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  • National Academy of Sciences  (3,012)
  • International Union of Crystallography (IUCr)  (2,646)
  • 1980-1984  (2,940)
  • 1975-1979  (2,718)
  • 1950-1954
  • 1984  (2,940)
  • 1977  (2,718)
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  • 1980-1984  (2,940)
  • 1975-1979  (2,718)
  • 1950-1954
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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
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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
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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 
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    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 
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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 
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  • 19
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 502-503 
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  • 20
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 503-503 
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  • 21
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 504-504 
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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
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  • 23
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 508-509 
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  • 24
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 509-510 
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  • 25
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 184-190 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    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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    Applied crystallography online 10 (1977), S. 200-200 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    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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  • 27
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    Applied crystallography online 10 (1977), S. 203-205 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    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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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 209-210 
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  • 29
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 211-211 
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  • 30
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    Applied crystallography online 10 (1977), S. 213-219 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    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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    Topics: Geosciences , Physics
    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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  • 32
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    Applied crystallography online 10 (1977), S. 256-261 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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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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  • 33
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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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    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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  • 40
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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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  • 41
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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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  • 42
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    Applied crystallography online 17 (1984), S. 249-256 
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    Topics: Geosciences , Physics
    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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  • 43
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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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  • 44
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    Applied crystallography online 17 (1984), S. 293-294 
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    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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  • 45
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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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  • 46
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    Applied crystallography online 17 (1984), S. 297-306 
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    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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  • 47
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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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  • 48
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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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  • 49
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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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    Applied crystallography online 17 (1984), S. 22-26 
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    Notes: Several procedures for Fourier analysis of single diffraction peaks for microstrains and mosaic sizes are compared. A simple new method works well, especially when the size distribution is broad, and/or when the strains vary in an unusual manner.
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    Applied crystallography online 17 (1984), S. 39-42 
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    Notes: A computer program for unambiguous and consistent indexing of a series of single-crystal diffraction patterns is discussed which allows determination of the unit triangle and therefore tilting of the specimen into any special orientation even if no Kikuchi patterns can be observed.
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    Applied crystallography online 17 (1984), S. 47-54 
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    Notes: For the application of Rietveld profile analysis to neutron powder diffraction data a precise knowledge of the peak profile, in both shape and position, is required. The method now in use employs a Gaussian-shaped profile with a semi-empirical asymmetry correction for low-angle peaks. The integrated intensity is taken to be proportional to the classical Lorentz factor calculated for the X-ray case. In this paper an exact expression is given for the peak profile based upon the geometrical dimensions of the diffractometer. It is shown that the asymmetry of observed peaks is well reproduced by this expression. The angular displacement of the experimental profile with respect to the nominal Bragg angle value is larger than expected. Values for the correction to the classical Lorentz factor for the integrated intensity are given. The exact peak profile expression has been incorporated into a Rietveld profile analysis refinement program.
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    Applied crystallography online 17 (1984), S. 77-84 
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    Notes: In the solvent contrast variation technique, the structure factor is a linear function of contrast, and the intensity is therefore a parabolic function of contrast. A method has been developed which scales together data measured at different contrasts on different samples. Once the data are scaled, structure-factor amplitudes or intensities for any other contrast can be obtained by inter- (or extra-)polation. The magnitude of the relative phase change between contrast, an important piece of phasing information in single-crystal studies, can be determined.
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    Applied crystallography online 17 (1984), S. 100-102 
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    Notes: These studies on phase transition and crystal structure in a single-crystal of CTFP by X-ray diffraction in the temperature range 88 to 293 K show that CTFP undergoes successive phase transitions at about 123, 143 and 178 K and that the existence of another phase transition at about 168 K is possible. In the lowest-temperature phase, various commensurate and incommensurate structures were found. The appearance of the structure depends on storage duration at 293 K after undergoing the successive phase transitions. The original crystal, just crystallized from acetone solution, showed commensurate structure of which the modulation wave number, δ, is 0.20. The samples stored for a week and a month were both incommensurate. However, the crystals stored for two months were commensurate (δ = 0.20). Therefore, the storage duration of two months is a relaxation time long enough to recover the genuine structure. In the other temperature phases below 178 K, the crystals were all incommensurate. Consequently, the complicated structural change of CTFP crystals during storage is now well understood as a thermal relaxation phenomenon.
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    Applied crystallography online 17 (1984), S. 120-121 
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    Applied crystallography online 17 (1984), S. 122-122 
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    Applied crystallography online 17 (1984), S. 131-133 
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    Notes: A procedure based on successive convolution operations is developed and associated with the Warren–Averbach method. Applications to cold-worked lithium fluoride samples are presented. The microstrain results are compared with those obtained in the classical way including the use of the direct deconvolution method. It is shown experimentally that the difficulties involved in the deconvolution operation have no significant effect on the evaluation of microstrains by the classical X-ray line-profile analysis.
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    Applied crystallography online 17 (1984), S. 134-139 
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    Notes: Equations are derived for the intensity ratio R of secondary to primary scattering in the fixed-sample transmission case with an incident-beam monochromator crystal that results in a polarization ratio Km for unpolarized incident radiation. A table allows R to be calculated for Km = 0. This table, along with the one for Km = 1 published previously for the case without a monochromator, allows R to be estimated before an experiment is done and thus an optimum experiment can be designed. A simple computer program allows direct calculation of R once experimental data are obtained if it is desired to avoid interpolation. Determination of R involves some approximation, but the values obtained are sufficiently accurate for most purposes. However, R can be improved using exact equations, if desired. If synchrotron radiation polarized normal to the plane of scattering of the monochromator is used, R does not depend on Km, and the value of R for Km = 0 applies despite the true value of Km. As is expected from symmetry considerations, R is independent of Km in the small-angle scattering limit.
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    Applied crystallography online 17 (1984), S. 159-166 
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    Notes: A new linear position-sensitive scintillation detector has been developed and installed on a neutron powder diffractometer. The physical properties of this detector are described; test powder diffractograms are presented. The special advantages of this detector for neutron powder diffractometry are discussed: high detection efficiency also in the region of short wavelengths (75% for λ = 1.3 Å), good spatial resolution (FWHM = 2.5 mm), and high data-point density (Δ2θ = 0.02°). Special emphasis is placed on peak-profile analysis for the powder diffraction patterns. Examples for position and intensity separation of overlapping reflections are presented.
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    Applied crystallography online 17 (1984), S. 167-171 
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    Notes: A 3 mm long double-polytype region was found in a ZnS needle. The region consists of a fine mixture of 47% of each of the polytypes 20L(5753) and 60R(5537)3 and about 6% of the polytype 60R(511553)3. The latter polytype is assumed to be the parent polytype transforming into the two others. It is the first time that traces of the parent polytype have been found in a double-polytype region. The formation mechanism of double-polytype regions is explained as a two-stage transformation process: expansion of grown-in stacking faults taking place at temperatures just below the hexagonal-to-cubic transition temperature, followed by a sudden formation and expansion of stacking faults at a lower temperature. It is suggested that a similar mechanism operates in the formation of single polytype regions and disordered ones.
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    Applied crystallography online 17 (1984), S. 189-195 
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    Notes: The optimum parameters and properties of a new type of neutron monochromator based on elastically bent silicon single crystals are theoretically evaluated. The use of a fully asymmetrical geometry permits the achievement of rocking-curve widths exceeding 20' in the incident beam. The associated effect of the reflected-beam widening is compensated for by the second crystal in the parallel (1,−1) setting in opposite geometry. Reflectivity calculations indicate that this double-crystal system might be compared with the best single-crystal mosaic monochromators. Besides the easy control of the effective mosaicity and the corresponding integrated reflectivity by variation of the bending radius, a further advantage of this system is its simultaneous action as a neutron filter improving the ratio of the thermal neutron signal to the background.
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    Applied crystallography online 17 (1984), S. 209-210 
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    Notes: A simple technique has been developed to lessen radiation damage of protein crystals grown from low-ionic strength solution. The technique consists of replacing the mother liquor with solutions containing 10–20% of polyethylene glycol 4000 or 20000.
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    Applied crystallography online 17 (1984), S. 210-211 
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    Notes: The powder data at 293 K of p-dichlorobenzene (pDCB), C6H4Cl2, p-dibromobenzene (pDBB), C6H4Br2, and of their mixed crystals [pDCB]x [pDBB](1−x) are reported. Their thermal stability at 293 K is given. They are all isomorphous, monoclinic, P21/a with Z = 2. a = 14.792 (6), b = 5.839 (2), c = 4.036 (2) Å, β = 112.52 (4)°, V = 322.0 (5) Å3, Dx = 1.516 Mg m−3 for pDCB; a = 15.487 (2), b = 5.836 (2), c = 4.108 (1) Å, β = 112.74 (1), V = 342.5 (3) Å3, Dx = 2.289 Mg m−3 for pDBB; a = 14.893 (10), b = 5.837 (7), c = 4.046 (3) Å, β = 112.48 (5)°, V = 325.1 (9) Å3, Dx = 1.592 Mg m−3 for [pDCB]0.90[pDBB]0.10; a = 14.942 (5), b = 5.840 (2), c = 4.048 (1) Å, β = 112.53 (3)°, V = 326.3 (4) Å3, Dx = 1.677 Mg m−3 for [pDCB]0.80 [pDBB]0.20; a = 15.018 (4), b = 5.846 (1), c = 4.061 (1) Å, β = 112.50 (2)°, V = 329.4 (3) Å3, Dx = 1.751 Mg m−3 for [pDCB]0.70 [pDBB]0.30; a = 15.138 (11), b = 5.843 (3), c = 4.067 (3) Å3, β = 112.57 (8)°, V = 332.2 (8) Å3, Dx = 1.825 Mg m−3 for [pDCB]0.60 [pDBB]0.40; a = 15.247 (6), b = 5.842 (2), c = 4.081 (2) Å, β = 112.66 (5)°, V = 335.4 (5) Å3, Dx = 1.896 Mg m−3 for [pDCB]0.50 [pDBB]0.50; a = 15.300 (5), b = 5.838 (1), c = 4.081 (1) Å, β = 112.54 (4)°, V = 336.7 (3) Å3, Dx = 1.976 Mg m−3 for [pDCB]0.40 [pDBB]0.60; a = 15.373 (4), b = 5.842 (1), c = 4.094 (1) Å, β = 112.64 (3), V = 339.3 (3) Å3, Dx = 2.048 Mg m−3 for [pDCB]0.30 [pDBB]0.70; a = 15.410 (7), b = 5.838 (4), c = 4.098 (2) Å, β = 112.65 (3)°, V = 340.2 (6) Å3, Dx = 2.129 Mg m−3 for [pDCB]0.20 [pDBB]0.80; and a = 15.450 (3), b = 5.838 (1), c = 4.103 (1) Å, β = 112.69 (1)°, V = 341.4 (2) Å3, Dx = 2.208 Mg m−3 for [pDCB]0.10 [pDBB]0.90. The JCPDS Diffraction File Nos. are: 34-1987 for pDCB; 34-1985 for pDBB; 34-1986 for [pDCB]0.50 [pDBB]0.50.
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    Applied crystallography online 17 (1984), S. 212-214 
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    Notes: A microcomputer-based method has been developed for rapid search/match analysis of X-ray diffraction patterns from compound mixtures with user-assembled data bases for specific sample types, e.g. air pollutants, asbestiform minerals, mineral sands and coal ash. Accidental coincidences between stored and measured patterns are largely eliminated using status parameters which are assigned to each data base and measured line. Scaling factors determined for each of the identified compounds afford calculation of relative concentration values. The list of unassigned lines is presented for subsequent analysis with other compact data bases or using more complex search/match methods.
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    Applied crystallography online 17 (1984), S. 217-218 
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    Applied crystallography online 17 (1984), S. 231-237 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    Notes: The potential of synchrotron radiation sources (SRS) for the topography of polytypic structures has been assessed. Thus it has been found that the white radiation and low divergence characteristics of the SRS are uniquely suitable for accommodating the complications of mixed polytypes, heavily distorted crystals and edge-reflection topography. Computer aids are particularly important for separating contributions from mixed polytype crystals. All these techniques are illustrated in this paper using silicon carbide as a prototype polytypic crystal.
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  • 71
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    Applied crystallography online 17 (1984), S. 244-248 
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    Notes: A single-stage computer procedure to calculate an electron density map suitable to detect errors in a tentative macromolecular model has been developed. In this procedure, an atom of the tentative model does not contribute to the phases used to calculate electron density values at or near its current position, that is within the region containing it and a neighborhood surrounding that region. In this way, the phases used to calculate electron density values within a region are not biased by the model atoms contained within that region or its neighborhood. The number of atoms which are omitted for a given region is maintained at a small fraction of the total structure so that the phases used to calculate electron density values may still be a good approximation to the phases of the complete structure. The procedure was used to improve the model of the Fab portion of the mouse galactan-binding immunoglobulin J539 (IgA2, κ), which contains 431 residues.
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  • 72
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    Applied crystallography online 17 (1984), S. 352-357 
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    Notes: A modification of the Rietveld refinement program is introduced by replacing the Gaussian profile function with a Voigtian. The performance of the program is tested by an application to simulated and measured diffraction patterns of NaTaO3, and excellent results are obtained when the tails of the reflections are included at distances 10 to 20 times the half-width of the reflections. Comparison between Gaussian and Voigtian refinements show large differences in the thermal parameters. This is due to the high level of background that is assumed in the Gaussian refinement to compensate for the lack of tail overlap. Application of the Voigtian refinement to the measured pattern of Ni yields a thermal parameter that is close to the literature values, while the Gaussian analysis gives a value which is 35% too large. The isotropic temperature factors of the room-temperature structure of NaTaO3, respectively, drop down to the values B(Ta) = 0.09(2), B(Na) = 1.21(3), B(O1) = 0.59(3) and B(O2) = 0.61(3) Å2, when the Voigtian analysis with a proper background is carried out. The role of background in powder pattern refinement is discussed, and it is suggested that a calculation of the average TDS should be included in the refinement.
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  • 73
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    Applied crystallography online 17 (1984), S. 361-362 
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    Notes: The segment description of the unique set of reflections of Gabe [In Crystallographic Computing, edited by F.R. Ahmed. Copenhagen: Munksgaard (1969)] and Le Page & Gabe [J. Appl. Cryst. (1979). 12, 464–466] has been extended to the non-centrosymmetric crystal classes. To the reflections generated by the unique set of the Laue group are added the anti-reflections generated by a set of anti-segments. These anti-segments are defined by the same lattice rows as those of the corresponding Laue group but their origins are shifted to avoid generating anti-reflections in any centrosymmetric section of reciprocal space.
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    Applied crystallography online 17 (1984), S. 362-362 
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    Notes: In the paper by Kervarec, Baudet, Caulet, Auvrey, Emery & Regreny [J. Appl. Cryst. (1984). 17, 196–205], an error has been introduced. On page 202, right-hand column, the second paragraph should read: The uneven surfaces of the interfaces can be taken into account by assuming that the SL is made of a juxtaposition of perfect crystallites of the same composition x whose period varies between the extreme values of 2n1d1 + 2n2d2. In this hypothesis, the experimental diagram is the sum of the X-ray diagrams given by each crystallite; the value of n1 + n2 deduced from such a diagram is an average value in the sample zone analyzed, therefore most of the time not integer.
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    Applied crystallography online 17 (1984), S. 366-368 
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    Notes: The well known molecular drawing program, PLUTO78, has been made more widely available in the UK by mounting it on the minicomputers of the Joint Academic Network (JANET) wide-area network. Enhancements include interaction, faster plotting, use of colour and simulated half tone, easy access to centrally maintained high-quality-output devices, and the provision of comprehensive documentation in the form of on-line help and a user manual.
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    Applied crystallography online 17 (1984), S. 369-369 
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    Applied crystallography online 17 (1984), S. 371-371 
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    Applied crystallography online 17 (1984), S. 372-373 
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    Applied crystallography online 17 (1984), S. 373-374 
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    Applied crystallography online 17 (1984), S. 375-384 
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    Notes: The calculated intensities of the X-ray beam behind the exit surface of the Ge [440, {\bar 2}20] dispersive monolithic monochromator for Co Kα1 radiation are presented. The reflections 440 and {\bar 2}20 resulting from Ge {000, 440, 260, {\bar 2}20} coplanar four-beam diffraction were taken into account and the influence of the π/2 reflection 260 is discussed. The four-surface arrangement of Ge [440, {\bar 2}20] providing the resultant beam parallel to the incident beam is considered. For comparison Ge [440, {\bar 2}20] in the two-beam approximation is solved. The obtained results demonstrate that the four-beam basis of diffraction affects the resultant beam. The quality of the X-ray beam evaluated behind the examined monochromators also provided the conditions for the use of such monochromators as wavelength standards.
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    Applied crystallography online 17 (1984), S. 13-17 
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    Notes: It is shown by a diffraction technique that 2-naphthalenethiol (C10H8S) undergoes a first-order phase transition which occurs at 313 ± 0.5 K. The low-temperature form III is ordered whereas the high-temperature one I is disordered and may be kept as metastable far below 313 K. Thermal expansion studies, performed on the stable form III and on both metastable and stable forms I, indicate that: form III exhibits a pronounced `pretransitional' effect; discontinuities of the lattice constants occur at the III →I transition; the lattice-constant thermal-expansion curves of form I do not show any inflexion at the metastable to stable transition; in every case a large thermal expansion anisotropy is observed. Curiously, the compacity of the ordered low-temperature form is lower than that of the disordered high-temperature form. The JCPDS Diffraction File No. of the low-temperature form III is 34–1990.
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    Applied crystallography online 17 (1984), S. 33-38 
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    Notes: The energy-dispersive X-ray diffraction method has been used to measure integrated Bragg intensities from TaDx single crystals as a function of D concentration and scattering vector. The observed attenuation can be described in terms of a thermal and a static Debye–Waller factor (DWF). From the static DWF, for small scattering vectors, the displacements u1 = 0.095 (5) Å of the four Ta atoms closest to the D atom are obtained and, for large scattering vectors, the change of the mean-square thermal displacement of these atoms Δv21 = −[5.5 (3)] × 10–4 Å2.
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    Applied crystallography online 17 (1984), S. 42-46 
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    Notes: A new direct-methods computer program, MITHRIL, is described. Written in a neutral subset of Fortran IV, it is built around a heavily modified MULTAN80 system. It incorporates many recent theoretical developments in direct methods including the use of quartet and quintet invariants, a new method for estimating triplets, YZARC and MAGEX, and random-phase tangent refinement. It can be run as a menu-driven interactive real-time package or in the more conventional batch mode. Several levels of user-program interaction are provided.
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  • 84
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    Applied crystallography online 17 (1984), S. 61-76 
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    Notes: X-ray diffraction experiments on amorphous Fe40Ni40P20 were performed in association with a detailed study of the accuracy, limitations and reproducibility of experimental radial distribution functions (RDF's). A novel counting strategy is presented, and the merits of conducting two diffraction experiments in different geometries as a test for internal consistency are argued. Experimental methods and data-processing techniques are treated in detail. The results indicate that the RDF can be determined virtually with pre-selected accuracy. The positions of the co-ordination shells for Fe40Ni40P20 show a remarkable periodicity.
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  • 85
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    Applied crystallography online 17 (1984), S. 85-95 
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    Notes: The application of synchrotron X-ray radiation to powder diffraction is described. A perfect Si double-crystal monochromator at the Cornell High Energy Synchrotron Source (CHESS) was used in conjunction with Si, Ge, LiF and Al2O3 analyzers to investigate resolution and intensity characteristics at selected wavelengths between 1.07 and 1.54 Å. The results obtained with Ge at 1.54 Å gave a resolution Δd/d of 5 × 10−4 at 2θ = 30°, falling to 2 × 10−4 at 2θ = 140°. Analysis of the peak shapes is described in detail, with particular emphasis on the asymmetry observed at angles below 50° due to axial divergence effects. With a simple correction to allow for these, the peak shapes are found to be well represented by a convolution of Gaussian and Lorentzian components, the respective peak widths of these being related to the intrinsic resolution and sample broadening effects. It is pointed out that the use of a crystal analyzer should eliminate shifts in the Bragg-peak positions owing to the displacement-type aberrations which occur with conventional focusing-type diffractometers. Except for a constant zero error, the mean discrepancy in observed and calculated peak positions between 0 and 90° for reference samples CeO2, Al2O3 and NiO is found to be only about 0.003°. Finally, some general remarks are made about the application of the Rietveld profile technique to structural analysis from synchrotron powder diffraction data.
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    Applied crystallography online 17 (1984), S. 103-110 
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    Notes: In diffusion-grown layers of compounds with the orthorhombic FeB- and ZrSi2-type structures a pronounced texture is present which is rotationally symmetric with respect to the direction of diffusion. In FeB-type structures a [101]-fibre texture is observed for ZrSi and HfSi and a [100]-fan texture for TiSi and ZrGe. In ZrSi2-type compounds the b axis is always perpendicular to the direction of diffusion; different compounds, however, show different textures: a [010]-fan texture for ZrSi2, a [100]-fibre texture for HfSi2 and a 〈101〉-fibre texture for ZrGe2. The textures are explained in terms of preferential diffusion along specific crystallographic directions which can be derived from the crystal structures. At high temperatures a second texture component has been observed in HfSi layers and in layers with the ZrSi2 structure. This second component can be considered as a recrystallization texture and can be derived from the original texture by a rotation of the crystallites of 20–25° around a simple crystallographic direction. The existence of a so-called TiGe phase with the FeB structure in the temperature region 1070–1270 K is questionable. Only Ti6Ge5 has been produced and this compound showed a [001]-fibre texture.
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    Applied crystallography online 17 (1984), S. 120-120 
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    Applied crystallography online 17 (1984), S. 122-122 
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    Applied crystallography online 17 (1984), S. 122-122 
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    Applied crystallography online 17 (1984), S. 123-124 
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    Applied crystallography online 17 (1984), S. 140-146 
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    Notes: One of the main sources of error when studying weakly absorbing materials using an X-ray diffractometer with Bragg–Brentano focusing geometry is specimen transparency. Its undesired effects (peak displacement and loss of intensity in the medium angular range) can be avoided by using a slab-like specimen in symmetrical transmission, in which case the type of diffractometer is more or less immaterial except that a non-focusing diffractometer offers a series of advantages, e.g. a very simple lining-up procedure and the possibility of using rod-like specimens. Such a diffractometer with a flat monochromator in the primary beam is described and both the equatorial aberration and the axial aberration are discussed.
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    Applied crystallography online 17 (1984), S. 147-153 
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    Notes: Hexagonal crystals of cowpea mosaic virus in space group P6122 (or its enantiomorph) with a = 451 and c = 1038 Å have been analyzed using the synchrotron X-ray source at LURE. Data were collected using an X-ray beam of size 0.18 × 0.22 mm permitting resolution of reflections along the c* direction. The beam was defined using a multiple slit system at both the monochromator and at the collimator. A helium path was used to reduce air scatter as a specimen-to-film distance of 175 mm was employed. Data to 4.3 Å resolution were recorded on each film. The films were processed to 6 Å resolution using the oscillation photograph processing package developed by Rossmann [J. Appl. Cryst. (1979), 12, 225–238] with minor modifications to the normal procedures. One film was processed to 4.3 Å resolution establishing the feasibility of intermediate resolution analysis. Statistical analysis of the processed data showed excellent correlation between symmetry-equivalent reflections. Post refinement of oscillation angles and lattice constants was very stable and led to significantly improved scaling of partially recorded reflections.
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    Applied crystallography online 17 (1984), S. 172-178 
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    Notes: Structural disorder in severely ground MgCl2 is interpreted on the basis of a Bernoullian model with four parameters, specifying both the size and the statistical sequence of the layers; their choice completely dictates the intrinsic shape of all the X-ray diffraction lines. The X-ray powder spectra are reasonably well reproduced over a wide range of Bragg distances (1̃.7 〈 d 〈 9̃ Å) for samples with various degrees of disorder. Practical criteria are given to derive the statistical parameters from the X-ray spectra. Average crystal thickness as small as 20 Å across the layers is observed. From the usual structure with cubic packing in the Cl atoms, with increasing disorder at first the probabilities of cubic and hexagonal-like interlayer shifts (Pcub and Phex) tend to become equal; the ultimate degree of disorder also involves ± 60° rotations between adjacent layers, with about the same probability as Pcub and Phex.
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    Applied crystallography online 17 (1984), S. 206-207 
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    Notes: The usual powder diffraction technique needs about one hour for the registration of a pattern with either a Guinier camera or a powder diffractometer. This is a long time compared to the time of most chemical reactions and phase transitions. Quite recently the recording time has been reduced by using position-sensitive detectors and rotating-anode X-ray sources, but the registration time is still about one minute. With a Guinier diffractometer in front of a synchrotron-radiation source a time resolution of 300 μs can be achieved.
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    Applied crystallography online 17 (1984), S. 196-205 
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    Notes: The study of the physical properties of the superlattices (Ga 1− xAlxAs)n1(GaAs)n2/GaAs(001) requires the knowledge of their structural parameters n1, n2 and x and of their crystalline state. X-ray diffraction enables the existence of such artificial superstructures, obtained by molecular-beam epitaxy, to be revealed and their characteristic parameters to be determined. Two experimental techniques are used: a standard powder goniometer and a double-crystal diffractometer. The diffraction diagrams directly yield the superperiod and the average Al concentration in the superlattice. The value of x is determined by refinement between observed and calculated structure factors. The results are even more accurate when the number of observed satellite peaks for a given periodicity is greater; this number depends at the same time on the Al composition x, the n1/n2 ratio, the periodicity and its dispersion and the shape of the interfaces. This method is illustrated by a few examples with superlattice periods varying from 51 to 397 Å. The consequence on the X-ray diagrams of various defects (dispersion in n1 and n2, superperiod gradient, Al diffusion) is discussed.
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    Applied crystallography online 17 (1984), S. 211-211 
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    Notes: The powder data for 1,3,5-trichlorobenzene (1,3,5-TCB) and 1,3,5-tribromobenze (1,3,5-TBB) at 293 K are reported. These compounds are isomorphous, orthorhombic, P212121, Z = 4 with a = 13.971 (3), b = 13.251 (2), c = 3.921 (1) Å, V = 725.9 (4) Å3, Dx = 1.660 Mg m−3 for 1,3,5-TCB and a = 14.244 (3), b = 13.577 (2), c = 4.084 (1) Å, V = 789.8 (5) Å3, Dx = 2.648 Mg m−3 for 1,3,5-TBB. These values are more accurate than those previously reported [Milledge & Pant (1960). Acta Cryst. 13, 285–290]. The JCPDS Diffraction File Nos. are 34-1989 for 1,3,5-TCB and 34-1988 for 1,3,5-TBB.
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    Applied crystallography online 17 (1984), S. 215-215 
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    Notes: The ROCKS crystallographic computing system is a general complete integrated and easy-to-use system of programs for macromolecular structure determination. However, it has heretofore been available only on IBM 360 or 370 main-frame computers. This note reports the successful adaptation of the ROCKS system to the DEC VAX series of superminicomputers.
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    Applied crystallography online 17 (1984), S. 218-218 
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    Applied crystallography online 17 (1984), S. 219-225 
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    Notes: The present paper describes a newly built four-circle diffractometer for X-ray structure analysis with synchrotron radiation and particularly with the use of anomalous scattering due to a relatively heavy atom or atoms. This system is installed on beamline 10A in the Photon Factory (PF) in the National Laboratory for High Energy Physics, and its 2θ arm is rotated in the plane perpendicular to the floor. In this system, a one-crystal monochromator of the horizontal dispersion type is used. When the 111 reflection from a Si monochromator is used, the energy resolution is about 14 eV for X-rays of about 10 keV. The four-circle diffractometer, a monochromator and an alignment carriage for the four-circle diffractometer can be controlled by a computer and, therefore, once the optical system has been adjusted for X-rays of a certain wavelength then the diffraction intensity values from a single-crystal sample can be automatically measured at any wavelength chosen. An NaI(Tl) scintillation counter is used for both the intensity measurements of the diffraction beam and the monitoring of the incident-beam intensity. In order to make the counting rate high, 50 MHz counters are used for the above two intensity measurements. In this set-up, about 1% counting loss occurs at a counting rate of 55000 s−1. With this diffractometer, diffraction intensities have been measured for a hemimorphite single-crystal at two energy values on both sides of the Zn K absorption edge. The variation of the intensities, after being normalized by the standard reflection and the monitor counts, was somewhat large in comparison with the data measured in the usual laboratory. From these data measured at two energy values, a so-called two-wavelength anomalous-scattering difference Patterson function was calculated, where the Zn–Zn vector peaks were twice as high as in an ordinary Patterson.
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  • 100
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    Acta crystallographica 40 (1984), S. 722-725 
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    Topics: Chemistry and Pharmacology , Geosciences , Physics
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