of the evidence is in favour of a structure containing mixed bilayers.
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    Applied crystallography online 13 (1980), S. 529-532 
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    Topics: Geosciences , Physics
    Notes: At the IA magnetic transition (12.5 K) in CeBi a theoretical study has suggested that either the Ce or Bi sublattices should move with respect to the other. Such a distortion would give rise to extra reflections of the form 0′0′l+½. A search for such a reflection using a Bond X-ray diffraction technique was conducted. The 007½ line was not found and an upper limit of 2̃ × 10−4 was set for the magnitude of the expected internal distortion. This limit is valid for a surface layer whose thickness equals the penetration depth of the X-radiation which is about 0.4 × 10−2 mm.
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    Acta crystallographica 33 (1977), S. 685-685 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 78
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    Notes: This report lists nomenclature recommendations resulting from a six-year study of problems common to the disciplines of mineralogy and crystallography. The recommendations include definitions of polytypism, topotaxy, syntaxy, and epitaxy, certain criteria for mineral names, preferred format for chemical formulae, and preferred symbols for crystallographic axes and repeat distances. Two recommended systems of structural symbols to be used to differentiate polytypes are presented.
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    Acta crystallographica 33 (1977), S. 685-686 
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    Acta crystallographica 33 (1977), S. 687-689 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A faulted 4H structure of CdI2 gives a diffraction pattern composed of sharp spots and diffuse streaks. The structure was solved by assuming that the sharp spots come from the 'average structure' and the diffuse streaks from one-dimensional disorder.
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    Acta crystallographica 33 (1977), S. 697-701 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is assumed that a crystal structure P1 is fixed and that the 31 non-negative numbers R1, R2, . . ., R6; R12, R13, . . ., R56; R123, R124, . . ., R156 are also specified. The random variables (vectors) h, k, 1, m, n, p are assumed to be uniformly and independently distributed in the regions of reciprocal space defined by and |Eh| = R1, |Ek| = R2, . . . , |Ep| = R6; (1) |Eh + k| = R12, |Eh + l| = R13, . . . , |En + p| = R56; (2) |Eh + k + l| = R123, |Eh + k + m| = R124, . . . , |Eh + n + p| = R156; (3) and h + k + l + m + n + p = 0. (4) Then the structure invariant φ = φh + φk + φl + φm + φn + φp, as a function of the primitive random variables h, k, l, m, n, p, is itself a random variable, and its conditional probability distribution, given (1), (2) and (3), is derived from the main result of the previous paper [equation (3.1); Fortier & Hauptman (1977). Acta Cryst. A33, 694-696.] and compared with the distribution when only (1) is given. The distribution leads to estimates for cos φ in terms of the 31 magnitudes (1), (2) and (3).
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    Acta crystallographica 33 (1977), S. 704-705 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: This paper points out an indirect but convincing experimental verification of certain theories concerning elastic-wave propagation in anisotropic media. Relations between the Poynting vector and the propagation vector for longitudinal wave propagation near, but not exactly parallel to, certain pure-mode axes permit the calculation of the ultrasonic diffraction from large single apertures (transducers) oriented for propagation exactly along the pure-mode axes. Then the diffraction loss versus distance, measured in oriented single crystals, permits the verification of the theories concerning these relations and concerning ultrasonic diffraction. It is shown that five items of theory have been verified by the ultrasonic diffraction measurements.
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    Acta crystallographica 33 (1977), S. 717-722 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The observed habits of crystals of the compounds α-Ca(NO3)2.2H2O and β-Ca(NO3)2.2H2O can be explained in terms of the crystal structure with the Hartman theory. A structural interpretation is given of the growth of α-phase crystals from a β-phase crystal with the [110] directions of the two phases parallel.
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    Acta crystallographica 33 (1977), S. 730-739 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The structure of a dislocation network in a crystal boundary depends, among other parameters, on the two crystal structures and their relation, i.e. the linear transformation leading from one crystal lattice to the other. For the same transformation, the structure of the network is related to the crystal structure. The link between the transformation and the many possible dislocation networks is described by a theorem which states that the displacement field of the linear transformation in the boundary can be described in an infinite number of ways by continuous dislocation distributions. The discrete dislocations are then obtained by grouping the continuous dislocations. The O-lattice theory is discussed in relation to these new aspects, particularly with respect to the features which tend to be conserved in the boundary. A special discussion is given of the case where a common crystallographic axis, without the relaxation pattern being periodic, represents the preferred state.
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    Acta crystallographica 33 (1977), S. 750-757 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Magic integers enable several unknown phases to be expressed in terms of a single variable at the expense of introducing some error in the phases represented. The economy of variables is particularly useful in a multisolution direct-methods program like MULTAN where a successful solution may depend upon the ability to use a large number of phases at the beginning of phase determination and the computing time is proportional to the number of variables. Formulae are presented which give the phase errors in the magic-integer representation. A recipe is given for the generation of efficient magic-integer sequences in which the r.m.s. error is spread evenly over all the phases represented. These sequences minimize the overall phase error for a given maximum integer in the sequence. It is found that the errors are minimized when the differences between adjacent members of the magic-integer sequence form the terms of a geometric progression and the smallest integer is greater than half the largest.
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    Acta crystallographica 33 (1977), S. 758-767 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The simulation of section topographs of distorted crystals may be performed with good accuracy; it is now possible to use it as a tool to determine the quantitative parameters of various defects. The simulation of traverse topographs is a more complicated problem: it needs a tremendous amount of computation time, but the main difficulty arises from the numerical method itself. Reasons and a demonstration, with simulations of the contrast of a dislocation, are given why the only way to compute the traverse topograph is by the addition of the intensities due to an incident spherical wave. Nevertheless, the accuracy of the result remains poor.
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    Acta crystallographica 33 (1977), S. 767-770 
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    Notes: Electrical measurements have been carried out on a 4N5 Cr single crystal in multi-Q and single-Q domains. It was found that the Néel temperature depends neither on the direction of current, nor on the existence of a magnetic field. In the multi-Q domains, the transverse magnetoresistance G changes according to G = AHn. The magnetoresistance decreases with increasing temperature showing an anomaly at 40°C. For the single-Q domain specimen, measurements with H ⊥ Q showed in the antiferromagnetic state the existence of saturation in the curve G = G(H). In the paramagnetic state, the transverse magnetoresistance is an increasing function of the magnetic field. Measurements with H || Q showed that G increases according to G = AHn, and this is ascribed to the existence of open orbits along the direction of polarization.
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    Acta crystallographica 33 (1977), S. 771-776 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The method of calculation of crystal-lattice normal-mode frequencies of hydrocarbons was sharpened to include bond foreshortening of C-H bonds, dynamic derivatives, and atomic point charges. A test of the method was made with 118 observed structural parameters from 18 aromatic and saturated hydrocarbon crystal structures, and 58 observed crystal frequencies from five aromatic hydrocarbon crystal structures. The use of dynamic derivatives significantly improved the fit to the observed frequencies. The use of atomic point charges also significantly improved the fit to the observed frequencies; the optimum values found for the point charges were essentially identical to the optimum values obtained from structural data alone. The direct parameter-fit method, although giving reasonable results for the structural parameters, was found not to transfer well to the calculation of lattice frequencies. The force-fit method gave significantly better results for the lattice frequencies. The final optimum (exp-6-1) nonbonded interatomic potential functions derived from the combined structural and vibrational data were very similar to the functions derived from the structural data alone.
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    Acta crystallographica 33 (1977), S. 776-783 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: NaF gives rise to very weak odd-index reflexions in X-ray diffraction. These reflexions have been studied to determine the best models for electron distribution in the NaF crystal. Data were obtained from a single crystal at room temperature for Ag Kα, Mo Kα, Cu Kα and Co Kα radiations. Least-squares analyses were made on the Ag Kα and Mo Kα data. Scattering factors for Hartree-Fock singly charged free ions allowed a good fit to the data, and there was a marginal improvement of the fit with similar scattering factors modified for effects of crystalline environment. The mean thermal parameter for the ions was {\bar B} = 0.905 ± 0.025 Å2. An attempt to account for the wavelength dependence of intensities of the strong reflexions, with current theories of extinction, was unsuccessful.
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    Acta crystallographica 33 (1977), S. 784-788 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A qualitative study was made by electron diffraction of the short-range-order diffuse scattering from disordered Au-Pd, α phase Au-Zn, and α phase Ag-Mg alloys. Twofold and fourfold splittings of diffuse maxima are observed at 100, 110 and equivalent positions on the diffraction patterns with the incident beam parallel to [001], in the composition ranges from about 28 to 62 at.% Pd for Au-Pd, from about 7 to 24 at.% Zn for Au-Zn and from about 11 to 28 at.% Mg for Ag-Mg systems. The separation of the split diffuse maxima increases monotonically with Pd, Zn or Mg content. The results can be interpreted with the Fermi-surface-imaging concept proposed by Krivoglaz. The variation with composition of the Fermi wave vector parallel to the [110] directions is discussed with the nearly-free-electron and the rigid-band models. The latter model is also applied to the experimental results for disordered Cu-Pd and Cu-Pt alloys reported by the present authors [Ohshima & Watanabe (1973). Acta Cryst. A29, 520-526].
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    Acta crystallographica 33 (1977), S. 789-800 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Neutron diffraction and γ-ray rocking-curve measurements were performed on plastically deformed single crystals of Cu in order to compare the mosaic distribution parameters obtained from least-squares structural refinement with those seen by the γ-diffractometer. Neutron data were collected at two wavelengths (λ = 0.538 and 0.741 Å) for one crystal with very homogeneous mosaic distribution, and for another crystal with less good but more typical mosaic structure data were collected for λ = 0.538 Å. Maximum sin θ/λ was 1.52 Å-1 and maximum extinction correction was less than 30%. For the latter crystal no comparison could be made between the two sets of observed distribution parameters because of the uneven mosaic distribution. The former crystal was found from the γ-ray measurements to be bent, but when this was taken into consideration in the Coppens-Hamilton [Acta Cryst. (1970), A26, 71-83] formalism for anisotropic extinction correction good agreement was found between mosaic parameters obtained by γ-ray and neutron measurements for directions not affected by the crystal curvature, whereas the structure refinement did not, as expected, reflect the complete crystal curvature when this effect was dominant.
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    Acta crystallographica 33 (1977), S. 813-818 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In a spectrometer with finite beam divergence the vertical size of the reflections will increase with the order of reflection. This effect will be more pronounced if the mosaic width of the sample is large, which is often the case with biological samples. When the size of the reflection is greater than the detector size, a correction factor has to be introduced to account for this loss of intensity. A method of calculating this correction factor for a given beam divergence and mosaic width has been developed.
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    Acta crystallographica 33 (1977), S. 800-804 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A method of structure-factor least-squares refinement of constrained groups linked by distance restraints has been developed for the refinement of macromolecular structures. Each constrained group can have any number of variable dihedral rotation parameters within the group in addition to the rigid-body translational, rotational and thermal parameters. The matrix of normal equations may be either full or sparse and provision is made for solution by matrix inversion or the conjugate-gradient iterative method. This procedure has been successfully used for 3 Å data and should be applicable even for lower-resolution data and especially for cases with a poorer data-per-atom ratio. The structure of yeast phenylalanine tRNA has been refined with this procedure from a starting crystallographic R value of 42% to a final R value of 25% with isotropic 'group" thermal parameters and 22% with isotropic atomic thermal parameters for 8207 independent reflections at 2.7 Å resolution. The proper stereochemistry of bond distances, angles and van der Waals contacts for the restrained atoms was maintained within reasonable limits throughout the refinement. Although originally developed for nucleic acids, this procedure is directly applicable to the refinement of protein structures. In addition, a combination of applying distance restraints between groups and least-squares fitting of these groups to target coordinates has been used purely as an idealization process for imposing proper stereochemistry on an approximate model.
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    Acta crystallographica 33 (1977), S. 819-822 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A new method is described to express the Madelung parameter of a structure with a non-cubic Bravais lattice as a function of a lattice parameter by means of a power series expansion. The method is derived for the case of a cubic lattice which is tetragonally distorted. The generalization to other crystal structures is shown. For illustration the method is applied to the rutile and cadmium iodide structures.
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    Acta crystallographica 33 (1977), S. 834-837 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is shown that two vanadium oxides of unknown composition, i.e. VOx [Tilley & Hyde, J. Phys. Chem. Solids, (1970), 31, 1613-1619] and phase Q [Colpaert, PhD Thesis, Rijksuniv. Gent (1972)] are identical with V4O9 [Théobald, Cabala & Bernard, C. R. Acad. Sci. Paris, SOr. C, (1969), 269, 1209-1212]. The unit cell of V4O9 is orthorhombic, with a = 8.235, b = 10.32, c = 16.47 Å. A model for the structure of V4O9 is proposed. It is based on an ordered superlattice of O vacancies in V2O5, and is in agreement with X-ray oscillation photographs. The model accounts for the low density of this material, its low temperature of formation, the typical appearance of twins and the topotaxic growth of V4O9 on V2O5.
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    Acta crystallographica 33 (1977), S. 838-844 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Several criteria suitable for taking a decision regarding the centrosymmetry of crystals by the study of the goodness-of-fit of the observed and expected cumulative, semi-cumulative and distribution functions of normalized structure amplitudes are suggested. Practical aspects of the various tests are discussed. From a study of the performance of these tests on four crystals a safe method of conducting these tests is suggested.
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    Acta crystallographica 33 (1977), S. 823-828 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A full sphere of data extending to (sin θ/λ) = 1.2 Å-1 and containing 1595 reflections was measured with small, natural, single crystals of graphite and Mo Kα radiation. With the structure assumed to be in space group P63/mmc, the valence-charge density of the two independent C atoms was represented by a finite multipole expansion containing products of single-exponential radial functions and tesseral harmonics. A full-matrix, least-squares refinement of the model charge was computed where populations of the multipole deformations, thermal parameters, scale, and extinction parameters were refined. The experimental data fit a model charge density containing highly directional build-ups of charge to 2 e Å-3 between nearest neighbors in the basal plane. No structure was evident in the charge density between the basal planes. The refinable parameter, α, in the exponential radial function was 3.16 ± 0.05 bohr-1. The charge density and associated parameters are consistent with strong covalent bonding of sp2 character in the basal plane of graphite and minimal interactions between the layers.
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    Acta crystallographica 33 (1977), S. 829-833 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is assumed that a crystal structure in P{\bar 1} is fixed and that the 15 non-negative numbers R1, R2, R3, R4, R5; R12, R13, R14, R15, R23, R24, R25, R34, R35, R45 are also specified. The random vector (h, k, l, m, n) is assumed to be uniformly distributed over the subset of the fivefold Cartesian product W × W × W × W × W of reciprocal space W defined by |Eh| = R1, |Ek| = R2, |El| = R3, |Em| = R4, |En| = R5; (1) |Eh + k| = R12, |Eh + l| = R13, |Eh + m| = R14, |Eh + n| = R15, |Ek + l| = R23 |Ek + m| = R24, |Ek + n| = R25, |El + m| = R34, |El + m| = R35, |Em + n| = R45; (2) and h + k + l + m + n = 0. (3) Then the structure invariant (φ = φh + φk + φl + φm + φn, (4) as a function of the primitive random variables h, k, l, m, n, is itself a random variable, and its conditional probability distribution, given (1) and (2), is derived. The distribution yields reliable estimates for large numbers of quintets φ in terms of the 15 magnitudes (1) and (2).
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    Acta crystallographica 33 (1977), S. 844-846 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The calculation of wave functions of scattered electrons by the multislice method of Cowley and Moodie with a finite number of beams is shown to lead to the solution of a finite, closed set of differential equations in the limit that the slice thickness approaches zero. The solution is normalized but differs from the exact wave function unless sufficient beams are included in the calculation. Hence, normalization is not sufficient to ensure that the computed wave function equals the exact wave function. The implications of this result for numerical work are discussed.
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    Acta crystallographica 33 (1977), S. 856-858 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A new expression for the conditional probability distribution of quintet structure invariants is given, which in exponential approximation reduces to the exponential expression of Hauptman & Fortier [Acta Cryst. (1977), A33, 575-580]. In a practical example the expression gave promising results.
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