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  • International Union of Crystallography (IUCr)
  • Annual Reviews
  • 2005-2009
  • 1985-1989  (2,457)
  • 1980-1984
  • 1988  (2,457)
Collection
Publisher
Years
  • 2005-2009
  • 1985-1989  (2,457)
  • 1980-1984
Year
  • 1
    Electronic Resource
    Electronic Resource
    The statistical significance of difference densities (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 33-38 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The statistical properties of a difference density Δρ are not fully characterized by the standard deviation σ(Δρ), which relates to the density at a point. That is not sufficient information to assess the significance accurately for the density within a finite volume. The reliability of a complete Δρ map may be determined by applying standard statistical tests to the chi-square index \chi ^{2} = \Sigma_{{\bf S}} \sigma ^{2}({\bf S})[\Delta F({\bf S})]^{2} from a least-squares refinement, where ΔF is a structure-factor residual and σ2 is the variance in the structure factor, or equivalently to the goodness-of-fit index [(χ2/&ngr; - 1)/2]1/2, where &ngr; is the number of degrees of freedom in the structure refinement. A similar treatment applies to component volumes or features in the difference density for which the chi-square index is \chi ^{2} = \Sigma_{{\bf S}} \sigma^{-2}_{n}({\bf S})[\Delta F_{n}({\bf S})]^{2}, where ΔFn is obtained by Fourier transformation from the nth component of Δρ and on is the variance rescaled by the fraction of the cell volume occupied by the feature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767387008353
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  • 2
    Electronic Resource
    Electronic Resource
    Application of the molecular replacement method to multidomain proteins. 1. Determination of the orientation of an immunoglobulin Fab fragment (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 38-45 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Multidomain proteins provide special problems in the application of the molecular replacement method of structure determination. The structure of the Fab fragment from the autoimmune poly(dT)-specific antibody HED10 has been determined using molecular replacement. An analysis of the effects of varying the model and the parameters used in the rotation function indicates that dividing the molecule into individual relatively rigid domains simplifies interpretation of the results, and that the optimal parameters depend on the molecule under study.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767387008365
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  • 3
    Electronic Resource
    Electronic Resource
    On integrating the techniques of direct methods and isomorphous replacement. A new probabilistic formula for triplet invariants (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 45-51 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The mathematical technique recently used [Hauptman (1982). Acta Cryst. A38, 289-294] for integrating direct methods and isomorphous replacement techniques is reconsidered. The atomic positions are assumed to be the primitive random variables instead of the reciprocal vectors. A new probabilistic formula for estimating three-phase invariants given six magnitudes has been obtained which differs from the corresponding result of Hauptman. The first applications suggest that the formula is quite efficient for picking up triplet cosines near 0 or π.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767387008377
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  • 4
    Electronic Resource
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    Errors arising from numerical use of the Mott formula in electron image simulation (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 1-6 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The Mott formula relating the electron scattering factor to that for X-rays is inaccurate in its numerical form in the small-angle region. Effects of the inaccuracy on both kinematic and dynamical diffraction have been investigated. Some artifacts arising from the inaccuracy have been found in simulated electron microscopy images. A modified form for the Mott formula, which minimizes the error, is proposed. However, for any accurate and reliable calculation only those electron scattering factors derived directly from the atomic potential can be used.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767387006858
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  • 5
    Electronic Resource
    Electronic Resource
    Non-linear diffraction strain vs sin2ψ phenomena in specimens exhibiting rolling-type texture (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 163-167 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Non-linearities in measured diffraction strains are frequently observed in textured materials. For the case of textured cold-rolled low-carbon steel sheet specimens the diffraction strain is analysed in its constituents: single-crystallite strain and the orientation distribution function of the crystallites. With only macro-stresses σ1 and σ2 taken into account, a satisfactory explanation of practical measurements on these steel specimens is obtained.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767387009814
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  • 6
    Electronic Resource
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    The influence of texture on the strain measured by diffraction (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 157-163 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Strain, as determined by diffraction techniques, is calculated from its constituents. First, the fraction of the crystals that have the proper orientation for diffraction. One degree of freedom is present: the angle of rotation φ”2 about the scattering vector that the diffracting crystals have in common. The proper orientations, expressed in Euler angles, lie on a line ('trace') in orientation space. The density along the trace is asserted to be known as a Fourier series in φ”2. Second, the strain in the diffracting crystals. The simplest possible models are discussed: the Voigt and Reuss approximations. The symmetries of the crystal (m3 or m3m) and of the orientation distribution function (o.d.f.) are taken into account. The dilatation in spacing of the reflecting planes is found as a Fourier series in φ”2 also. Only the zeroth, first and second harmonic (including phase angles: five parameters) play a part. The diffraction strain is the average over the angle φ”2 of the dilatation, weighted with the product of the orientation density and the square of the structure factor. For each contributing trace, the corresponding Fourier coefficients have to be multiplied and added to obtain the diffraction strain. The symmetry of the diffraction pole figure is derived.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767387009802
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  • 7
    Electronic Resource
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    Distortion-induced scattering due to vacancies in NbC0.72 (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 167-176 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The diffuse X-ray (and electron) scattering from NbC0.72, previously thought to be due to vacancy octahedra, is shown to be dominated by the scattering due to mean-square atomic displacements with wave vectors near the Brillouin-zone boundary. The atomic displacements are similar to those produced by an optical phonon. On the basis of the sign and amplitude of the displacement parameters a model for the environment around a carbon vacancy is proposed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767387010043
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  • 8
    Electronic Resource
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    Direct methods and structures showing superstructure effects. IV. A new approach for phase solution (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 183-189 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The mathematical model proposed in paper III of this series [Cascarano, Giacovazzo & Luic (1988). Acta Cryst. A44, 176-183] for describing structures with superstructure effects has been used to derive probabilistic formulas for estimating triplet invariants. The formulas obtained proved sufficiently robust to be applied successfully to a wide range of structures with superstructure effects, in which deviations both of replacive and of displacive type from ideal pseudotranslational symmetry occur.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767387010493
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  • 9
    Electronic Resource
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    Direct methods and structures showing superstructure effects. III. A general mathematical model (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 176-183 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A general mathematical model is presented which can describe a large variety of structures showing superstructure effects. In particular the model can take into account deviations, both of displacive and of replacive type, of the substructural part from ideal pseudotranslational symmetry. The formulation is used to predict statistical effects of deviations on diffraction data. It is shown that the scattering power of the substructural part may be estimated via a statistical analysis of diffraction data for ideal pseudotranslational symmetry or for displacive deviation from it, while it is not estimable in the case of replacive deviation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767387010468
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  • 10
    Electronic Resource
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    Thermal phonons in the modified two-beam description of diffraction near a three-beam point (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 189-193 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The influence of thermal phonons on the dynamical diffraction 0H near a third reciprocal-lattice point L, in an otherwise perfect crystal, is investigated theoretically. It is shown that in the first-order modified two-beam description [Juretschke (1984). Acta Cryst. A40, 379-389] all effects due to phonon transitions are governed by the usual Bessel functions, but only of arguments involving H. With this proviso, the first-order modified two-beam description of diffraction near L incorporates phonon coupling entirely in the standard manner of the strict two-beam case. Therefore typical phonon influences, such as the Debye-Waller factor or thermal diffuse scattering, can be discussed directly in the neighborhood of n-beam diffraction merely by using the modified parameters, i.e. structure factors, absorption coefficients etc. in a traditional two-beam formulation. Some additional implications of this result about the effect of other deviations from crystal perfection on the modified two-beam description are also pointed out.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767387010481
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  • 11
    Electronic Resource
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    Wide-angle X-ray diffraction of materials comprising layer-type molecules (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 150-157 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The shapes of the wide-angle X-ray reflexion profiles produced by materials comprising layer-type molecules, such as carbon fibres and pyrolytic graphites, are affected by distortions, sizes and preferential orientation of the crystallites. In the present study, the diffraction intensity distribution of layer-type materials has been deduced theoretically as a function of structural parameters and measuring direction. The reflexion profiles of carbon fibres have been simulated to investigate the effects of structural parameters on the modulation of the diffraction pattern.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767387009796
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  • 12
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    Direct methods: the identification of space-group-specific inconsistent three-phase structure invariants (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 563-566 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Certain space groups often permit the generation of pairs of triple relationships involving the same three parent reflections in different symmetry forms, giving rise to two equally probable invariant estimates which, because of the space-group symmetry, must disagree by an a priori known phase shift. The 230 space groups have been examined to identify those which permit inconsistent triples, and the complete list which describes the forms of the pair of triples and their phase inconsistency is given.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S010876738800306X
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  • 13
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    Determination of structure-factor phase invariants and effective structure factors in non-centrosymmetric crystals (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 558-562 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Many-beam diffraction effects in non-centrosymmetric crystals have been studied with emphasis on three-beam interactions and determination of three-phase structure invariants in electron diffraction experiments. The effective structure factor has been determined both by numerical many-beam calculations and from the second Bethe approximation. The dependence of this factor on the phase invariant, the excitation errors and the magnitude of the structure factors involved has been discussed in detail. From the values of the effective structure factors at symmetrical positions on each side of a three-beam condition an asymmetry ratio is introduced. By a comparison of the observed variation in this ratio with theoretical profiles, it has been shown that the magnitude of three-phase invariants can be determined in the non-centrosymmetric case. This method may in principle be applied in any type of electron or X-ray three-beam experiments where variations in the effective structure factor are projected out. An example from electron channelling patterns is given.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767388003010
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  • 14
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    Evaluating finite Fourier transforms that respect group symmetries (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 467-478 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A general method for producing efficient algorithms to evaluate finite Fourier transforms that fully utilize symmetry to reduce both computing time and space requirements is described. The method is applicable to all space groups. The resulting algorithms retain the 'N log N' behavior of the fast Fourier transform while reducing the size of the data to approximately an asymmetric unit. The algorithm for the p3 and P3 groups is shown.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767388001758
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  • 15
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    On the treatment of primary extinction in diffraction theories based on intensity coupling (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 461-467 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Czochralski-grown silicon crystals of approximately 10 cm diameter and 1 cm thickness have been annealed at 1470 K in order to create a homogeneous defect structure, which is a basic condition for all statistical treatments of extinction. Absolute values of the integrated reflecting power of the 220, 440 and 660 reflections have been measured with 0.0392 Å γ-radiation in symmetrical Laue geometry for sample thicknesses between 1 and 3 cm. The amount of extinction in the experimental data varies between y ∼ 0.95 and y ∼ 0.05. Darwin's extinction theory has been used to describe the thickness dependence of the data sets. Despite some shortcomings of the model, it is shown that the assumption of a physically unrealistic Lorentzian mosaic distribution models the effect of primary extinction in an extinction theory based on the energy-transfer model. The sharp central part of the Lorentzian distribution produces a reduction of the effective sample thickness due to primary extinction, whereas the wings of the distribution dominate the correction for secondary extinction in the remaining part of the sample. A more flexible mosaic distribution function is proposed, which should be useful in cases of severe extinction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767388001734
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  • 16
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    Increasing the size of search fragments for use in Patterson method calculations – partial-fragment rotation function (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 478-481 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A method is described of expanding a molecular fragment for use in Patterson search procedures by the rotation of part of a model about a bond direction with respect to a fixed fragment, allowing the removal of an important degree of freedom in the model. The function has been incorporated into a computer program and it has been found possible to orient very small partial fragments in this way. The consequent expansion of a search model should assist in structural solution.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767388001813
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  • 17
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    Distribution fitting methods used as figures of merit for non-centrosymmetric structures (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 482-485 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Experimental tests have shown that for non-centrosymmetric structures distribution fitting methods can be used successfully as figures of merit for the determination of the most probable set of phases. This applies to both symbolic addition procedures and multisolution methods.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767388001898
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  • 18
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    Measurement of the coherence length of highly collimated X-rays from the visibility of equal-thickness fringes (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 496-499 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The coherence length of highly collimated X-rays produced by a collimator system, using successive asymmetric reflections, has been determined for the first time from the visibility of equal-thickness fringes in the Laue geometry. Observed transverse and longitudinal components of the coherent length were more than 220 and 40 μm, respectively, at a wavelength of 0.70 Å.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S010876738800234X
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  • 19
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    The isomorphous pseudo-derivative technique for phase refinement by density modification (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 485-495 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Density modification techniques try to improve the phases of poorly resolved electron density maps given by isomorphous replacement by correcting the systematic errors of the maps according to known physical properties. The phases computed from the corrected maps are combined with the observed moduli through a suitable weighting scheme. A new refinement strategy is proposed which considers the observed moduli and the moduli of the Fourier coefficients of the 'best' map as isomorphous pairs, the Fourier transform of the known systematic errors being a 'heavy-atom contribution'. The lack of closure is assumed to have a Gaussian probability law and gives the basis of the weighting scheme as in a true single isomorphous replacement. The application of the technique to the multiple isomorphous replacement map of E. coli methionyl-tRNA-synthetase yields dramatic improvements.
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    URL: http://dx.doi.org/10.1107/S0108767388002326
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  • 20
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    Small-angle-scattering-data treatment by the regularization method (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 244-250 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The application of Tikhonov's regularization method [Tikhonov & Arsenin (1977). Solution of Ill-Posed Problems. New York: Wiley] for the solution of ill-posed problems in small-angle-scattering-data treatment is considered. Simple regularization algorithms are proposed for solving convolution equations in data desmearing (slit-width and polychromaticity problems) as well as for polydispersity problems. A general indirect approach of data processing based on the regularization method is described. Comparison with other data-treatment methods is made.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767387011255
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  • 21
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    Direct methods: the identification of conditions which simplify the generation of inconsistent quadrupoles (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 657-661 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An algorithm is implemented to determine the form and phase shift for inconsistent type II quadrupoles for any space group having glide or screw-axis translations which are not a consequence of lattice centering. Cumulatively there are only six different Miller index restrictions and nine different phase shift forms common to all space groups of orthorhombic or lower symmetry. A similar analysis has been performed for a newly discovered type III class of quadrupoles. The configuration of the phase connections among the four triples of the type III quadrupole is different from the common configuration previously described for both normal (type I) and inconsistent (type II) quadrupoles. A knowledge of these constraint conditions for type II and III quadrupoles greatly simplifies a procedure for generating these relationships.
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    URL: http://dx.doi.org/10.1107/S0108767388003435
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  • 22
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    Quasiperiodic tilings: a generalized grid-projection method (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 667-672 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: We generalize the grid-projection method for the construction of quasiperiodic tilings. A rather general fundamental domain of the associated higher-dimensional lattice is used for the construction of the acceptance region. The arbitrariness of the fundamental domain allows for a choice which obeys all the symmetries of the lattice, which is important for the construction of tilings with a given non-trivial point-group symmetry in Fourier space. As an illustration, the construction of a two-dimensional quasiperiodic tiling with 12-fold orientational symmetry is described.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S010876738800368X
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  • 23
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    Aperiodic tilings with non-symmorphic space groups p2jgm (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 678-688 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: As an illustrative application of the general theory of quasicrystallographic space groups a non-symmorphic aperiodic tiling is constructed with space group p2Jgm using a generalization of the grid method.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767388003733
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  • 24
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    A maximum entropy derivation of the integrated direct methods–isomorphous replacement or integrated direct methods–anomalous scattering probability distributions (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 672-677 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is shown that taking the appropriate terms from a series expansion of the Shannon-Jaynes entropy of a density map subject to intensity constraints gives the standard direct methods structure factor probability distribution functions. The use of two maps, one to represent a native structure, the other to represent either heavy atoms or the number density of anomalous scatterers, and the application of a similar expansion to the total entropy of both maps rapidly gives either the integrated direct methods-single isomorphous replacement or the integrated direct methods-anomalous scattering probability densities.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S010876738800371X
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  • 25
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    The program SAPI and its applications. I. Automatic search of pseudo-systematic extinction for solving superstructures (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 688-691 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The name SAPI is an abbreviation of 'structure analysis programs with intelligent control'. It may also be read inversely as 'Institute of Physics, Academia Sinica'. SAPI is based on MULTANSO, but differs from it by a number of features. These will be described in a series of papers. The present paper describes an algorithm which can distinguish superstructures from ordinary structures by automatically discovering the pseudo-systematic extinction rule in reciprocal space. This algorithm enables SAPI to handle superstructures in a fully automatic way, leading to a complete solution in favourable cases.
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    The RED extinction model. I. An upgraded formalism (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 283-286 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Some modifications are introduced into the extinction correction formulae based on the RED (random elastic deformation) model developed earlier by the author [Kulda (1987). Acta Cryst. A43, 167-173]. The scattering cross section, which includes a correction for primary extinction, has been extended to allow for a more general angular variation of the effective deformation gradient. The originally proposed cos θ dependence is included as one of the limiting cases corresponding to a pure lattice-plane misorientation. A modified expression has been derived for the angular reflection curve profile which describes properly the broadening due to the finite dimensions of the diffracting region. This version of the RED model containing three free parameters has been employed in experimental tests reported in paper II [Kulda (1988). Acta Cryst. A44, 286-290].
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    Report of the Executive Committee for 1987 (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 741-764 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    The Kathleen Lonsdale Lecture (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 765-765 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    International tables for crystallography Volume A - Brief Teaching Edition (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 765-765 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    On the breakdown of Friedel's law in electron backscattering channelling patterns (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 700-707 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The theory for electron channelling patterns has been investigated. Calculations based on existing intensity expressions have been found inadequate for explaining recently observed deviations from Friedel's law. This is found to be the case even if the calculations are based on the full non-Hermitian eigenvalue matrix. The theory has hence been reinvestigated and a new intensity expression has been derived which includes the inter Bloch-wave coupling terms and is valid for non-centrosymmetric polyatomic crystals. The expression explains the observed asymmetries for GaSb which make it possible to determine unambiguously the correct non-centrosymmetric point group for this crystal. It is further found that the same effects in GaAs should be very weak in accordance with the non-observed deviation from Friedel's law in this case.
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    Determination of crystal symmetry from electron channelling patterns (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 693-700 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The application of selected area channelling patterns in crystal symmetry determination has been investigated. In addition to rotation axes and mirror planes diffraction effects have been observed which may be ascribed to violation of Friedel's law. It has thus been possible for the first time with this technique to distinguish uniquely between centrosymmetric and non-centrosymmetric point groups. This is demonstrated with examples taken from the cubic and the hexagonal crystal systems, of which some show very detailed zone-axis absorption HOLZ patterns. Owing to the large tilt angle represented in each diagram the directions of double scattering may easily be localized. It is shown that this may be utilized in determination of the extinctions and the space group. The sensitivity to deviation from centrosymmetry is discussed and found to be somewhat less than for the convergent-beam technique. Crystal sizes from micrometres to bulk may be used. The strength of the channelling method is its simplicity and in particular the possible use of bulk specimens which may also be directly studied by other techniques, for example by X-rays.
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    Grain boundaries in sintered YBa2Cu3O7–δ (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 772-775 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: High-resolution electron microscopy has been carried out on grain boundaries of 92% dense YBa2Cu3O7-δ in the tetragonal form. Grain boundaries were found to be predominantly parallel to (001) of one of the adjacent grains. No amorphous interlayer was observed at the grain boundaries. At some grain boundaries highly localized strains were detected.
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    A contribution to the unsolved problem of a high-tilt fully eucentric goniometer stage (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 775-780 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A design is presented of a high-angle fully eucentric tilting stage (±45. ±23° around two orthogonal axes) for a transmission electron microscope. The concurrent design of suitable objective-lens pole pieces has been made in order to get the best compromise between electron optical performances and the difficulties inherent in the construction of the stage. A resolution of 3 Å is calculated at an accelerating voltage of 200 kV. The design exploits the space both above and in the middle of the objective pole-piece assembly: the space above is used to house the stage, while the space in the lens gap serves the purpose of loading the specimen. In this way the incorporation of an x, y, z specimen traverse stage inside the high-tilt universal suspension appears to be a practical proposition. Although the details of the stage have been worked out with the JEOL JEM 100 and 2000 series of electron microscopes in mind, the working principles of the design can be applied to other types of microscopes.
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    Results of multislice matrix calculations for convergent-beam RHEED patterns (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 780-788 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Results of dynamical convergent-beam reflection high-energy electron diffraction (CBRHEED) calculations are presented for the (001) surface of magnesium oxide, the (111) surface of silicon and the (001) surface of molybdenum disulfide. These double rocking calculations are performed using a dynamical scattering approach. This is based on the evaluation of the surface parallel multislice matrix for the reflection (i.e. Bragg) geometry with account taken of the boundary conditions. Comparison with experimental results reported in the literature for these surfaces shows that only a full dynamical calculation with an appropriate number of beams is capable of a detailed description of the experimental contrast distributions. In particular, the nature of surface-wave-resonance effects is discussed.
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    Symmetries and beam reduction in the matrix formulation of the dynamical theory of fast electrons (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 789-798 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A survey of the symmetries of the two-dimensional scattering Hamiltonian of fast electrons is presented in order to minimize the size of the effective diffraction matrix. The principle of the method is to decompose the initial Hilbert space into the irreducible invariant subspaces corresponding to the different irreducible representations of the symmetry group of the Hamiltonian and to perform the diagonalization only into those subspaces which share common representations with the initial state. Both unitary and anti-unitary operators are considered. The analysis is based on space-group representations and applies for both symmorphic and non-symmorphic space groups.
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    Imaging tunnel atoms in intergrowth tungsten bronzes (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 798-805 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Crystals of caesium and rubidium intergrowth tungsten bronze (ITB) have been studied by high-resolution electron microscopy. The structure of these consists of slabs of WO3 type, intergrown with slabs of hexagonal tungsten bronze (HTB) type containing hexagonal tunnels in which the alkali atoms are accommodated. In the images of thicker parts the hexagonal tunnels are clearly revealed. In the thin parts the HTB structure mostly appears as a hexagonal pattern of dots of equal intensity and the tunnels are not recognizable. This applies particularly to Cs ITB but also in many instances to Rb ITB. Image simulations, assuming known or estimated degrees of filling of the tunnels with alkali, show a clear difference in contrast at the tungsten and alkali positions, especially at certain focus settings. The discrepancy is thought to be due to surface reconstruction, whereby tungsten atoms are trapped in the tunnels or the surface layers are shifted, making the surface structure similar to that of WO3.H2O.
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    Compilation of temperature factors of cubic elements (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 396-399 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A compilation is presented of the temperature factors of 22 cubic elements. This represents the first stage of a Temperature Factor project initiated by the Neutron Diffraction Commission of the International Union of Crystallography [Acta Cryst. (1985), B41, 374].
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    Microfiche version of Acta Crystallographica and Journal of Applied Crystallography (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 399-399 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Solid state science: past, present and predicted edited by D. L. Weaire and C. G. Windsor (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 399-399 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Quantum aspects of molecular motions in solids. edited by A. Heidemann, A. Magerl, M. Prager, D. Richter and T. Springer (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 399-400 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Effect of RHEED resonances on secondary and Auger electron emission of Pt(111) surfaces (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 853-857 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The secondary electron emission, the Auger electron emission and the elastically reflected intensity have been measured simultaneously in a reflection high-energy electron diffraction (RHEED) beam rocking experiment with Pt(111). Secondary and Auger yield depend similarly on incident angle. In particular, both quantities exhibit maxima at primary beam orientations around the resonance maxima of the specular beam intensity. The corresponding primary wave field arising by diffraction in the crystal has been calculated using the dynamical theory of RHEED. It turns out that, in contrast to previous suppositions, at the resonances there is a minimum of primary electron density at the atomic sites. It is shown that the behaviour of both yields is rather a consequence of total primary electron intensity near the surface, which is enhanced at the resonances.
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    The octagonal quasilattice and electron diffraction patterns of the octagonal phase (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 857-863 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An analysis is given of the dual transformation and also the strip method which can yield the ideal octagonal quasilattice as well as its approximants. An ideal octagonal tiling consisting of 45° rhombi and squares can be derived from the projection of a 4D cubic lattice within an irrational 2D subspace onto an irrational 2D hyperplane, and its Fourier transform matches well the eightfold electron diffraction pattern of the Cr-Ni-Si octagonal quasicrystal. The approximant of an octagonal tiling corresponds to the rearrangement of two kinds of tiles in an ideal quasilattice which destroys the exact quasiperiodic sequence. It is shown that the defects introduced to change the aperiodic order into a regular approximant correspond to a linear phason strain along certain directions, and this will break the eightfold rotational symmetry. The Fourier transform agrees well with the experimental electron diffraction pattern displaying only fourfold symmetry.
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    Electron microscope analyses of domains and discommensurations in ferroelectric Rb2ZnCl4 (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 864-870 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Image contrast of domains and discommensurations in ferroelectric Rb2ZnCl4 is studied. Six domains of the two orientation variants and three translation variants in the ferroelectric phase are observed. However, only two kinds of domain boundaries, which are the discommensurations in the incommensurate phase of Rb2ZnCl4, are observed. These observations agree with group-theoretical considerations of the phase transition from the normal phase to the commensurate phase of Rb2ZnCl4.
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    Electron diffraction analysis of polycrystalline and amorphous thin films (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 870-878 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A rapid analytical technique has been developed for obtaining the reduced density function, G(r), from polycrystalline and amorphous thin films, using post-specimen scanning and an energy loss spectrometer on a transmission electron microscope. The technique gives on-line analysis of nearest-neighbour distances to an accuracy of 0.02 Å, together with coordination numbers. It has the advantage over X-ray and neutron techniques that the information can be obtained from small (≲ 1 μm diameter) chosen regions of the specimen. Results from neighbouring selected regions can be compared.
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    Validity domain of the weak-phase-object approximation for electron diffraction of thin protein cystals (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 878-884 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The domain of validity of the weak-phase-object (WPO) approximation is evaluated for high-energy electrons (100 keV, 500 keV and 1 MeV) scattered by crystalline biological macromolecules. Cytochrome b5 is used as an example in which calculated dynamical diffraction intensities are used to simulate observed diffraction intensities, which are then compared with intensities calculated by the weak-phase-object approximation. Three criteria of validity are used, namely the crystallographic residual (R value), the interpretability of difference Patterson maps, and the results of phasing by the heavy-atom isomorphous replacement method. The present calculations indicate that the error associated with the WPO approximation is quite acceptable up to a specimen thickness of 200 Å for 100 keV electrons, which is two to four times the thickness limit for crystalline organic structures with much smaller unit-cell dimensions. An equally acceptable thickness limit at 500keV and 1 MeV is about 300-350 Å.
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    Linearization of the basic equations of the dynamical theory of electron diffraction by crystals (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 885-890 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The dynamical theory based on the plane-wave expansion of Bloch waves is applied to problems of electron diffraction by reflection at crystal surfaces. It is mainly aimed at reflection high-energy electron diffraction (RHEED), but may be applied also to medium- and low-energy electron diffraction (MEED and LEED). It is shown that the theory leads to a quadratic matrix eigenvalue problem, which can be expanded into a linear matrix eigenvalue problem by applying two alternative standard methods of matrix theory, the Günther expansion and the diagonal expansion of Falk. These methods are shown to be equivalent to the expansions of the system of second-order ordinary differential equations, obtained by the two-dimensional Fourier expansion of the Schrödinger equation, into systems of first-order differential equations. The equivalence enables a physical interpretation of the quantities introduced in the matrix methods to be given.
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    A rigorous treatment of the asymptotic development of the probability density of a structure factor in P\overline{1} (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 14-17 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is shown that an asymptotic development up to order N-2 exists for the density of the structure factor in P\bar 1. An upper bound for the error is calculated.
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    X-ray topography of bent crystals (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 8-14 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The influence of the bending of a crystal on the formation of diffracted images in Bragg section topographs as well as in Laue section and traverse topographs is studied. In the case of Bragg section topography the interferometric fringes due to the interference of waves once and twice internally reflected inside a bent crystal are described. It is established that the maximum positions of diffracted intensity satisfy the law xn = [16π(2n - 1)/5B2]1/3, where xn is the distance from the incidence slit of the X-rays, B is the uniform strain gradient and n is the fringe number. This dependence is found to be in good agreement with experimental data. The Laue section topograph of a bent crystal with a screw dislocation parallel to the diffraction vector is considered. The effects of asymmetry in the Pendellösung fringe pattern and of 'splitting' of the direct image with respect to the dislocation line in both the experimental and simulated topographs are accounted for. The variation of the contrast of dislocations with depth inside a bent crystal in Laue traverse topographs is studied by computer simulations using the reciprocity theorem.
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    Multiple scattering and the 200 reflection in silicon and germanium (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 22-25 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Absolute measurements of the 200 reflection in Si and Ge at various azimuthal orientations are compared with N-beam calculations of the integrated intensity. All of the non-zero integrated intensity is accounted for by multiple-beam scattering. The measurements match the calculations on the assumption that F200 = 0.
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    Phase observation in an organic crystal (benzil: C14H10O2) using long-wavelength X-rays (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 17-22 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The phase-related asymmetry effect near a multi-beam excitation point has been observed for a noncentrosymmetric organic crystal, benzil (C14H10O2), by using 3.5 keV X-ray synchrotron radiation. A multi-beam theoretical calculation shows good agreement with the experimental data when mosaic spread of the crystal is taken into account. A practical method to extract the cosine of the phase triplet for noncentrosymmetric crystals is also discussed.
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    On the X-ray analysis of thin subsurface layers. Bicrystal diffraction analogues (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 25-33 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: X-ray determination of strain and damage distributions in thin subsurface layers from rocking curves is an ambiguous procedure. In the case of N distorted layers, all equivalent profiles can be obtained in the kinematical limit and their total number may be sufficiently large, being of the order of magnitude 2N [Afanas'ev & Fanchenko (1986). Dokl. Akad. Nauk SSSR, 287, 1395-1399]. A more detailed theoretical treatment of the problem and the analytical expressions of all bicrystal-equivalent crystal structures (crystal structures leading to the same diffraction intensity as the crystal with constant spacing deviation in the subsurface layer) are presented.
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    Electron diffraction patterns of chrysotile: effect of specimen orientation (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 70-76 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Electron diffraction patterns of chrysotile asbestos fibrils should have the 2mm symmetry of a rotation photograph because the layers in the structure are curled as cylinders. The way in which the fibril orientation affects the diffraction patterns is considered theoretically. The departures from ideal symmetry attributable to specimen orientation are noted: they affect mainly the h00 reflections close to the fibre axis. Actual diffraction patterns show a consistent difference in the separation of the h0l-h0\bar l pairs on the upper levels, and \bar h0l-\bar h0\bar l pairs on the lower. The experimental conclusion [Yada (1979). Can. Mineral. 17, 679-691] that this difference is not an effect of specimen orientation is confirmed theoretically but its cause remains obscure.
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    A lattice-dynamical comparison of nonbonded potential parameters for hydrocarbons (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 76-79 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The crystallographic thermal parameters have been calculated with a lattice-dynamical procedure for some hydrocarbons in the rigid-body appproximation using some sets of potential parameters taken from the literature. A comparison with experimental data confirms that the well known Williams IVb set is very good for describing vibrational properties in polycyclic hydrocarbons.
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    Normalization factors for spherical harmonic density functions (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 6-8 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Normalization factors Nlm for spherical harmonic density functions clmp defined by ∫ Nlm|clmp| dτ = 2- δl0 have been derived for l ≤ 7, from both analytical and numerical integration methods.
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    On the application of phase relationships to complex structures. XXVII. Phase extension for small proteins (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 410-413 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An example is given of phase extension for a small protein, avian pancreatic polypeptide, by the use of the Sayre-equation tangent formula (SETF). Initial data were the phase estimates of 129 reflexions with large values of |E| within the 3 Å resolution sphere. The mean error of these phases, estimated by a combination of isomorphous replacement and anomalous scattering, was 26.5°. Random values were then given to 1371 other phases out to 1 Å resolution and refinement was carried out with SETF. In 20 trials, 11 gave mean phase errors less than 34° for all 1500 reflexions with the best set having a mean phase error of 31-9°. Maps computed with these phases showed the general form of the molecule.
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    Exact random-walk models in crystallographic statistics. IV. P.d.f.'s of E allowing for atoms in special positions (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 413-417 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The effect of scatterers, located in variable special positions, on the probability density function of the magnitude of the normalized structure factor has been investigated. Exact characteristic functions have been obtained for all the statistically different variable special positions in triclinic, monoclinic and orthorhombic space groups except in Fdd2 and in the space groups based on the point group 222, and the probability density functions have been evaluated from their Fourier or Fourier-Bessel series expansions. It is seen that the effect of heavy scatterers, located in the special positions investigated, is very marked and should be accounted for in cases of space-group ambiguities.
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    The ambiguity problem of vector-set interpretation (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 422-425 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An efficient method based on the utilization of crystal symmetry for the interpretation of vector sets of crystals belonging to space groups of higher symmetry than P1 is described. New equality relations between atomic position vectors which are sufficient to provide the possibility of an ambiguous solution are derived. It is proved that for many space groups of the polar kind these conditions of ambiguity are necessarily satisfied. A complete list of such space groups is given.
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    X-ray diffraction by a low-angle twist boundary perpendicular to crystal surface. III. The integral characteristics (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 425-432 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: For X-ray diffraction by a pure low-angle twist boundary perpendicular to a crystal surface, within the framework of the kinematic and dynamical theories, the following integral characteristics are calculated: (a) the bicrystal reflectivity in the vicinity of the lth reflection; (b) the integrated intensity of the lth reflection; (c) the bicrystal total reflectivity, i.e. the sum of the integrated intensities over all reflections. The case for even h. b (b is the diffraction vector, b is the Burgers vector of the boundary screw dislocations) is considered. In dynamical theory an increase of the total reflectivity of a bicrystal due to the boundary dislocation structure is obtained.
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    Observation of Pendellösung fringes induced by X-ray resonant scattering (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 433-436 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A new type of Pendellösung fringe has been observed which is induced by X-ray resonant scattering very near the absorption edge. Integrated intensities of the 600 reflection from GaAs(100) across the Ga K absorption edge, at which X-ray resonant scattering is expected to be significant, were measured. The integrated intensities showed modulations which were caused by the change of the extinction distance due to X-ray resonant scattering or to the change of anomalous scattering factor.
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    Some thoughts on Harker–Kasper inequalities (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 222-226 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The approach by Harker & Kasper [Acta Cryst. (1948), 1, 70-75] which led to the first inequality relationships between structure factors has not previously been applied to the space group P1 and there seems to have been a view that it could not give useful results for that space group. The idea has also been advanced that Harker-Kasper inequalities are contained within the complete set of determinantal inequalities derived by Karle & Hauptman [Acta Cryst. (1950), 3, 181-187]. It is shown that the Harker-Kasper approach can be applied in space group P1 and gives inequality relationships which are distinct, at least in form, from those derived from determinants. Indeed, in some cases, a simple Harker-Kasper inequality can be more effective than an equally simple determinantal inequality in restricting the allowed values of three-phase invariants.
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    Commission on Powder Diffraction (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 230-230 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Improving the interpretation of translation functions by a simple map-correlation procedure (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 226-230 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A procedure for improving the reliability of the Q (translation) functions is presented. The procedure involves correlating between maps calculated using different sections of the reflection data, these being spherical shells divided according to d spacing. The peaks in the Q function representing the true shifts of the fragment are found to be the most stable under such a correlation procedure. The modification has been incorporated into a computer program.
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    Crystallographic Databases (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 230-231 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    LOGO DESIGN CONTEST (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 231-232 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Standard Crystallographic File Structure-87 (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 232-232 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Publish your Crystallographic Computer Programs (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 232-232 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Etching of crystals. Theory, experiment, and application by K. Sangwal (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 233-233 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    The diffraction of X-rays by close-packed polytypic crystals containing single stacking faults. I. General theory (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 640-649 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A general theory of X-ray diffraction by one-dimensionally disordered close-packed polytypic crystals has been developed. A random distribution of all possible single stacking faults for arbitrary n-periodical structure is taken into consideration. Exact expressions for measurable parameters of changes in the intensity distribution caused by faults are given. Initial equations of the theory have been formulated by applying Holloway's [J. Appl. Phys. (1969), 40, 4313-4321] method of analytic solution. For the mathematical description of disorder, successive layers of the perfect structure have been denoted by an additional subscript j. The probability of occurrence of the faulted layer with subscript k after the layer with subscript j has been denoted by αjk. A set of recurrence relations is developed for average phase factors of layers with subscript j on m positions. An arbitrary sequence of layers is written in these relations by using the so-called phase-change factors (after layer with subscript j), determined by Hägg's structure symbols. Terms of the coefficients of the characteristic equation and boundary conditions which are necessary to describe the change in the intensity distribution are given for small values of αjk. Finally, the shifts and broadenings of the reciprocal-lattice points and the changes in the intensity of peak maxima are derived in terms of αjk.
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    The diffraction of X-rays by close-packed polytypic crystals containing single stacking faults. II. Theory for hexagonal and rhombohedral structures (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 650-657 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The general theory developed by Michalski [Acta Cryst. (1988), A44, 640-649] has been applied to the cases of hexagonal and rhombohedral structures. The symbols of stacking faults based on Zhdanov's symbols of local structure near the faults have been introduced and assigned to the formal subscripts j, k used in general theory. On this basis the regularities, according to which the faults with different subscripts j, k have the same structures, have been characterized. Then these regularities have been taken into consideration in the derivation of expressions for measurable parameters of changes (caused by faults) in the X-ray intensity distribution. The results obtained for structures 2H, 4H, 6H(33), 8H(44), 10H(55), 12H(66), 3C, 9R(12)3, 12R(13)3 and 15R(23)3 are given. Some results are compared with published data. The physical meaning of the assumption of small values of fault probabilities is discussed.
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    Application of the molecular replacement method to multidomain proteins. 2. Comparison of various methods for positioning an oriented fragment in the unit cell (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 300-309 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The capabilities of several different methods to determine the correct translation of a model for the application of the molecular replacement method of structure determination to multidomain proteins have been analyzed. The structure of the Fab fragment of the autoimmune anti-poly(dT)-specific antibody HED10 was determined using molecular replacement and provides an example for comparing different methods of determining the correct translation of the model and for evaluating the importance of the parameters used. Expansion to space group P1 and phasing with a correctly oriented randomly positioned model was found to be superior to either the Crowther-Blow translation function [Crowther & Blow (1967). Acta Cryst. 23, 544-548] or a brute-force search when only a small part of the molecule was used as a model.
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    Anisotropic bond polarizabilities in birefringent crystals (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 320-326 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Based on the concept of bond polarizabilities the optical properties of a crystal are traced back to the orientation of its bonds and to the individual bond parameters \bar \betaand Δβ. A test with various sulfate compounds shows that the average bond parameters \bar \beta describe the average refractive indices with a higher accuracy than the traditional model of ionic polarizabilities. The anisotropic part of a bond polarizability, Δβ, is considered to be the origin of optical birefringence. Values for Δβ are evaluated in MCO3 and MSO4 (M = Mg, Ca, Sr, Ba). In different crystals the same values for Δβ have been discovered for the C-O and the S-O bonds. Δβ of the M-O bonds depends in a characteristic way on the bond length. A simple method is used to calculate local field effects in accordance with the model of bond polarizabilities. Possible applications of bond polarizabilities for the determination of structural quantities are discussed.
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    Analysis of solvent structure and hydrogen exchange in proteins on the basis of neutron diffraction data from deuterated and hydrogenous crystals (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 309-320 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A method has been developed to determine the structure of bound solvent and the positions of exchanged hydrogens in proteins, on the basis of neutron diffraction from hydrogenous and deuterated crystals. In this method phases for the hydrogenous and for the deuterated model are refined simultaneously, and an average model is imposed in the volume occupied by non-hydrogen atoms. The densities in the areas of bulk solvent are replaced by their average values, while no modifications are performed in the vicinity of ordered solvents and potentially exchangeable hydrogens. The method was tested on 1.8 Å neutron diffraction data collected from two crystals of bovine pancreatic trypsin inhibitor, one of them deuterated and the other hydrogenous. Significant improvement was observed for the densities corresponding to many partially occupied solvent sites, as well as to partially exchanged hydrogens. The algorithm presented here has been compared with a different approach published recently by Shpungin & Kossiakoff [Methods Enzymol. (1986), 127, 329-342].
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    Definition of an asymmetric domain for intercrystalline misorientation in cubic materials in the space of Euler angles (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 326-336 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A new asymmetric domain for intercrystalline misorientation is defined in the space of Euler angles for materials exhibiting cubic (Oh point-group) lattice symmetry. The invariant measure for this new domain is nearly constant; this is in significant contrast to the previous domain defined by MacKenzie [Biometrika (1958), 45, 229-240]. Distribution functions in the misorientation can now be represented with greater clarity and convenience in the new domain. A detailed theoretical analysis of special misorientations exhibiting multiplicities m 〉 1 is described. It is demonstrated that all such special misorientations fall upon the surfaces separating distinct asymmetric domains. This result convincingly proves that the derived asymmetric domain is correct. The location of all possible coincidence site lattice boundaries for Σ ≤ 49 are identified in the asymmetric domain, and their characteristic multiplicities are given.
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    The Gram–Charlier and multipole expansions in accurate X-ray diffraction studies: can they be distinguished? (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 336-343 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The Gram-Charlier temperature factor formalism has been applied to a set of accurate low-temperature data on bis(pyridine)(meso-tetraphenylporphinato)iron(II), and to a theoretical set of static structure factors on the hexaaquairon(lI) ion. The refinements are compared with the multipole treatment for atomic asphericity due to chemical bonding. In a treatment of the experimental data in which only the iron atom asphericity is considered, the 'thermal motion' formalism is as efficient as the multipole formalism in accounting for the observations. It is slightly less efficient when applied to the static theoretical data, though model maps based on the two treatments are remarkably similar. A high-order Gram-Charlier refinement of the porphyrin data, followed by a multipole refinement of all data with the Gram-Charlier parameters initially fixed, and later varied, shows that simultaneous refinement of anharmonic and aspherical effects is possible, though the resulting separation may not be accurate. A combined Gram-Charlier multipole refinement on the static data, however, leads to non-significant thermal parameters. It is concluded that the statistical Gram-Charlier formalism is remarkably successful in representing bonding effects in the valence charge density if these are not specifically accounted for in the scattering formalism. Statistical anharmonic thermal motion formalisms should only be used for X-ray data analysis in combination with a formalism accounting for the effect of bonding on the atomic charge density.
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    On the application of phase relationships to complex structures. XXV. TRITAN – or recycled failure (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 349-353 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Even when direct methods fail to give a clear solution, E maps from some of the phase sets obtained contain correctly oriented fragments. It is shown that such information from several phase sets can be amalgamated by an automatic procedure to give estimates of the values of three-phase invariants. These estimates are incorporated into a modified tangent formula which is used in a new run of a multisolution direct-methods procedure. Tests of the total process, called TRITAN, reveal that it is very effective in determining structures which otherwise would not be found routinely.
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    Negative volumes and central moments of anharmonic probability density functions (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 343-349 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The suitability of different functions used for anharmonic temperature factors in describing anharmonicity in structures is studied from the point of view of whether the respective probability density functions (p.d.f.'s) (isolated-atom-potential expansions, Gram-Charlier series, p.d.f. of the cumulant expansion of the temperature factor) are non-negative for the final values of the parameters. Certain central moments of the p.d.f. should also not become negative. Analytical formulae have been derived for the central moments of several (different) p.d.f.'s. 20 p.d.f.'s of atoms in published structures have been examined, their negative volumes determined by numerical integration and their central moments calculated. For Al(4) in the structure of VAl10.42 at 293 K (strong anharmonicity) the negative volume was -0.238 and several moments were negative. Here the temperature factor and p.d.f. used are not acceptable. For the remaining structures (weak anharmonicity) the negative volumes found were 〈|-0.03| and the respective central moments were positive. Temperature factors and p.d.f.'s proved to be acceptable except for the cumulant expansion. In some cases its p.d.f. is not accessible, and, if it is, its negative volumes were found to be relatively large. A p.d.f. is proposed which is non-negative for all values of the parameters and whose Fourier transform can be derived in analytical form. An explicit formula is given for the cubic site symmetries.
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    On the application of phase relationships to complex structures. XXVI. Developments of the Sayre-equation tangent formula (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 353-357 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The Sayre-equation tangent formula (SETF) develops sets of phases tending to satisfy Sayre's equations for both large and small normalized structure factors. There are two components in the SETF, corresponding to contributions from phase triplets and quartets respectively. The development of objective algorithms for properly weighting these components and for gradually building up the quartet contribution has enabled the SETF, within the procedure SAYTAN, to be incorporated into MULTAN87, the latest version of the package. Examples of tests of MULTAN87 and its use in solving unknown structures are given.
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    A reconciliation of extinction theories (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 368-374 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The differences between previous theoretical treatments of extinction based on the Darwin intensity equations arise because of the different functional form chosen for the coupling constant σ. When the same function is used these theories make closely similar predictions. It is shown that a limiting condition on integrated intensity as the crystal size increases puts restrictions on the functions which may be used. A Lorentzian or Fresnellian function can be used for primary extinction while secondary extinction requires a Gaussian, rectangular or triangular function. An analytical expression is given for the variation in the value of the extinction factor with scattering angle.
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    Combining direct methods with isomorphous replacement or anomalous scattering data. VI. Incorporation of heavy-atom information into Hauptman's distributions (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 379-383 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Heavy-atom information has been incorporated into Hauptman's distributions of three-phase structure invariants for both isomorphous replacement and anomalous scattering cases. Reliable estimates of individual phases can be obtained by introducing the phase doublet expression φH = φ′H ± |ΔφH|. A test calculation with error-free data of insulin showed results better than previous methods.
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    Quantitative determination of phases of X-ray reflections from three-beam diffraction. II. Experiments for perfect crystals (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 1073-1078 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The method proposed by Chang & Tang [Acta Cryst. (1988). A44, 1065-1072] of quantitative determination of X-ray reflection phases from multiple diffraction profiles is applied to nearly perfect crystals of gallium arsenide. The detailed intensity-profile-analysis procedures are given. Multiple diffraction profiles obtained with a conventional X-ray source and synchrotron radiation are subjected to this analysis. It is found that, for this particular diffraction example, errors as small as 15° in phase determination are achieved. Errors due to the theoretical approximation, peak position measurement and scaling factor are also discussed.
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    Refinement at 1.4 Å resolution of a model of erabutoxin b: treatment of ordered solvent and discrete disorder (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 357-368 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The latter stages in the refinement of the protein erabutoxin b are described. The crystal structure of the 62-residue protein has been refined to a conventional R factor of 0.144 by stereochemically restrained least-squares methods using diffraction data to a limit of 1.4 Å spacings. Emphasis was placed on determining as accurately as possible the solvent structure and the structures of heterogeneous groups in the protein. The final model includes two conformers for each of seven side chains and for an octapeptide segment. A total of 111 sites for water molecules have been located as well as one sulfate ion with a total of 68 site occupancies. 65 of the solvent sites overlap either with protein atoms belonging to groups in two alternative conformations or with other solvent sites. Dual protein conformers and overlapping solvent sites were both included in the least-squares refinement. Individual thermal and occupancy parameters were refined for solvent molecules. An analysis of these parameters has provided useful structural information.
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    Streaks of instrumental origin near a Bragg point (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 383-392 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is predicted that, at some sufficiently high level of sensitivity, two streaks intersecting at the Bragg point and symmetrically situated about the reciprocal-lattice vector G should always be observable in a diffraction experiment.
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    Determination of planar group orientation in Patterson methods (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 1078-1082 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An adaptation of the I(θ, φ) function for location of planar group orientation is presented. More accurate determination of this orientation is found to be possible in some cases using the modified functions, based on rings and hoops rather than disks in Patterson space. In some examples incorrect orientations are corrected by the new functions. A possible extension to the simultaneous detection of several small group orientations in one molecule, based on bond length selection, is discussed.
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    A comment on 'Determining skewness in atomic probability density functions for non-centrosymmetric structures' (Nelmes & Tun, 1987) (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 1097-1097 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In a recent publication Nelmes & Tun [Acta Cryst. (1987). A43, 635-638] give the impression that the atomic probability density function can be determined unambiguously even for non-centrosymmetric crystal structures when using the Gram-Charlier expansion technique. This is only true under a tacit assumption of rapid convergence of the Gram-Charlier series. A similar problem exists in the analysis of electron deformation densities for non-centrosymmetric structures.
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    Synchrotron X-ray diffraction on a CaF2 microcrystal with 2.2 cubic micrometres volume (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 1099-1101 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The photon flux generated by the six-pole wiggler at CHESS combined with a focusing mirror and a focusing monochromator allowed diffraction experiments at 1.56 Å, wavelength with a 2.2 (5) μm3 CaF2 single crystal. The crystal was oriented by means of a multiwire proportional area counter. Reflection profiles and Bragg intensities were collected with a scintillation counter. The Bragg intensities were used for a structure refinement. The results demonstrate that crystals composed of light elements with volumes down to only 0.5 μm3 can be mounted and used for single-crystal X-ray diffraction experiments. Until now such crystals have been considered as powder grains. Besides the possibility of applying single-crystal methods to materials of which larger crystals are not available the essentially extinction-free data from microcrystals allow a high- precision determination of electron densities and vibrational amplitudes.
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    Prices of Acta Crystallographica and Journal of Applied Crystallography (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 1102-1103 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
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    Interpretation of the shape of electron diffraction spots from small polyhedral crystals by means of the crystal shape amplitude (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 890-897 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The influence of the crystal shape on the fine structure of transmission electron diffraction (TED) patterns described by the crystal shape amplitude is discussed. A general algebraic expression for the crystal shape amplitude of any crystal polyhedron is used for computing the intensity distribution of TED reflections. The computer simulation method is applied to the analysis of the fine structure of TED patterns of small gold and palladium crystals having octahedral and tetrahedral habits.
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    Stacking disorder in a stage-4 FeCl3–graphite intercalation compound (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 897-904 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: X-ray scattering measurements have been made in situ on highly oriented pyrolytic graphite intercalated with FeCl3 to stage 4 in a sealed glass tube at 620 K. It has been found that the FeCl3 is in a two-dimensional liquid state at this temperature and that the stacking of the sets of ordered graphite layers (ABAB) that bound the intercalant is nearly random. Through a novel modeling of L scans for 10.L, 11.L, 20.L, 21.L, 30.L and 22.L, a good fit has been achieved by using 60% of a preferred (A-A) stacking of sets with a broad lateral distribution about the ordered position. The remaining 40% of the sets are stacked with complete translational randomness, without regard to the normal graphite crystallography. This sliding randomness remains compatible with Daumas-Herold domain formation.
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    Electron microscope observations of lattice imperfections and their movement on the atomic scale (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 928-938 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The effect of partial coherence of illuminating electron waves on the image contrast is discussed. It is shown that aberration-free focus (AFF) imaging in axial and symmetrical illumination, which is generally believed to be the best imaging condition for coherent illumination, produces an inherent contrast which does not correspond to the atomic structure of the specimen. However, tilted-illumination AFF imaging produces the correct image. Atomic structure images of gold crystals containing edge, screw and mixed dislocations have been photographed, with tilted-illumination AFF imaging and by adjusting the specimen thickness to the optimum value, and the atomic structure of the core of these dislocations is discussed. The movement of these dislocations has been recorded on an atomic scale using a TV system, and the movement of atom columns accompanying the movement of dislocations is analyzed.
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    Visibility of single heavy atoms on thin crystalline silicon in simulated annular dark-field STEM images (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 912-927 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The multislice method, pioneered by Cowley and Moodie, has recently been adapted to simulate annular dark-field scanning transmission electron-microscope (ADF STEM) images. This paper presents a series of calculations using this new approach with experimental parameters appropriate for a VG-HB501 STEM to investigate the visibility of single heavy adatoms on thin crystalline silicon membranes. The tendency for electrons to channel along columns of atoms in crystals can greatly increase the intensity incident on an adatom on the exit surface, thereby increasing the adatom visibility. The simulations indicate that an adatom on the exit surface on a column of crystal atoms is up to three times as visible as an adatom on the entrance surface, and that the adatom remains highly visible as the crystal thickness is increased. Tilting the specimen or displacing the adatom from the column appears to lower the visibility of the adatom dramatically. These calculations suggest that, with the appropriate imaging conditions, a single gold adatom may be visible on at least 235 of (111) silicon.
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    Multiple scattering theory for fast electrons in single crystals and Kikuchi patterns (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 51-61 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: By solving the kinetic equation for the density matrix of fast electrons the intensity distribution of the diffuse background and the Kikuchi lines and bands is found. It is shown that the Kikuchi pattern contrast depends on the angle of deflection of the scattered particles and the crystal thickness. For a thin crystal the expressions obtained coincide with the usual results found in the single inelastic scattering approximation. The theory takes into account both the dynamical diffraction and multiple inelastic scattering of electrons and gives a simple interpretation of a variety of contrast effects observed in thick single crystals.
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    Image processing in high-resolution electron microscopy using the direct method. III. Structure-factor extrapolation (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 61-63 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A resolution-enhancement method has been proposed which makes use of the Sayre equation [Sayre (1952). Acta Cryst. 5, 60-65] to extrapolate both phases and magnitudes of structure factors. The starting point of the procedure is just a single deconvoluted electron microscopic image. No preliminary knowledge other than the chemical composition of the sample is necessary. A simulation on a theoretical image of copper perchlorophthalocyanine shows that the image resolution can be enhanced from 2 to 1 Å, resolving clearly individual atoms.
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    Phase determination using high-order multiple diffraction of X-rays (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 63-70 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The effect of invariant phases on the intensity profiles of high-order N-beam X-ray diffractions, with N 〉 3, is investigated. Theoretically, the second-order Bethe approximation and the graphic analysis of the structure-factor multiplets involved in the dispersion equation of the dynamical theory of X-ray diffraction are employed to reveal the dominant invariant phases in the multiple diffraction processes. It is shown that the phases of the triplets or the quartets are the effective phases which affect the multiply diffracted intensities. Experimentally, the intensity profiles of four-, five-, six- and eight-beam cases provide clear evidence to support the theoretical considerations.
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    Atomic physics, 10 edited by H. Narumi and I. Shimamura (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 104-104 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Some generalized Penrose patterns from projections of n-dimensional lattices (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 105-112 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Examples of quasi-periodic plane tilings, consisting of rhombi and having n-fold rotational symmetry, are obtained by the projection method from a slice of an n-dimensional cubic lattice (with n = 5, 7, 8, 9, 10 and 12). A simplified geometrical description of the method is given. A comparison is made of some of the properties of these tilings with those of the Penrose patterns. The applicability of recursions (composition/decomposition or deflation/inflation) to generalized Penrose patterns with n = 5 is discussed.
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    Etude des représentations des groupes ponctuels bicolores cristallographiques sous-tendues par des familles de points équivalents (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 124-128 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The representations spanned by sets of equivalent (general or special) positions of the crystallographic coloured point groups are reduced to irreducible representations using a property proposed by Bates [Labarre (1978). Thdorie des Groupes. Paris: Presses Universitaires de France]. The derivation has been carded out fully for the 32 one-coloured point groups, for the 58 true two-coloured point groups and the 32 grey point groups. Examples of groups \bar 42 m, 2/m 2/m 2/m; 2'/m 2'/m 2/m'; 2'/m' 2'/m'2/m; 2/m' 2/m' 2/m' and 2/m 2/m 2/m 1' are given.
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    Restrained refinement of two crystalline forms of yeast aspartic acid and phenylalanine transfer RNA crystals (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 112-124 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Four transfer RNA crystals, the monoclinic and orthorhombic forms of yeast tRNAPhe as well as forms A and B of yeast tRNAAsp, have been submitted to the same restrained least-squares refinement program and refined to an R factor well below 20% for about 4500 reflections between 10 and 3 Å. In yeast tRNAAsp crystals the molecules exist as dimers with base pairings of the anticodon (AC) triplets and labilization of the tertiary interaction between one invariant guanine of the dihydrouridine (D) loop and the invariant cytosine of the thymine (T) loop (G 19-C56). In yeast tRNAPhe crystals, the molecules exist as monomers with only weak intermolecular packing contacts between symmetry-related molecules. Despite this, the tertiary folds of the L-shaped tRNA structures are identical when allowance is made for base sequence changes between tRNAPhe and tRNAAsp. However, the relative mobilities of two regions are inverse in the two structures with the AC loop more mobile than the D loop in tRNAPhe and the D loop more mobile than the AC loop in tRNAAsp. In addition, the T loop becomes mobile in tRNAAsp. The present refinements were performed to exclude packing effects or refinement bias as possible sources of such differential dynamic behavior. It is concluded that the transfer of flexibility from the anticodon to the D- and T-loop region in tRNAAsp is not a crystalline artefact. Further, analysis of the four structures supports a mechanism for the flexibility transfer through base stacking in the AC loop and concomitant variations in twist angles between base pairs of the anticodon helix which propagate up to the D- and T-loop region.
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    Short-range-order correlations in the orientationally disordered phase of hexachloroethane. I. Diffuse X-ray scattering (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 128-136 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Diffuse X-ray scattering has been measured in the orientationally disordered phase of C2C16 at 453 K. A model structure, which was generated in a Monte Carlo program, reproduces the dominant single-molecule part of the diffuse scattering. In addition the existence of a one-dimensional orientational order along the body diagonals of the cubic lattice is demonstrated. The related planes of diffuse scattering are calculated in terms of the Naya model [Naya (1974). J. Phys. Soc. Jpn, 37, 340-347].
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    The Debye–Waller factor for polyatomic solids. Relationships between X-ray and specific-heat Debye temperatures. The Debye–Einstein model (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 136-142 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The Debye-Waller factor for a polyatomic crystal is derived in the Debye approximation. If the crystal has a basis of p atoms per lattice point, it is shown that the specific-heat Debye temperature, ΘD, and the X-ray Debye temperature, ΘM, are related by ΘD ∼ ΘMP1/2 in the classical limit. The Debye and Einstein theories are then combined to yield an expression for the Debye-Waller factor of a polyatomic solid. The acoustic phonon modes are described with a Debye approximation, and the optic modes with an Einstein model. For temperatures above the Debye temperature, the Debye and Einstein parts of the Debye-Waller factor have the same dependence on temperature and diffraction vector. Thus, the two parts cannot be distinguished.
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    Simple statistics for intensity data from twinned specimens (1988)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 44 (1988), S. 142-144 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The statistics of intensity data from hemihedrally twinned specimens are analyzed in terms of a new parameter and are shown to take a simple form in both the centrosymmetric and non-centrosymmetric cases. This analysis provides a sensitive method for determining the twinning fraction. The effects of intensity measurement errors on the observed statistics are discussed.
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