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  • Wiley  (20,272)
  • Geological Society of America (GSA)
  • International Union of Crystallography (IUCr)
  • 1980-1984  (22,487)
  • 1984  (11,924)
  • 1981  (10,563)
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  • 1980-1984  (22,487)
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  • 1
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 3-7 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: There has recently been an increased amount of interest in laboratory equipment for extended X-ray absorption fine-structure (EXAFS) measurements. In an earlier paper a facility for performing such measurements easily in a few hours was described in some detail. In this paper, further development of the techniques is reported. The use of high-power rotating-anode X-ray generator, the design of an X-ray curved-crystal spectrometer, the factors governing the proper choice of monochromator crystals and the design of fast-counting detectors and electronics are discussed. In addition, it is shown how errors caused by characteristic lines in the spectrum can be eliminated by use of a flux stabilization circuit.
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  • 2
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 24-27 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Photographic neutron intensity measurements on a 2 mm3 single-crystal of triclinic lysozyme are compared with conventional neutron diffractometer data. The reflections were recorded with the oscillation technique. The structure factors were derived from the optical densities scanned by an automatic microdensitometer and processed by computer programs. A statistical analysis shows that the photographic data are of about the same accuracy as recently collected diffractometer data. For the same flux and crystal volume the data collection time is reduced by up to two orders of magnitude.
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  • 3
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 43-50 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A device is described which uses liquid nitrogen to generate a cold or hot gas stream which is blown directly onto the crystal mounted on an X-ray diffractometer. With a specially constructed high-vacuum (〈 1.3 × 10–5 Pa) jacketed silica Dewar tube, it can operate between 83 and 1120 K with a stability over long time periods of ± ¼ K below and ± 1 K above ambient temperature. In the latter case the short-time stability is ± ¼ K for hours. It gives an inert atmosphere at elevated temperatures and allows extensive data collection for the accurate determination of crystal structures. The evaporator, the automatic refilling system and the silica Dewar tube are described.
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  • 4
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 60-61 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A fitting method is proposed for X-ray diffraction profiles which requires only a small number of parameters.
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  • 5
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 61-62 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Complete solid solution is observed amongst the compounds of the type M1MgCr2(VO4)3 (where M1 = K, Rb or Tl). The solid-solution series obtained show linear changes in peak positions and peak intensities in their X-ray diffraction patterns. The crystallographic behaviour with respect to the value of x for (RbxK1−xMgCr2(VO4)3 and (TlxK1−x)MgCr2(VO4)3 solid solutions is discussed.
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  • 6
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 64-65 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Indexed powder patterns of the monoclinic phase, stable at room temperature, and of the orthorhombic phase, stable for T 〉 786 K, are given. The cell dimensions are: a = 15.737 (8), b = 9.231 (5), c = 18.224 (9) Å, β = 125.46 (2)° at 295 K; a = 9.330 (3), b = 12.868 (5), c = 9.242 (3) Å at 818.6 K. Linear equations describing the thermal expansion of the two phases of ferric molybdate are reported. To verify if ferric molybdate can incorporate excess MoO3, measurements on non-stoichiometric samples were also made: no evidence of the presence of excess MoO3 was found.
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  • 7
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 68-68 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: High-temperature As2O5 at 768 ± 5 K is tetragonal with a = 8.577 (1), c = 4.637 (1) Å, V = 341.2 Å3, Z = 4, Dc = 4.475 Mg m−3. Indexed powder data are given.
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  • 8
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 85-93 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A flat, relatively thin (9 mm) xenon-filled multiwire proportional counter with two-dimensional, 2 μs delay line readout of a 270 × 300 mm active area has been developed for use as a position-sensitive area X-ray detector in the 8 keV energy region (Cu Kα) used in crystallographic structure work with large biological molecules. Its quantum detection efficiency for 8 keV X-ray photons is about 0.5, a value which is spatially uniform to within ± 2%. Its dead-time loss fraction at a typical data collection rate of 30000 photons s−1 is 12%. The detector has spatial resolution for X-rays of 0.6 mm FWHM in the horizontal direction and 2 mm, the anode wire spacing, in the vertical direction. The effects of parallax are found to be limited and do not seriously increase the apparent size of the diffracted beams. The position sensitivity of this detector is geometrically linear to within 0.5 mm across its active surface. Routine maintenance of the detector requires the attention of a skilled technician but is not time consuming. For four years, this detector has been used to measure millions of reflection intensities from crystals of many different proteins. The down time due to the detector has averaged less than four days per year, considerably less than the down time of other components of the data collection system. Four new protein structures have now been solved using data from this detector. Also, a considerable amount of data have been collected at higher resolution or at different temperatures with crystals of other proteins.
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  • 9
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 124-130 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The sources of systematic error in the X-ray diffractometer method of stress measurement are discussed. The errors in stress measurement and correction procedures for their elimination are described and quantitatively assessed.
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  • 10
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 117-123 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The diffraction aspect and rough cell parameters of a crystal to be studied are usually known before mounting the crystal on a diffractometer. Such data are used for orientation purposes in the method described. An ordered list of indexed 2θ values is calculated from which an optimum 2θ for a peak search is derived. The peaks found are centered, their possible unique indices are derived by comparison of the observed 2θ values with the calculated values. Two intense reflections with few indexing ambiguities are selected to define the crystal orientation. Their possible pairs of indices are found by considering combinations of indices related to the unique indices by the Laue-group symmetry and for which the calculated angle between the reciprocal vectors is comparable with the observed angle. Only the non-equivalent orientations defined by these initial pairs will be accepted. An initial pair will be rejected if the indices of both reflections can be transformed into those of a previously accepted pair by a single Laue-group symmetry operator of the first kind. The correct setting is usually obvious from a very short list of possible orientations and is retained. This simple approach has been programmed and treats all symmetries without branching of the logic. Rhombohedral crystals can be referred to hR or rP cells. The possible non-equivalent settings due to metric ambiguities, to pseudo-symmetry or to symmetry are exhaustively listed, but no symmetry-related setting is indicated. This scheme has automatically oriented more than 40 crystals from the seven crystal systems in 20–50 min each. The wavelength, the cell parameters and the short symbol of a valid space group in the correct diffraction aspect are the only input.
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  • 11
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 137-139 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Neutron time-of-flight powder data have been collected for forsterite [Mg2SiO4; Pbnm (D162h, No. 62); Z = 4, a = 4.7534 (1), b = 10.1989 (2), c = 5.9813 (1) Å] with the ZING-P′ high-resolution diffractometer at Argonne National Laboratory. Forty-seven variables, including anisotropic temperature-factor coefficients, were refined with a profile-fitting procedure to R(profile) = 2.10% and R(Rietveld) = 3.53%. Positional and thermal parameters are in good agreement with those determined from recent X-ray and neutron single-crystal diffraction experiments. Results suggest that powder data can be used to obtain accurate positional parameters and reasonable temperature factors for moderately complex structures.
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  • 12
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 139-141 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A Kratky small-angle X-ray camera was modified so as to allow the use of a one-dimensional (one-dimensional) position-sensitive detector (PSD) with it. The modification was designed in such a way that most of the calibrations, necessary for subsequent correction of the collected intensity data, can be performed with little alteration in the collimation geometry or the electronics settings. In particular, the calibration of the uniformity of detector efficiency can be performed by repeated vertical travel of the detector at a constant speed across a beam scattered from a sample.
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  • 13
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 145-148 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A computer program for the separation of a composite peak into the components and the background has been written in Fortran. Each component is approximated by a function of the form y(x) = c [1 + ({{x - d}\over{e}})^{2}]^{-f}, where c, d, e and f are the parameters to be determined by a least-squares method. Arbitrary, known or assumed relations among the parameters of different components may be read in to reduce the number of independent parameters and save computation time or, in other cases, to offer a better chance of separating a complex composite peak by varying a subset of the parameters.
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  • 14
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 151-153 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A computer program has been written with the aim of calculating the domain of an atom in a structure. The domain may be limited by planes situated half-way between the atoms, or at a distance that takes into account the relative radii of the atoms. Data concerning this domain and the corresponding coordination polyhedron are computed and printed. Punched cards, to be used in a special plotting program, can also be produced. Three types of weighted coordination number are calculated.
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  • 15
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 160-168 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Intensity can often be measured as a continuous function of scattering angle in diffraction from non-crystalline specimens. An analysis of the amount of information contained in one-dimensional continuous diffraction data is presented here. This analysis, based on the sampling theorem and the theory of entire functions, indicates that there is a limited but often rather large number of possible phase solutions to any given continuous intensity distribution. A refinement technique has been developed which allows phase solutions to be found that are consistent with the diffraction data and with other physical and chemical data. In favourable cases, when diffraction data is used in coordination with other kinds of data, there can be enough information in a diffraction pattern to identify a unique structural solution.
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  • 16
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 370-382 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A small-angle neutron scattering (SANS) spectrometer has been constructed at the University of Missouri Research Reactor Facility (MURR). The design of the MURR–SANS is unusual in that the size of the reactor containment building constrains the flight path to be vertical. This is achieved by Bragg scattering upward through 90° from a set of slightly misaligned pyrolytic graphite crystals to provide a neutron beam at 4.75 Å with a wavelength spread of approximately 4.1%. The beam incident on the sample is defined by two matched variable apertures located either 3.0 or 4.5 m apart. The evacuated scattered flight path is designed with removable extensions to match the primary flight path in length. The instrument has an automatic sample handling capability provided by its own dedicated PDP 11/03 computer. The detector is a large assembly of commercially available linear 3He detectors as an economic alternative to a crossed-wire two-dimensional multi-detector. An array of 43 position-sensitive proportional counters, 24 in (609.6 mm) long and 0.5 in (12.7 mm) in diameter, using charge division gives a spatial resolution of 5 x 12.7 mm. The area-averaged detector efficiency is about 84% at a wavelength of 4.75 Å. The range of scattering vectors that can be measured is 0.005 〈 Q〈 0.15 Å−1. The instrument is well suited to a wide variety of experiments on specimens having characteristic dimensions between 20 and 500 Å. MURR–SANS is designed as a user-oriented facility which provides both reasonable resolution and intensity on sample at a modest cost, and forms part of a neutron scattering center.
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  • 17
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Two-dimensional antiphase domain structures existing in the composition range 20–23 at.% Mn were investigated by a high-resolution structure-imaging technique with a 1 MV electron microscope. The structures are based on the DO22 structure and consist of parallelogram-shaped domains containing 4 × 3 columns of Mn atoms and lozenge-shaped domains with 4 × 4 and 3 × 3 columns, and the domains are separated by two-dimensional antiphase boundaries parallel to the ({\bar 2}40) and (240) planes of the fundamental face-centred structure. The configuration of the domains changes delicately with a slight change of composition or annealing temperature, and the symmetry of the structure is lowered below about 670 K. The ideal structure models have compositions of about 22.7 at.% Mn. The images of about half of the specimen area of the 22.6 at.% Mn alloy annealed at 570 K do not correspond to these new structures, but bear a resemblance to the image expected from the two-dimensional antiphase structure proposed by Watanabe [J. Phys. Soc. Jpn (1960), 15, 1030–1040] for Au3Mn, which is based on the L12 structure and has boundaries parallel to the (100) and (010) planes.
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  • 18
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 432-436 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Two methods for the numerical resolution of the Takagi-Taupin equations are compared. It is shown that for a small integration step Taupin's [Acta Cryst. (1967), 23, 25–35] extension to two dimensions of the one-dimensional Runge–Kutta third-order method is more accurate than the algorithm of Authier, Malgrange & Tournarie [Acta Cryst. (1968), A24, 126–136] but, for a given precision, Authier, Malgrange & Tournarie's method is faster than Taupin's so the former will usually be preferred for numerical calculation.
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  • 19
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 444-446 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The phase separation of an 80% B2O3–15% PbO–5% Al2O3 (weight %) glass after splat-cooling was studied by small-angle X-ray scattering. The wavenumber which receives maximal amplification and the scattering intensity corresponding to this wavenumber were determined as a function of the cooling rate of the glass. The experimental results qualitatively agree with the main features of previous calculations based on the linear theory of spinodal decomposition, but no quantitative agreement is obtained.
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  • 20
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 455-461 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: An analysis of published X-ray diffraction data from nerve myelin is given based on the properties of analytic functions. Functions defined by a finite Fourier transform may be described by their distribution of zeros. This description allows a phase function to be determined from real data, which is unique in principle. A solution to the phase assignment is given and compared with corresponding published solutions derived by other methods. The strong measure of agreement for the phases of the first nine diffraction orders, and the stability of this agreement against the efforts of experimental error, leads to the conclusion that these phases are probably correct.
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  • 21
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 465-465 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The powder data for [2.1.2.1]paracyclophan is reported. This compound is monoclinic with a = 21.44 (4), b = 17.22 (6), c = 9.70 (4) Å, γ = 141.30 (3)°, Z = 4.
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  • 22
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 14 (1981), S. 491-491 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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  • 23
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    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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  • 24
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    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 17 (1984), S. 238-243 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    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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  • 25
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    Applied crystallography online 17 (1984), S. 249-256 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    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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  • 26
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    Applied crystallography online 17 (1984), S. 257-264 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    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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  • 27
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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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  • 28
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    Applied crystallography online 17 (1984), S. 273-285 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    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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  • 29
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    Applied crystallography online 17 (1984), S. 297-306 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    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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  • 30
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    Applied crystallography online 17 (1984), S. 307-314 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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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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  • 31
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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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  • 32
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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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  • 33
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    Applied crystallography online 17 (1984), S. 369-370 
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  • 34
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    Applied crystallography online 17 (1984), S. 370-370 
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  • 35
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    Applied crystallography online 17 (1984), S. 372-372 
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  • 36
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    Applied crystallography online 17 (1984), S. 373-373 
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  • 37
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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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  • 38
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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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  • 39
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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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  • 40
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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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  • 41
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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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  • 42
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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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  • 44
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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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  • 45
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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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  • 46
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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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  • 47
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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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  • 48
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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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  • 49
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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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  • 50
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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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    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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    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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  • 55
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    Applied crystallography online 14 (1981), S. 247-252 
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    Notes: The effect of refraction on the observed diffractometer angles θr, ωr and χr of a centered reflection is discussed for general asymmetric reflection from a finitely absorbing extended-face crystal. Formulae are presented for θr, ωr and χr for symmetric reflection and for θr and ωr for the two extreme positions of asymmetric reflection in terms of the angles calculated assuming that refraction can be neglected.
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    Applied crystallography online 14 (1981), S. 265-269 
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    Notes: A modification of the Berg-Barrett X-ray topographic method was developed which enables measurement of all three misorientation parameters of a general subgrain boundary. The described method is based on the evaluation of three topographs of a single, arbitrarily oriented crystal surface taken by symmetric zero-layer reflections. In practical tests performed on an as-grown Fe–3 Wt% Si single crystal, relative accuracy of 10% and angular sensitivity better than 1′ were achieved.
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    Applied crystallography online 14 (1981), S. 274-279 
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    Notes: An interactive graphics program is described for use with the Evans & Sutherland Picture System 2 which is suitable for the comparison and refinement of protein structures. Several protein molecules and electron density maps can be viewed simultaneously, while great flexibility exists in creating, modifying and manipulating the picture on the screen. As the program is file oriented, it can be run on a small computer system with only 32 K memory.
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    Applied crystallography online 14 (1981), S. 281-284 
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    Notes: A precise X-ray determination of the lattice parameters of silver gallium telluride, AgGaTe2, has been made in the temperature range 301 to 667 K with a Unicam 190 mm high-temperature powder camera. The data have been used to evaluate the coefficients of thermal expansion, α⊥ and α\parallel, at various temperatures. It has been observed that the `c' parameter decreases while the `a' parameter increases with increasing temperature as in the case of AgGaSe2 and AgGaS2, which also have a chalcopyrite-type structure. The magnitudes of both α⊥ and α\parallel increase non-linearly with increasing temperature.
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    Applied crystallography online 14 (1981), S. 285-290 
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    Notes: Berg–Barrett topography gives information on large-scale dislocation distribution in deformed single crystals. The Burgers vectors are determined by disappearance of the dark `extinction contrast'. This contrast cannot be extinguished by any single diffracting condition when different types of non-coplanar Burgers vectors are present simultaneously; complementary information by electron transmission microscopy agrees well with this inference. These general remarks are illustrated by some examples on deformed single crystals of α-Al2O3, (Al2O3)1.8MgO, NaCl and Cu2O.
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    Applied crystallography online 14 (1981), S. 300-304 
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    Notes: A description is given of a mathematical formalism which allows the study of diffraction by stacks which contain, simultaneously or not, (i) layers of different kinds, (ii) different translations or rotations between layers and (iii) different thicknesses. The formalism allows the calculation of intensities for all the hkl reflections or (h,k) reciprocal rods, and not just the 00l reflections. It accounts for an Sth-neighbour interaction (S≥1).
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    Applied crystallography online 14 (1981), S. 315-320 
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    Notes: The results of a trial powder diffraction experiment on the ADONE storage ring at the Frascati National Laboratories (Italy) are described. A (220) channel-cut Si crystal was used to provide a monochromatic beam and it was demonstrated that Debye–Scherrer photographs could be obtained in several hours. It was also shown that very high resolution can be obtained simply by increasing the camera dimensions. Finally, it is shown that with the higher fluxes of the SRS at the Daresbury Laboratory (England) very fast exposures will be possible without the need for focusing.
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    Applied crystallography online 14 (1981), S. 329-336 
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    Notes: Satellite spots appear around every fundamental spot in electron diffraction patterns of iron-carbon martensite (α′) tempered in a temperature range from 273 to 363 K. High-resolution electron microscopic observations show that they are due to the formation of a modulated structure, in which interstitial carbon atom clusters smaller than 10 Å are distributed randomly in a plane nearly parallel to (102)α′, with inter-cluster distance of 10–20 Å and such carbon-rich planar regions are spaced periodically with intervals of 10–20 Å, depending on the carbon content of the martensite. A structure analysis was made by measuring the satellite spot intensity of the electron diffraction patterns, and the displacements of atoms from their average positions in martensite were determined. The result supports the above cluster model for the modulated structure.
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    Applied crystallography online 14 (1981), S. 348-349 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
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    Notes: MSbO4, (M = Cr, Fe) are tetragonal, rutile-type compounds, P42/mnm (No. 136); Z = 1. M and Sb are randomly distributed in 2(a) positions and O in 4(f). For CrSbO4, Dm = 5.31, Dx = 5.35 Mg m−1, a = 4.5899 (2), c = 3.0525 (2) Å, U = 64.307 (8) Å3, x = 0.318, R = 4.6; Cr and Sb are coordinated with two O atoms at 2.064 and four O atoms at 1.930 Å.
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    Applied crystallography online 14 (1981), S. 352-353 
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    Applied crystallography online 14 (1981), S. 354-354 
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    Applied crystallography online 14 (1981), S. 355-356 
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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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    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 14 (1981), S. 475-477 
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    Notes: Single crystals of [Cu(bpy)2NO2]ClO4, [Cu(C10H8N2)2NO2]ClO4, C20H16CuN5O2.ClO4, with space group P21/c, were grown from the reacting mixture of bis(2,2′-bipyridyl)copper(I) perchlorate with molecular oxygen in nitroalkanes. The unit-cell parameters are a = 10.83 (2), b = 12.51 (1), c = 17.04 (1) Å, β = 112.0 (2)°, Z = 4, Dm = 1.62 (1) Mg m−3.
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    Applied crystallography online 14 (1981), S. 472-473 
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    Notes: Indexed powder patterns of four monoclinic compounds of general formula (BH)3H3V10O28 .xH2O, where x = 1-2.5 and B = 2,4-dimethylpyridine (J8), (C7H10N)3H3V10O28.1.0H2O, 3,4-dimethylpyridine (J10), (C7H10N)3H3V10O28.1.0H2O, 3,5-dimethylpyridine (J11), (C7H10N)3H3V10O28.2.5H2O, and 2,4,6-trimethylpyridine (J12), (C8H12N)3H3V10O28.1.0H2O are given. The cell dimensions of the unit cells with two molecules are: J8 a = 18.360 (2), b = 10.438 (2), c = 13.503 (2) Å, β = 95.99 (1)°; J10 a = 12.538 (3), b = 13.634 (5), c = 16.662 (5) Å, β = 111.82 (2)°; J11 a = 19.483 (5), b = 10.711 (2), c = 13.020 (4) Å, β = 101.52 (2)°; J12 a = 12.776 (3), b = 22.470 (4), c = 11.790 (3) Å, β = 91.54 (1)°.
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    Applied crystallography online 14 (1981), S. 487-489 
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    Applied crystallography online 14 (1981), S. 490-490 
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    Applied crystallography online 14 (1981), S. 491-492 
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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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    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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    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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  • 98
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 17 (1984), S. 218-218 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
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  • 99
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 17 (1984), S. 219-225 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    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
    Electronic Resource
    Electronic Resource
    Oxford [u.a.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 722-725 
    ISSN: 1600-5759
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
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