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  • International Union of Crystallography (IUCr)
  • 1985-1989  (6,250)
  • 1
    Electronic Resource
    Electronic Resource
    A new approach to the measurement of X-ray structure amplitudes determined by the Pendellösung method (1985)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 41 (1985), S. 48-55 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The Laue-case rocking curve from two thin crystals is known to exhibit fine structure which can be used to determine the corresponding structure amplitude Fh. Thus F444 and F777 have been measured for crystalline silicon to a standard deviation of 0.2%. F444 is in excellent agreement with published experimental values. There are no previous high-precision measurements of F777 in the literature. The values measured seem to indicate that the accepted theoretical dispersion corrections are somewhat too low. A highly stable monolithic thin-wafer silicon dittractometer of a novel design was employed. Using an energy-dispersive solid-state detector and the white spectrum of a tungsten X-ray tube, rocking curves from a number of planes of the hhh family could be recorded simultaneously, thus reducing measurement time considerably and minimizing the influence of some sources of systematic and random error. Least-squares computer fitting of the theoretical curve to the complete measured one is employed to determine the value of Fh from the data. A complete discussion of the method is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767385000083
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  • 2
    Electronic Resource
    Electronic Resource
    Exact random-walk models in crystallographic statistics. II. The bicentric distribution for the space group P\overline{1} (1985)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 41 (1985), S. 55-59 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An exact probability density function for the magnitude of the normalized structure factor |E| has been derived for the space group P\bar 1, taking account of the presence of one non-crystallographic center of symmetry. The function is based on the exact solution of the corresponding random-walk model and its expansion into a Fourier series. The above result is compared with simulated semi-cumulative distributions based on hypothetical structures and very good agreement is obtained for the equal-atom case, as well as for a heterogeneous asymmetric subunit containing fourteen carbon atoms and one uranium atom. The new exact bicentric probability density functions of |E|, for the space group P\bar 1, reduce to the well known asymptotic expressions that are valid for equal-atom structures and a large number of atoms in the asymmetric unit of the space group.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767385000095
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  • 3
    Electronic Resource
    Electronic Resource
    The estimation of average molecular dimensions. 2. Hypothesis testing with weighted and unweighted means (1985)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 41 (1985), S. 85-89 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The average value (μ) of a molecular dimension may be estimated by a weighted (\bar xw) or unweighted (\bar xu) mean. Computer simulations show that \bar xu can be used in hypothesis tests, since the distribution of (\bar xu - μ)/σ(\bar xu) is closely approximated by Student's t distribution. In contrast, hypothesis tests based on the weighted mean are inexact and potentially misleading.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767385000150
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  • 4
    Electronic Resource
    Electronic Resource
    Modern crystallography. Vol. III. Crystal growth by A. A. Chernov (1985)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 41 (1985), S. 112-112 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767385000216
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  • 5
    Electronic Resource
    Electronic Resource
    Local density approximations in quantum chemistry and solid state physics edited by J. P. Dahl and J. Avery (1985)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 41 (1985), S. 112-112 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S010876738500023X
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  • 6
    Electronic Resource
    Electronic Resource
    Simulation of multiple diffraction (1985)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 41 (1985), S. 128-133 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The multiple diffraction of X-rays and neutrons is discussed on the basis of the kinematical theory; a program for the simulation of ψ scanning and λ scanning is developed, where the influence of the wavelength width of incident beams on the Ewald construction is properly taken into account. The effect of higher-order diffraction (n-beam interaction, n 〉 3) is treated as the sum of those of (n-2) pairs of relevant double diffractions (three-beam interactions). Applications are made for some examples for which experimental data are available; it is shown that the results are in very good agreement with experiment. This suggests that the kinematical approach is appropriate. The simulation is useful in planning ψ-scanning experiments for precise structure determination and for examining experimental data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767385000265
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  • 7
    Electronic Resource
    Electronic Resource
    On the classical interpretation of thermal probability ellipsoids and the Debye–Waller factor (1985)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 41 (1985), S. 156-157 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In many standard text books on crystallography, the Gaussian expression for the Debye-Waller factor is derived from a classical point of view. The physical model normally used is too simple in that it is incompatible with the concept of probability ellipsoids used to depict atomic thermal motion. A classical derivation of the Debye-Waller factor expression is given that corrects some misconceptions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767385000307
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  • 8
    Electronic Resource
    Electronic Resource
    X-ray diffraction lenses with spherical focusing (1985)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 41 (1985), S. 17-25 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: X-ray spherical-wave focusing in multibeam dynamical diffraction by a biaxially bent single crystal has been considered. In contrast to cylindrical lenses already studied in the two-beam case, which presented a line focus, here wave packets focusing in two directions into a single point are dealt with. The conditions for focusing of the trajectories of the X-ray Bloch waves are established and the algorithm for the determination of the parameters of corresponding X-ray optical systems is described. Possible sets of parameters are calculated. The X-ray wave field distribution in a crystal is simulated numerically. The calculated topographs confirm the existence of the focusing effect.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767385000034
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  • 9
    Electronic Resource
    Electronic Resource
    The extension and refinement of the 1.9 Å spacing isomorphous phases to 1.5 Å spacing in 2Zn insulin by determinantal methods (1985)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 41 (1985), S. 3-17 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A new mathematical description of phase relationships which connects different approaches, both in reciprocal and direct space, is formulated. It leads to the development of a novel algorithm for phase extension and refinement based on a probability function for atomic presence. This function, calculated from the elements of the Karle-Hauptman inverse matrix, is used in an iterative procedure. Various tests have been performed on an idealized set of calculated structure factors for an insulin model structure. The method has been applied to experimental data, Fobs, and the isomorphous phases for 2Zn insulin. An assessment of the quality of the phase refinement and calculation has been made by comparison with the crystallographically refined phases.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767385000022
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  • 10
    Electronic Resource
    Electronic Resource
    Multilayer thin films as polarizing monochromators for neutrons (1985)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 41 (1985), S. 41-48 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Recent progress in the development of polarizing multilayer thin-film monochromators for neutrons is reported. In particular, multilayers consisting of thin films of Fe and Ge with bilayer thicknesses of the order of 50 Å have been made with high peak reflectives and polarizing efficiencies. The results of measurements of multibilayer reflectivity and polarizing efficiency are compared with other neutron polarizers. A picture of the microscopic structure of the individual Fe and Ge films has also been obtained. Finally, practical applications of polarizing multibilayers are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767385000071
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