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  • Oxford University Press  (16,836)
  • Cambridge University Press  (9,518)
  • International Union of Crystallography (IUCr)
  • 1980-1984  (32,317)
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  • 101
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 39-47 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The extended structure of the Pt L3 X-ray absorption edge for some platinum compounds was measured and Fourier inverted. It is shown that the way in which the contribution of the isolated atom is subtracted from the absorption spectrum may influence the shape of the first peak in the Fourier transform and possibly lead to erroneous structural interpretations concerning the first-neighbour shell of the absorbing atom. These errors are the more important the smaller the separation between the absorbing atom and its nearest neighbours. An improved method of subtraction of the isolated-atom absorption is proposed and checked.
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  • 102
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 13 (1980), S. 605-607 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    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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  • 103
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 13 (1980), S. 611-611 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    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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  • 104
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 251-254 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Changes in the microporosity of several types of Brazilian high-ash coals have been analyzed by small-angle X-ray scattering as a function of thermal treatment. Pore size, determined using Guinier plots, decreased following thermal treatment of the samples. The modification of porosity induced by thermal treatment was similar for high-ash and low-ash coals, from which it may be inferred that mineral content is not a determining factor in the process of pore formation. Internal surface area for the samples was calculated with SAXS data. A correlation is established between the internal surface area values obtained and the carbon content and heat-treatment temperatures of the samples.
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  • 105
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 279-281 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Crystal data for the series of piperazinium bis(n-alkanoates) of the type 2[CxH2x − 1O2−] [C4H12N22+] for x = 1 to 16 are presented. Most of these compounds have triclinic unit cells whose dimensions, obtained from single-crystal work, have been used to index their powder diagrams. The a ({\bar a} = 5.718 Å) and b ({\bar b} = 7.481 Å) cell dimensions remain nearly constant while the c dimension and the volume of the unit cell increase linearly with x, the number of carbon atoms in the alkanoate anion. Presumably the aliphatic sequence in the alkanoate anion adopts the fully extended conformation and lies in the bc plane and nearly along the c direction.
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  • 106
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 226-229 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A numerical procedure using orthogonal polynomials is used for extracting the background in measured spectra. An X-ray spectrum obtained with the energy-dispersive method is considered as an example.
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  • 107
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 461-462 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Lattice-constant thermal expansions, Δa/a and Δc/c, have been measured on α-HgI2 single crystals from 293 K up to the red-to-yellow transition temperature. The curves Δa/a(T) and Δc/c(T) are linear. The thermal expansion coefficients are determined: α[100] = (1.0 ± 0.1) × 10−5 K−1 and α[001] = (4.6 ± 0.1) × 10−5 K−1. No evidence of a red-to-orange phase transition could be found.
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  • 108
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 452-460 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Total neutron scattering data were collected on sputtered YFe2 at 298 K and TbFe2 at 423 K with a wavelength of 0.7 Å. The TbFe2 data were collected above the magnetic ordering temperature of 383 K. In addition, the elastic neutron scattering of TbFe2 was measured with the use of a pyrolytic graphite analyzer at a wavelength of 1.5 Å, and its total X-ray scattering was measured with Mo radiation and a silicon-lithium drifted detector. Experimental radial distribution functions, with statistical error limits, were calculated. Errors due to an incorrect background, scaling of the data and termination effects were minimized. The scale and shape of the experimental background and the coordination numbers, internuclear distances and disorder parameters, for the first six coordination spheres, have been determined. The contribution of paramagnetic inelastic scattering from TbFe2 to the total neutron scattering is quite appreciable. The shape of the background scattering, which goes through a maximum, is indicative of residual coherence and suggests short-range magnetic ordering where neighboring atom spins are aligned. These effects are not observed in YFe2, nor in the elastic TbFe2 data. The metallic glasses have a structural topology which is quite different from that found in their crystalline analogues. The transition-metal substructure, consisting of corner-sharing tetrahedra, is the only aspect of the crystalline topology preserved in the amorphous phase. The structural parameters suggest a tendency of the rare-earth atoms to cluster, thereby decreasing the number of Fe nearest neighbors relative to the crystalline structure.
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  • 109
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 55-59 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: By using data from the Wallace & Ward [J. Appl. Cryst. (1975), 8, 255–260] cylindrical texture camera integrated with other traditional X-ray powder film techniques for very low and very high diffraction angles, a new high-temperature modification of CeO2 was identified and indexed resulting in a trigonal or hexagonal unit cell with a = 8.36(2) and c = 10.42(2) Å, (axial ratio 1.264) and Z = 16. The new phase is an oxidation product of CeO2-doped hot-pressed silicon nitride. It can be quenched to room temperature under appropriate conditions. Its stability on reheating is strictly related to its interaction with the silicate phase.
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  • 110
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 60-64 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Multiple electron scattering between weak beams has been used as the basis for a simple method to determine the absolute polarity of some non-centrosymmetric crystals. For crystals with the sphalerite structure many orientations have been found in which small departures from centrosymmetry produce large effects on the convergent-beam diffraction patterns (microdiffraction). The effects are reasonably independent of thickness and so can be analyzed qualitatively without the use of computers.
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  • 111
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 570-571 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A method is proposed for unambiguously indexing X-ray multiple diffraction patterns obtained with irregularly shaped crystals on a four-circle diffractometer. The method is derived from the orientation matrix together with the operation of crystal rotation. It is shown that this method facilitates the procedures in the experimental method for phase determination.
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  • 112
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 574-576 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Unit-cell constants for synthetic fluoroalleghanyite compounds have been determined by the refinement of X-ray powder diffraction data. Manganese norbergite: Pbnm, a = 4.862(2), b = 10.797(3), c = 9.188(2) Å, U = 482.4(4) Å3, Z = 4; alleghanyite: P21/b, a = 4.871(2), b = 10.818(6), c = 8.206(5) Å, α = 108.58(5)°, U = 414.1(7) Å3, Z = 2; manganhumite: Pbnm, a = 4.888 (2), b = 10.712(2), c = 21.749(6), U = 1138.8(5) Å3, Z = 4; sonolite: P21/b, a = 4.889(1), b = 10.668(3), c = 14.239(5), α = 100.83(5)°, U = 731.9(5) Å3, Z = 2. The JCPDS Powder Diffraction File numbers are: for manganese norbergerite 33–1500; for alleghanyite 33–1499; for manganhumite 33–1498; for sonolite 33–1497.
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  • 113
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 577-577 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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  • 114
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 577-578 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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  • 115
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 579-579 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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  • 116
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 579-580 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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  • 117
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 116-119 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Three members of a new family of phases, Li3Mg2XO6: X = Nb, Ta, Sb, have been prepared by solid-state reaction of MgO and Li3XO4. Unit-cell parameters of Li3Mg2TaO6 determined by electron diffraction and refined by X-ray powder diffraction are a = 8.883(2), b = 5.802(2), c = 17.437(6) Å, with space group Fddd; z = 8. Powder diffraction data of Li3Mg2NbO6 and Li3Mg2SbO6 were indexed by analogy with those of Li3Mg2TaO6 and unit-cell dimensions obtained by least-squares refinement: Li3Mg2NbO6 a = 8.552(2), b = 5.897(1), c = 17.721(5) Å; Li3Mg2SbO6 a = 8.614 (1), b = 5.908(1), c = 17.759(5) Å.
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  • 118
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 590-593 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The structure of the transition phase M′ in AlZnMg alloys has been studied by Buerger X-ray precession photographs and transmission electron microscopy. The M′ phase, also called η′ or R, has a hexagonal or pseudo-hexagonal cell with a = 4.96, c = 14.03 Å. It is confirmed that M′ precipitates are hexagonal platelets with the following epitaxy: (00.1)M′//(111)Al and |10.0|M′//1{\bar 1}0|Al. It is shown that none of the known models of the unit cell accounts for the observed scattered intensities.
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  • 119
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 602-604 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: An improved `substitution method' for the separation of X-ray diffraction α1−α2 doublets is based on the possibility of refining both the proportionality constant between the α1 and α2 contributions to the line intensity and the doublet separation expressed in terms of the diffraction angle or of a related variable in the reciprocal space. Optimization of these two parameters, with a proper evaluation of the error, clearly shows the statistical nature of the oscillations appearing on the high-angle side of the pure α1 component after correction; their elimination by polynomial smoothing can therefore be performed and a procedure is suggested for achieving this result. A computer program, based on these principles, has been written and tested in many practical cases.
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  • 120
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 611-614 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The precipitation behavior in the Ti–27at.%Nb–6at.%Ta–6at.%Zr alloy aged at 643 K was examined by means of time-of-flight neutron small-angle scattering. When the alloy was aged, fine-scale α precipitates appeared. During the precipitation process, each precipitate grew in size, but the total number of precipitates remained nearly constant up to 1 Ms of ageing. The increase of critical superconducting current density could be connected with increasing size of α precipitates. The specimen for the measurements consisted of a composite containing 3721 filaments in a copper matrix. The use of neutrons was shown to give great advantages for the investigation of precipitation phenomena.
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  • 121
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    Applied crystallography online 15 (1982), S. 632-632 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The wrong original was used for Fig. 6(b) of Gastaldi, Jourdan, Marzo, Allasia & Jullien [J. Appl. Cryst. (1982), 15, 391–395]. The correct version of Fig. 6(b) is given. In Fig. 6(c) the growth time increases from right to left.
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  • 122
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    Applied crystallography online 15 (1982), S. 635-636 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The high-temperature paraelectric phase of dicalcium lead propionate, DCLP, at 363 ± 5 K is tetragonal, with a = 12.574 (6), c = 17.403 (9) Å, V = 2751.4 Å3, Z = 4 and corresponds to the space group P41212 (or P43212). The thermal expansion curve shows the transition somewhere between 328 and 343 K.
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  • 123
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    Applied crystallography online 15 (1982), S. 638-638 
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  • 124
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    Applied crystallography online 15 (1982), S. 638-639 
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  • 125
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 15 (1982), S. 639-639 
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  • 126
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    Applied crystallography online 15 (1982), S. 640-676 
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  • 127
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    Applied crystallography online 15 (1982), S. 182-189 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The method of determining the thickness of a diffuse phase boundary with density profile governed by equilibrium conditions is proposed. It follows the well known procedure of analyzing the deviations from Porod's law. Errors in the obtained boundary thickness, owing to the statistical scatter in the intensity data and to the difficulty of separating the effect of density fluctuations within the phases, are examined. For this purpose, scattering curves are synthesized on the basis of a well defined model structure with known boundary thicknesses. These synthesized curves, when analyzed according to the proposed method, yield the correct boundary thickness under favorable conditions, but are also shown to lead to very erroneous results in some cases.
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  • 128
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    Applied crystallography online 15 (1982), S. 211-215 
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    Notes: Monoclinic Gd2SiO5 has been investigated by high-resolution electron microscopy (HREM) at 100 kV. Structure images are observed in the [100] and [001] projections and calculations of the image contrast using the multislice approximation are carried out to interpret the observations. For thin samples the image fit is improved by Fourier filtering of the observed images. For thicker samples the fit is found to be very sensitive to small tilt deviations of the zone axis to the electron beam. For defects observed in a fast-cooled specimen, a model in which Gd atoms replace Si atoms in segments along the (100) plane is proposed and tested by contrast calculations. These defects may be one of the reasons for deviations from stoichiometry as frequently observed in rare-earth silicates.
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  • 129
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    Applied crystallography online 15 (1982), S. 241-244 
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    Notes: A low-temperature camera for single-crystal X-ray investigation between 28 and 350 K has been developed for a modified Weissenberg goniometer. Cooling is provided by a double-stage cryorefrigerator which is firmly attached to the low-temperature camera. Stranded copper cables ensure a flexible heat transfer between crystal and cold finger allowing a crystal rotation of 180°. Compared to the usual Weissenberg technique almost no further limitation is given on the accessible angular range of the scattered radiation.
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  • 130
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    Applied crystallography online 15 (1982), S. 248-248 
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    Notes: The X-ray powder data have been obtained for a new phase MoGa5(CuS2)0.0556. The powder patterns were indexed on the basis of a tetragonal body-centred unit cell with the lattice constants a = 12.872, c = 5.284 Å, c/a = 0.410, dm = 6.825(8), dx = 6.869 Mg m−3, Z = 8.
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  • 131
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    Applied crystallography online 15 (1982), S. 249-249 
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  • 132
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    Applied crystallography online 15 (1982), S. 250-250 
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  • 133
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    Applied crystallography online 15 (1982), S. 255-259 
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    Notes: The identification of twofold axes is straightforward if the cell is based on three of the shortest lattice translations. The distribution of twofold axes in space fixes the lattice symmetry and most conventional cell edges. A program based on this approach has been written. It works for the seven cases with minimum branching of the algorithm.
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  • 134
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    Applied crystallography online 15 (1982), S. 266-274 
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    Notes: The commonest natural diamonds (classification type Ia) contain platelet defects on {100}, usually with mean diameters in the 20 to 100 nm range, which generate disorder diffuse X-ray reflections in the form of (100) spikes through reciprocal-lattice points. Corresponding spikes in electron diffraction patterns of diamonds, recorded with 100 and 120 kV electrons, have been investigated. It has been established that (100) spikes appear most strongly in electron diffraction patterns of diamonds very rich in platelets, and are absent in the case of platelet-free diamonds (types IIa and IIb). Spikes from diamonds containing relatively large platelets are sharper than those from diamonds containing very small platelets. Multiple scattering of electrons prevents measurement of the dependence of spike intensity upon diffraction order as can be done in the X-ray case. Some experiments with spike electron reflections have been performed that are impossible in the X-ray case. It has been shown that when the crystal volume sampled by the electron beam contains only one platelet, then the spike distribution reduces to a pattern containing only spikes normal to the platelet concerned. Also, dark-field weak-beam microscopy has been applied, by selecting one spike only in the diffraction pattern, to identify orientations of small platelets through the imaging of only those platelets normal to the spike selected.
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  • 135
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    Applied crystallography online 15 (1982), S. 301-307 
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    Notes: The amplitude and phase of a wave scattered by an adsorbed layer can be deduced from the modifications of the Bragg peak of the powder by adsorption. The treatment of experimental data involves the interpretation of a function of the extra intensity detected and has the advantages of an integrated intensity method. Powders containing crystallites limited by several types of faces can be studied. The Co3O4 sample studied is limited only by (110) and (111) faces of respective areas 11 and 27 m2 g−1. The (111) planes terminating crystallites contain oxygen covering a layer of cobalt in octahedral sites. Other possible terminating layers are ruled out for (111) planes. The structures of the bidimensional layers of argon on (110) and (111) planes are determined.
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    Applied crystallography online 15 (1982), S. 315-322 
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    Notes: A correction for the unavoidable truncation of line profiles and an improved correction for the inherent curvature of the variance–range function are discussed. An analytical procedure for making the corrections is proposed and applied to theoretical and practical small-crystallite line profiles. An alternative procedure for obtaining the integrated intensity, and hence the integral breadth, and the variance (Fourier) apparent size is also presented.
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  • 137
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    Applied crystallography online 15 (1982), S. 289-300 
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    Notes: As a consequence of several technological developments, the speed and sensitivity of solution X-ray (and neutron) scattering studies have recently risen by factors which may be as large as 105; formal problems – like assessing the information content of the data and retrieving that information in structural terms – have thus become of immediate practical interest. The general problem of the information content of scattering experiments is discussed; its mathematical expression is derived, which depends on both the experimental data (observed values and estimated accuracy) and the a priori stochastic assumptions on the structure of the sample. The practical application of these notions to solution scattering studies involves several steps, three of which – choice of the degrees of freedom, data reduction and error analysis – are dealt with in this work. The first step is to specify the minimal number of independent parameters necessary and sufficient to describe the whole of the scattering properties of the system. Whenever the solute particles are of finite dimensions the entire scattering curve is defined by its values at a one-dimensional lattice; if, moreover, the asymptotic trend of the scattering curves is known, then the degrees of freedom are the ideal intensities at a finite number of points plus a small number of parameters describing the asymptotic trend. It is also possible to include among the degrees of freedom a few subsidiary parameters like the normalization factors. The next step is, starting from a composite set of data, to determine the most probable numerical value of each degree of freedom and to evaluate its range of uncertainty. This is discussed within the framework of variable-contrast studies, assuming that the invariant-volume hypothesis is fulfilled. An algorithm is formulated which treats all the experimental observations and determines simultaneously all the degrees of freedom and the error matrix. The algorithm also allows one to introduce additional linear constraints on the degrees of freedom. As an example, the algorithm is applied to solution X-ray scattering data recorded with a low-density serum lipoprotein. The determination of the maximal chord of the particle – an important parameter in the informational analysis – turns out to be rather tricky.
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  • 138
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    Applied crystallography online 15 (1982), S. 330-337 
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    Notes: The design and operation of a focusing camera for high-resolution macromolecular crystallography with synchrotron radiation (SR) are described. The performance of this service-oriented instrument is evaluated on the basis of five years of use. Standard procedures for data collection, data processing and data reduction have been modified to take unusual features of the SR source into account; the effect of polarization is thoroughly discussed.
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  • 139
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    Applied crystallography online 15 (1982), S. 338-351 
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    Notes: Expressions are developed for reflecting range from various models, and the practical application of these in terms of reflection prediction and calculation of partiality is discussed here for conventional sources. Synchrotron sources utilizing focusing monochromator systems, where the wavelength of the radiation incident at the sample is correlated with the angular direction of that radiation, are dealt with elsewhere. Even for conventional sources spectral dispersion is shown to be an important factor, particularly in the case of high-resolution data. The various published methods are discussed and expressions are derived from first principles showing that all are inherently equivalent, differing only when approximations are used, and revealing a missing factor of two in the treatments of Rossmann [J. Appl. Cryst. (1979), 12, 225–238] and Rossmann, Leslie, Abdel-Meguid & Tsukihara [J. Appl. Cryst. (1979), 12, 570–581]. Particular emphasis is placed on the method which uses a spherical reciprocal-lattice volume element whose dimensions are designed to reproduce the expected or observed reflecting ranges, showing that for all practical purposes the effects of beam cross-fire, mosaic spread and spectral dispersion can be adequately simulated by such a volume. The equations for reflecting range are of particular interest in the electronic stationary-picture method and in the use of electronic area detectors.
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  • 140
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    Applied crystallography online 15 (1982), S. 361-374 
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    Notes: In the Rietveld method of analysing powder diffraction data, the crystal structure is refined by fitting the entire profile of the diffraction pattern to a calculated profile. There is no intermediate step of extracting structure factors. The method was applied first to diffraction patterns recorded with neutrons at a fixed wavelength. It has now been used successfully for the treatment of results from the four categories of experimental technique, with neutrons or X-rays as the primary radiation and with the scattered intensity measured at a fixed wavelength or at a fixed scattering angle. In this article we discuss the application of the Rietveld method to each of these techniques.
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  • 141
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    Applied crystallography online 16 (1983), S. 251-256 
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    Notes: Integrated X-ray Bragg intensities from Nb single crystals have been measured as a function of H concentration and scattering vector along high-symmetry directions. The observed attenuation can be described in terms of a static Debye–Waller factor (DWF) due to the static displacements of the Nb lattice atoms around the dissolved H. The crystals were loaded with H in situ on the X-ray diffractometer. The static DWF was determined from the ratio of the intensities from a crystal with and without H. Corrections are necessary for several magnitudes which change with the lattice-parameter change and the change of the thermal displacements upon the uptake of H. The measured static DWF is proportional to the H concentration c(H/Nb) for c≤0.30 and the square of the scattering vector K2 for small K, as expected from theory. From analysis of the static DWF the static displacements of the Nb atoms closest to the H impurity u1 = 0.1 Å were determined.
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  • 142
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    Applied crystallography online 16 (1983), S. 276-276 
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    Notes: Crystal data for the Schiff bases 2-(4-methyl-2-pyridinyliminomethyl)phenol (salampy) and 2-(4,6-dimethyl-2-pyridyliminomethyl)phenol (saladimpy) are reported. Salampy: C13H12N2O, Mr = 212.0, P21/c, a = 11.772(4), b = 7.5386(7), c = 15.154(5) Å, β = 124.89 (2)°, V = 1103.1(4) Å3, Dm = 1.24, Dx = 1.28 g cm−3, Z = 4; saladimpy: C14H14N2O, Mr = 226.0, P212121, a = 21.585(5), b = 8.8285(8), c = 6.2952(6) Å, V = 1199.6(3) Å3 Dm = 1.22, Dx = 1.25 g cm−3, Z = 4. The JCPDS Diffraction File No. for C13H12N2O is 33-1999, for C14H14N2O 33-1998.
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  • 143
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    Applied crystallography online 16 (1983), S. 277-277 
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    Notes: The air-sensitive title hydride [Hf(C9H13)2H2]2 was prepared by hydrogenolysis of (t-BuCp)2HfMe2 [Couturier, Tainturier & Gautheron (1980). J. Organomet. Chem. 195, 291–306] and crystallized from a solution in heptane or toluene and hexane. The compound is triclinic with the following cell parameters: a = 7.22(3), b = 10.31(4), c = 12.47(6) Å, α = 80.4(4), β = 74.6(4)°, γ = 72.1(3)°, V = 847 Å3, Z = 1, Dx = 1.67 Mg m− 3. Powder diffraction data were obtained from Debye–Scherrer photographs. The extended version of the paper examines several examples where Zr or Hf hydrides are monomers or dimers and shows that this structure is very similar to the related [(MeCp)2ZrH2]2 compound [Jones & Petersen (1981). Inorg. Chem. 20, 2889–2894]. This similarity and spectroscopic data show that the molecule probably has a center of symmetry situated between the two Hf atoms and the two bridging H atoms. The JCPDS Diffraction File No. for [Hf(C9H13)2H2]2 is 33-1996.
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    Applied crystallography online 16 (1983), S. 284-284 
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    Applied crystallography online 16 (1983), S. 295-303 
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    Notes: The integrated X-ray reflectivity of elastically bent, but otherwise perfect, crystals are calculated for Laue-type reflections covering the entire curvature range. Anomalous transmission and elastic anisotropy are taken into account, and it is shown that both these effects affect appreciably the reflected intensities for both weakly and strongly bent crystals. The validity of the reflectivity-versus-curvature relation thus obtained is experimentally confirmed for a number of reflections from silicon crystals. The implications of the experimental results to brittle fracture are discussed.
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    Applied crystallography online 16 (1983), S. 317-324 
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    Notes: A sequence of steps is given for making use of the information available on convergent-beam electron diffraction patterns from high-symmetry zone axes for crystal point- and space-group determination.
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    Applied crystallography online 16 (1983), S. 354-356 
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    Notes: By use of a high-temperature camera designed for in situ synchrotron-radiation X-ray topography the α−β transition was investigated in a Z-cut synthetic-quartz sample. Preliminary dynamical observations on movement of the boundaries between α and β phases and α1 and α2 domains are reported.
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  • 148
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    Applied crystallography online 16 (1983), S. 357-357 
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    Notes: The crystal structure of α-HgO2 was determined using X-ray powder diffraction. The crystal is C-centered monoclinic, C2/m, a = 4.470(2), b = 5.459(2), c = 3.519(2) Å, β = 108.45°, Z = 2, Dx = 9.48 g cm− 3. The variable coordinates of oxygen atoms from the peroxide dumbbell were determined as x = 0.476, z = 0.280. The final R is 0.047. The JCPDS Diffraction File No. for α-HgO2 is 33-1489.
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    Applied crystallography online 16 (1983), S. 358-358 
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    Notes: PROMETHEUS is a Fortran IV program system for investigation of anharmonic thermal vibrations in crystals, using elastic X-ray or neutron data. Three different anharmonic temperature-factor equations can be used, one of them up to sixth-order terms. The corresponding probability densities and mean effective atomic potentials can be calculated. Programs for correction of isotropic thermal diffuse scattering and of isotropic and anisotropic extinction are included. Fast averaging of symmetrically equivalent reflections combined with extended reflection statistics is possible. Calculations of Fourier syntheses and their errors and of Patterson functions can be carried out and the corresponding contour maps can be plotted.
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    Applied crystallography online 15 (1982), S. 540-541 
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    Notes: Second-harmonic generator tests for non-centrosymmetry have been applied to five monoclinic potassium feldspars. Four samples yielded null results; one specimen displayed a positive test for non-centrosymmetry.
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    Applied crystallography online 15 (1982), S. 564-566 
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    Notes: Clays prepared by sedimentation from aqueous suspension were shown to contain aggregates much larger than individual clay particles. Small-angle X-ray scattering data were obtained with an automated Bonse–Hart instrument. The distance distribution functions for the clay aggregates were obtained using the Glatter indirect Fourier transform method. The distance distribution function extended to over 200000 Å, and the average radius of gyration was over 50000 Å in the most extreme cases.
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    Applied crystallography online 15 (1982), S. 567-569 
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    Notes: The intensity of the small-angle X-ray and neutron scattering from a polydisperse system of randomly oriented independently scattering particles is shown to be proportional to h−α for all values of the scattering vector h when the distribution of particle dimensions is proportional to r−(2d + 1 − α), where h = 4πλ−1 sin(θ/2);θ is the scattering angle; λ is the wavelength; r is the maximum dimension of a particle; and d is the number of dimensions of the particles. The value of α lies in the interval 0 〈 α 〈 ω, where ω = 4, 2, and 1 for d = 3, 2, and 1 respectively. This relationship between the scattered intensity and the particle-dimension distribution does not depend on the shape of the particles in the polydisperse system, provided that the particle-shape distribution is independent of the distribution of particle dimensions.
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    Applied crystallography online 15 (1982), S. 571-573 
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    Notes: A simple method is described for increasing the lifetimes for several crystals of proteins and viruses during collection of high-resolution X-ray diffraction data on film. Crystals are mounted in a capillary filled with mother liquor, with cotton lint fibers nested against them to prevent movement. Some results and limitations of the method are discussed.
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    Applied crystallography online 16 (1983), S. 508-511 
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    Notes: The finite height of the detector slit of a powder diffractometer has three observable effects: (1) the peak is shifted by an amount that is proportional to −cot 2θ; (2) the peak is broadened by an amount proportional to cot22θ; (3) an asymmetry is introduced into the peak shape that depends on cot32θ. If the underlying instrumental resolution function has a Gaussian shape, the slit height effects can be approximated, over a rather broad range of shapes for the vertical resolution function, by an Edgeworth series making use of one adjustable parameter. This peak shape function is compared with experimental observations and with other shape functions that have been proposed. A well characterized peak shape is at least as important as resolution when the data are to be used for structure refinement by the Rietveld technique, and diffractometers should be designed so as to keep the slit height within the range that leads to shapes that can be described mathematically. Diffractometers with multiple detectors can usefully have different vertical divergences for detectors that cover different angular ranges.
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    Applied crystallography online 15 (1982), S. 578-578 
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    Applied crystallography online 15 (1982), S. 579-579 
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    Applied crystallography online 15 (1982), S. 580-580 
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    Applied crystallography online 15 (1982), S. 594-601 
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    Notes: Dislocation-free silicon crystals of ({\bar 2 \bar 1 \bar 1}) orientation with a hyperbolic notch, subjected to tensile deformation at 1073 K, were used as model material for the analysis of the induced plastic zone. The results obtained by X-ray topography and X-ray rocking-curve measurements were compared to theoretical calculations and predictions based on continuum mechanics. Good agreement between experiment and theory was obtained regarding the shape of the plastic zone, the contribution of the active slip systems to the size of the plastic zone and the direction of the maximum plastic strain trajectory in the zone. Discrepancies between experiment and theory regarding the symmetry relation of the plastic zone lobes and the dislocation density near the notch tip were attributed to the interactions and resulting work-hardening. These aspects were not taken into account in calculations of continuum mechanics.
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    Applied crystallography online 15 (1982), S. 605-610 
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    Notes: In two previous papers, algorithms designed for searching, matching and identifying phases in qualitative X-ray powder analysis were reported [Schreiner, Surdukowski & Jenkins (1982). J. Appl. Cryst. 15, 513–523, 524–530]. This paper extends the searching technique to unknowns containing isotypical phases and solid solutions. Methods are described for distinguishing among isotypical compounds in qualitative analyses, and for permitting the retrieval of phases in a data base that differ from measured patterns by a uniform Δd/d scale factor.
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    Applied crystallography online 15 (1982), S. 615-620 
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    Notes: Neutron powder diffractometers operating with large vertical counter acceptance angles produce diffraction patterns in which the peaks are shifted and broadened, and may become distinctly asymmetric. It is shown that a sum of Gaussians generally provides a good approximation to these peaks. The sum of Gaussians has been included as an optional description of the profile within an existing profile refinement program. In this application, the sum incorporates a single asymmetry parameter which can be estimated from the diffractometer geometry or determined in the refinement. It is recommended that, in describing asymmetric peaks, the sum of Gaussians be used in place of the simple Gaussian multiplied by Rietveld's semi-empirical asymmetry factor.
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    Applied crystallography online 15 (1982), S. 621-625 
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    Notes: Two distinct X-ray diffraction studies of 2̃ mol dm−3 ZnSO4 solutions were carried out in order to assess the possibility of studying the state of the sulfate ion in water and to verify whether and how structural information depends on possible differences in experimental data. It is shown that the X-ray diffraction method is unable to provide results capable of unambiguous modelling of the SO2−4 coordination; even the complete omission of sulfate-water interactions does not lead to unsatisfactory synthetic structure functions. This conclusion does not seem to be affected by differences in the experimental data; however, some characteristics of the `best' models can be affected by differences in experimental data that are within the limits of experimental uncertainties.
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  • 162
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    Notes: A coordinate X-ray diffractometer based on a flat two-coordinate multiwire proportional chamber 350 × 350 × 10 mm and information readout from cathode printed-circuit delay lines is described. The diffractometer is used to study macromolecular single crystals. The chamber is placed on a two-circle goniometer at distances of 300, 500 and 750 mm from the crystal. The diffractometer operates on-line with an SM-2 computer. A diffraction picture is collected in a 64 K 16-bit word computer core memory with the maximum count rate of 250000 events s−1. The detection quantum efficiency of Cu Kα radiation is about 70%. The number of spatial resolution elements of a diffraction picture is ̃50000, the resolution time ̃0.5 μs. The general case of inclined geometry is used. The diffraction picture is collected during quasi-continuous scanning. Control of the diffractometer, data collection and pre-computing X,Z are performed at the same time. The diffractometer makes it possible to study single crystals having unit cells up to 250 Å at a resolution of 3.5 Å and to 84 Å at a resolution of 1.5 Å.
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    Applied crystallography online 15 (1982), S. 632-635 
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    Notes: X-ray powder data have been recorded for the 32 complexes: CuL2X2 [L = 2-(2-pyridyl)acetamide (aapH), N-(2-pyridyl)benzamide (bapH), N,N′-diacetyl-2,6-diaminopyridine (daapH), N,N′-dibenzoyl-2,6-diaminopyridine (dbapH); X = Cl, Br]; ML2X2(M = Ni, Co; L = aapH, bapH, daapH, dbapH; X = Cl, Br, I). These data indicate that, in all cases: (a) complexes ML2X2(M = Ni, Co; X = Cl, Br, I) are isomorphous; (b) complexes CuL2X2(X = Cl, Br) are isomorphous; (c) the Cu complexes are not isomorphous with the Ni and Co analogues (a difference ascribed to the Jahn–Teller effect); and (d) the three points (a)–(c) above are consistent with the available spectral and magnetic data.
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    Applied crystallography online 15 (1982), S. 636-637 
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    Notes: To allow a convenient three-dimensional visualization of protein features and particularly of binding sites van der Waals contour levels are produced on large-scale stereopairs (about 0.3 to 1.0 cm Å-1). A chain of Fortran programs has been written and give screen or paper graphic outputs. The large-scale stereopairs can be easily viewed with only two ordinary mirrors.
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    Applied crystallography online 15 (1982), S. 639-639 
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    Applied crystallography online 16 (1983), S. 11-13 
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    Notes: A procedure for collecting protein data by a simulated step-scan method is described for a CAD4 diffractometer. The method allows one to collect 100–150 reflections per hour without any significant loss in accuracy, thus reducing the time spent on each reflection by a factor of 2–3 compared with the ω–2θ scan technique. A comparison between data collected for the protein phospholipase C by the present method and by the ω–2θ technique gives a reliability factor of 2%. Prior to this work the homogeneity of the X-ray beam from a graphite monochromator was determined, and the intensity cross section of the monochromatized X-ray beam showed that the total X-ray illumination of the crystal is practically unchanged regardless of the orientation of the crystal in the primary beam.
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    Applied crystallography online 17 (1984), S. 238-243 
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    Notes: Selective application of only parameter shifts with large eigenvalues permits singular or near-singular least-squares problems to be solved. This `eigenvalue filtering' process has been applied to the refinement of the unit-cell, crystal-orientation and reflecting-range parameters needed to process oscillation films. Eigenvalue filtering permits automatic identification and determination of just those combinations of parameters that are most relevant for processing films from a particular crystal in a particular setting. The procedure may be carried out with alignment information from an individual data film or from multiple alignment films taken at different spindle angles. Eigenvalue filtering has been incorporated in a refinement program that minimizes discrepancies between observed and calculated fractions recorded for partially recorded reflections. This permits the reflecting range (combined mosaic spread, beam cross-fire, and wavelength spread) to be refined along with the unit-cell and orientation parameters. Observed fractions recorded may be obtained by visual estimation prior to film scanning, or the program may be used in a `post-refinement' mode with data obtained from actual intensity measurements. The relatively unreliable information represented by the positions of spots on the film is not used except for indexing. The program handles crystals of any symmetry in any setting and the oscillation spindle may be inclined from normal beam geometry. The mis-setting angles are defined in a crystal-fixed coordinate system, making it easy to compare refinement results for data taken at different spindle angles.
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  • 168
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    Applied crystallography online 17 (1984), S. 249-256 
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    Notes: The instrumental optimization conditions for most small-angle scattering experiments in which the data are azimuthally symmetric require that the scattered flight path be equal to the incident flight path. This is in contrast to a recent analysis which shows that under some conditions the incident and scattered flight paths are in a ratio of two to one. The equal flight-path condition is also valid for experiments measuring sharp (Bragg-like) peaks, or where the intensity is required at specific scattering vectors, as in low-angle diffraction of ordered or semiordered systems. The implications of the optimization conditions on the resolution and count rates at the detector are discussed for both types of experiment, and the dependence of the resolution on the spectrometer geometry is considered.
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  • 169
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    Applied crystallography online 17 (1984), S. 257-264 
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    Notes: With a compact arrangement using single-crystal collimator and analyzer (silicon 220 reflection) of channel-cut type, angle-resolved scattering (ARS) curves of X-rays (Cu Kα1) for optically flat mirrors have been measured at various glancing angles, ω, of X-rays to the mirror surface with an angular resolution of 4′′. Weak scattered intensity (10−3–10−5 of the specularly reflected beam) is observed over an angular range of a few hundred arc s below and above the direction of specular reflection. When ω is close to the critical angle for total reflection, the scattered intensity at the low-angle tail of ARS curves is higher than that at the high-angle tail. This asymmetric tail profile of ARS curves is explained by simply superposing intensities of specularly reflected beams from surface elements inclined to each other.
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  • 170
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    Applied crystallography online 17 (1984), S. 293-294 
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    Topics: Geosciences , Physics
    Notes: A recent comparison of data for a specific experiment taken on a time-of-flight small-angle scattering spectrometer on a pulsed neutron source and data for the same kind of experiment taken at a steady-state reactor suggests that the instruments have similar performances, and in particular comparable count rates at the detector. This similarity disappears when resolution is taken into account. The criterion for a valid comparison should be the detector count rate for instruments measuring over similar ranges of scattering vector and with similar resolutions, as required by the particular experiment.
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  • 171
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    Applied crystallography online 17 (1984), S. 273-285 
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    Notes: The purpose of this paper is to establish an explicit correlation between the values and probable deviations of the observed intensities and the potential accuracy of the structure eventually determined using those intensities. This problem involves several steps: the choice in real space of a reference stochastic model defining the ideal state of ignorance; the determination in reciprocal space of the probability density corresponding to that model and of the probability law corresponding to the experiment; the determination of the information; the transfer of the information from reciprocal to real space; the interpretation of the information in terms of structural resolution. A rigorous treatment of this problem is hindered by the inadequate knowledge of the statistical correlations between different reflections; more realistically, it is convenient to address a simpler problem, corresponding to an ideal experiment in which the intensities relevant to each reflection are measured independently of all the other reflections. In this case the mathematical problems can easily be solved and a parameter introduced – called projection information – whose value can be determined. This information is the sum of two terms, one associated with the modulus, the other with the phase of each reflection. By resorting to a mathematical model it is possible to make use of the projection information to determine the value of a parameter akin to structure resolution. The final result of this work is an operational definition of resolution, based upon the stochastic properties of the experimental observations. This resolution can be used to assess and compare the intrinsic quality of different structure analyses and of different stages of one structure determination, before the structure is solved.
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  • 172
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    Applied crystallography online 17 (1984), S. 297-306 
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    Topics: Geosciences , Physics
    Notes: Crystal structure parameters have been obtained for α-Al2O3, β-PbO2 and (Mg, Fe)2SiO4 by Rietveld analysis of Cu Kα X-ray powder diffraction data collected on a conventional diffractometer using counting times ranging from 0.01 to 5 s per step. For all but the 0.01 s data collected on (Mg, Fe)2SiO4 the structural parameters obtained at different counting times are statistically identical at the 3 σ level, and the spread in the values is essentially the same as that obtained by sample repacking at a fixed counting time of 1 s per step. The parameter e.s.d.'s and conventional agreement indices Rwp and RB decrease to values limited by residual model errors as the counting time increases, but the goodness-of-fit parameter becomes unacceptably larger than its ideal value of unity. When more than a few thousand counts are accumulated for the maximum step intensity in the diffraction profile a weighting scheme based solely on counting variance is inappropriate and the parameter e.s.d.'s are no longer a reflection of their accuracy.
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  • 173
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    Applied crystallography online 17 (1984), S. 307-314 
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    Notes: The many-beam dynamical theory of moiré fringes of Gevers [Phys. Status Solidi (1963). 3, 2289–2297] has been applied to the parallel moiré case and the results have been verified on epitaxically grown Sn/SnTe specimens showing the sensitivity of moiré patterns to the presence of systematic reflections. The Fourier analysis of the calculated intensity profiles showed that each additional diffracted beam gives rise to a new term in the Fourier series so that the profile can be sinusoidal only in the two-beam approximation. The same result has been obtained recently by Pardo, Pariset & Renard [Phys. Status Solidi A. (1981). 64, 283–295], who applied a slightly different theoretical approach to Sb/Bi specimens. It is shown that the corrections to scattering factors for temperature effects are not important since they do not affect the general character of the moiré profiles. Some experiments were performed showing the dependence of the moiré pattern on the number of beams that are transmitted through the objective aperture. The effect of the gradual thinning of one layer on the moiré pattern could be reproduced by computer simulation.
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  • 174
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    Applied crystallography online 17 (1984), S. 331-333 
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    Notes: A combination of X-ray powder diffraction and single-crystal Laue photography was used to determine the unit-cell parameters for letovicite. μr = 247.25. The unit cell is triclinic, P1 or P{\bar 1}, and has dimensions a0 = 5.87(1), b0 = 10.17(3), c0 = 8.27(1) Å, α = 101.1(4), β = 111.1(1), γ = 89.9(2)°, V = 450.7 Å3, Z = 2, Dx = 1.82 Mg m−3. The M(20) cell reliability is 23.8. Many weak powder spectra were observed, which preclude the possibility of monoclinic cell symmetry previously reported. Projection of the letovicite Laue data along [001] reveals the striking pseudo-hexagonal symmetry observed optically. The JCPDS Diffraction File No. for ammonium hydrogen sulfate is 35-1500.
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    Applied crystallography online 17 (1984), S. 364-366 
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    Notes: A program is presented that solves crystal structures completely using advanced Fourier methods starting from a small known fraction (about 8%) of the structure. The program uses a statistical test based on the R2 factor to check the correctness of the model obtained so far. The results obtained with four test structures are given.
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    Applied crystallography online 17 (1984), S. 369-370 
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    Applied crystallography online 17 (1984), S. 370-370 
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    Applied crystallography online 17 (1984), S. 372-372 
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    Applied crystallography online 17 (1984), S. 373-373 
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  • 180
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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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  • 186
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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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  • 190
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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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    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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    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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    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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  • 194
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    Applied crystallography online 13 (1980), S. 529-532 
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    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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  • 195
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    Applied crystallography online 16 (1983), S. 577-578 
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    Notes: A simple Fortran computer program XRTDAT is described. This program generates data useful in the planning, execution and interpretation of an X-ray topographic experiment.
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    Applied crystallography online 16 (1983), S. 580-580 
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    Applied crystallography online 16 (1983), S. 581-589 
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    Notes: All information about the scattering sample obtainable from small-angle scattering data is shown to be contained in the discrete measured points of a scattering curve distorted by arbitrary collimation conditions, provided that the interval Δh between adjacent measured points fulfils the condition Δh ≤ π/L, where L is the largest correlation distance in the sample; h = 4πλ−1sinθ; λ is the scattered wavelength; and 2θ is the scattering angle. A simple technique has been developed for separation of part of the noise. It is shown that if the total time for measuring a scattering curve is held constant, a scattering curve recorded with Δh equal to the sampling-point interval π/L gives results like those obtained for a scattering curve recorded with a measured point distance smaller than that sampling-point interval. Therefore, Δh should be chosen to be small enough to guarantee that Δh ≤ π/L. Furthermore, a technique has been developed to calculate missing data points from the measured intensities. The condition Δh ≤ π/L has been found to be important for this calculation.
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    Applied crystallography online 16 (1983), S. 611-622 
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    Notes: The refinement of crystal structures using X-ray powder data in a two-stage method is described. (1) The integrated intensities of the individual reflections are derived by a profile fitting method in which the profile shapes are accurately defined using an experimentally determined instrument function and the sum of Lorentzian curves. (2) These values are then used in a powder least-squares refinement for structure determination. The results obtained with three simple structures (silicon, quartz and corundum) gave R(Bragg) values of 0.7 to 2.5%. The necessity of correcting for preferred orientation and the importance of proper specimen preparation are also discussed.
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    Applied crystallography online 16 (1983), S. 623-628 
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    Notes: Temperature dependences of PbHPO4 single-crystal lattice parameters were measured by the ratio method in the range 158–421 K. Below the ferroelectric phase transition (Tc = 310 K) lattice parameter b increases non-linearly with decreasing temperature while all others decrease linearly. The ratio method was generalized to the monoclinic crystal system for this purpose.
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    Applied crystallography online 16 (1983), S. 641-644 
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    Notes: X-ray spectra measured with a germanium detector can be corrected for escape. A numerical procedure to perform the correction has been implemented. The method employs a point by point subtraction and allows for energy-dependent detector resolution.
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