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  • International Union of Crystallography (IUCr)  (1,435)
  • 1980-1984
  • 1975-1979  (1,435)
  • 1950-1954
  • 1977  (1,435)
Collection
Publisher
Years
  • 1980-1984
  • 1975-1979  (1,435)
  • 1950-1954
Year
  • 1
    Electronic Resource
    Electronic Resource
    Quadratic electrostrictive effects in LiF, NaF, NaBr, KCl, KBr, KI, RbCl, RbBr and RbI (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 114-118 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Quadratic electrostrictive effects of both longitudinal and transverse type have been measured on LiF, NaF, NaCl, NaBr, KCl, KBr, KI, RbCl, RbBr and RbI single crystals with an improved frequency-modulation technique with a 100 MHz oscillator. The three independent tensor components d1111, d1122, d1212, defined as dijkl = ½ ∂2εij/∂Ek∂El have been determined for all these crystals. εij are the components of the strain tensor and Ek the components of the electric field vector. All longitudinal components d1111 and all 'shear' components d1212 are negative; all transverse components d1122 are positive. The electric field causes a volume contraction in these crystals. The theoretical values for d1111 as derived by Grindlay & Wong [Canad. J. Phys. (1969), 47, 1563-1571] agree well with the experimental data. Larger discrepancies are found between the predicted and observed behaviour of d1122 and d1212. Within the isotypic crystal group of alkali halides of rocksalt type the components d1111 show a positive correlation with the dielectric constants. The appreciable electrostrictive contribution to the total electro-optical effect is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739477000242
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  • 2
    Electronic Resource
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    Identification of enantiomorphously related space groups by electron diffraction (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 126-133 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A simple procedure is presented for the identification of enantiomorphous space groups by electron diffraction. This is based on the ability to determine the sense of an asymmetry in a projection, which is a property of N-beam dynamic diffraction. The steps in this procedure follow those of the X-ray procedure which depends upon anomalous dispersion instead of dynamic scattering. To illustrate and test the electron-diffraction procedure, a space-group determination was carried out on a sample of dextrorotatory α-quartz: space group P3221 was found, in agreement with the findings of de Vries [Nature, Lond. (1958), 181, 1193].
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739477000266
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  • 3
    Electronic Resource
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    Determination of strain distributions from X-ray Bragg reflexion by silicon single crystals (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 137-142 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Accurate determination of strain distributions normal to the crystal surface is attempted through best-fitting of intensity profiles recorded with a double-crystal diffractometer and corresponding theoretical curves computed within the frame of the dynamical diffraction theory. The strain distributions are assumed to be one-dimensional and their functional forms are supposed from the sample preparation process. The fits are quite satisfactory for three samples prepared by boron diffusion, phosphorus diffusion and epitaxic growth with germanium doping. For samples containing boron and phosphorus, sheet-resistivity measurements combined with anodic oxidation showed diffusant distributions which were reasonably proportional to the strain distributions assumed above. Ratios of strain to impurity concentration measured here are ca 40% larger than those defined for uniform doping; simple elasticity considerations give an interpretation of this fact and the factor can be expressed in terms of stiffness constants.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S056773947700028X
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  • 4
    Electronic Resource
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    Lattice dynamics, thermal expansion and bulk modulus of erbium (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 146-150 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The lattice dynamics and the temperature variation of the volume Grüneisen function and bulk modulus of erbium have been calculated with the nearest-neighbour central-interaction model of Srinivasan & Ramji Rao [Inelastic Scattering of Neutrons, IAEA, Vienna, (1965), 1, 325-342]. The even moments of the frequency distribution function of erbium have also been calculated. The agreement between the calculated in and that obtained from thermal data is good. The calculated Bs values agree with those obtained from the measured elastic constants of Fisher & Dever [Trans. Metall. Soc. A IME, (1967), 239, 48-57] at various temperatures up to 298 K, to within 2%.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739477000308
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  • 5
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    The intensity distribution of X-ray Pendellösung fringes (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 161-168 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The intensity distribution of X-ray diffraction topographs in Laue cases was measured for Si perfect crystals by microdensitometry with an accuracy of 1% at the maximum intensity. Both section and traverse topographs taken with {220} and {440} reflexions were studied, but the section topographs were the main interest. A least-squares analysis shows that the observed intensities can be represented well by the sum of two terms: the dynamical term based on the spherical wave theory and the kinematical term which has a form of attenuation because of the normal absorption. The latter term, however, is very small and its ratio to the former, evaluated at zero depth, was about 10-3 in the perfect crystals available.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739477000333
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  • 6
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    Analysis of the layer sequence for crystals with stacking faults by the method of structural models: comparison with the direct method of Farkas-Jahnke (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 172-177 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A development of a method of analysis based on models for crystals with complex polytypic structure is presented. The method is applicable to the analysis of structures consisting of the polytypes 2H, 4H 6H and 10H and of fragments of a DS structure appearing in ZnS and ZnS solid solutions. A discussion of the direct method of Farkas-Jahnke for characterizing structures with stacking faults is presented and it is shown that interpretation of the real sequence by the direct method is ambiguous. The mechanism of formation of the crystals investigated is discussed from the point of view of the results obtained.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739477000357
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  • 7
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    Prediction of density in organic crystals (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 216-218 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The possibility is explored of expressing the specific volume of organic substances in the crystal state as the sum of increments, including corrective terms. The unitary volumes listed herein allow an estimate of density with a confidence limit of 2-3%.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739477000448
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  • 8
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    Resolution of ambiguities in Zhdanov notation: actual examples of homometric structures (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 257-260 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Very often, ambiguities arise in deciding if two similar-looking Zhdanov symbols represent the same structure or two different structures. Starting from the concept of congruence of two structures, the effect of various symmetry operations on the crystal structure has been systematically analysed and the corresponding changes in Zhdanov symbol examined. Practical criteria have been evolved to decide quickly the congruence of two polytypic structures represented by similar-looking Zhdanov symbols. Pairs of structures in MX2-type and MX-type polytypic compounds have been identified which are fundamentally noncongruent but give the same set of X-ray intensities on calculation. These have been identified as first-found practical examples of homometric structures. Semi-empirical rules have been formulated to obtain theoretically an infinite number of such pairs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739477000655
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  • 9
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    Dielectric behaviour, Raman and IR spectra of Li1−xHxIO3 (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 264-267 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The low-frequency dielectric behaviour, the piezoelectric constant g33 and Raman and IR spectra of Li1 - xHxIO3 have been measured. The amount of hydrogen, x, in the formula Li1 - xHxIO3 varies from 0.0 to 0.35. The curves ε33 = ε33(f) and tan δ = tan δ(f) show anomalous behaviour with respect to frequency and composition. The piezoelectric constant (g33) tends to a maximum at the composition x ∼ 0.04. A model of the low-frequency relaxation spectra and the high value of ε is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739477000679
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  • 10
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    Spin point groups (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 279-287 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The 598 classes of nontrivial spin point groups are derived and tabulated. The relationship between classes of nontrivial spin point groups and the 32 classes of trivial magnetic point group and the 58 classes of nontrivial magnetic point groups is also given.
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    URL: http://dx.doi.org/10.1107/S0567739477000709
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  • 11
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    Decomposition and ordering in Fe1−xO (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 268-276 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Samples in the composition range Fe0.95O-Fe0.88O quenched from about 1000°C, have been studied by electron diffraction/microscopy with special emphasis on the dark-field technique. The set of morphologies obtained by quenching and annealing confirms a spinodal-like decomposition in an oxygen-rich and an almost stoichiometric component. The diffraction spots and dark-field images reveal a considerable degree of order which is enhanced upon heat treatment below 300°C. Previous diffraction studies have been interpreted in terms of an ordered phase with cubic symmetry. Dark-field microscopy, particularly at the symmetric composition, showed that the compositional fluctuation wave contained domains of tetragonal or orthorhombic symmetry at a very early stage. The range of order was found to increase during decomposition. An ordered phase of orthorhombic symmetry developed in oxygen-rich samples. Superstructure spots could be indexed by an A-faced orthorhombic cell with dimensions five times the fundamental cell. Extinctions lead to the space group Abm2 and a periodic antiphase relation. Only four types of clusters, all of which are based on three connected defect tetrahedra, are then possible.
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    URL: http://dx.doi.org/10.1107/S0567739477000680
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  • 12
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    Determination of stacking faults in the spinel-type lattice (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 386-389 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Full dislocations in a lattice of spinel type are dissociated into half and quaternary dislocations. In view of this, the common theory for f.c.c. crystals does not apply to stacking faults in spinel. Diffraction effects in the presence of half and quaternary dislocations are considered and expressions for the estimation of the probability of their appearance obtained. The results of calculations allow one to explain the broadening of lines on the magnesium spinel diffractogram and on that of γ-A12O3.
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    URL: http://dx.doi.org/10.1107/S056773947700103X
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  • 13
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    Direct phase determination of triple products from Bijvoet inequalities (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 382-385 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The classical method of phase determination from Bijvoet inequalities is applied to the phase {\bar \varphi}hk = ½(φhk -- \varphi_{\bar {\bf hk}}) of the triple product τhk ≡ FhFkF_{\bar {\bf h + k}}. The phase-determining formula is then (in the case of a centrosymmetric configuration of anomalous scatterers): sin {\bar \varphi}hk = {{|\tau_{hk}|^{2} - |\tau_{{\bar \bf hk}}|^{2}}\over{4\tau''_{\bf hk}|\tau^{0}_{\bf hk}}} in which τ”hk is the contribution from the imaginary part of the complex double Patterson function to τhk, and | τ0hk2 =½(| τhk|2 + | τ_{\bar hk}|2) - | τ”hk|2. It is shown that τ”hk contains an important term, i.e. the contribution from the origin peak of the double Patterson function, which is independent of the positions of the anomalous scatterers. A test calculation on a structure in P1, containing two Br ions, shows that, in fact, the phases of the triple products can be determined without introducing any a priori knowledge about the positions of the anomalous scatterers, provided an appropriate scaling procedure is applied.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739477001028
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  • 14
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    Technische Mineralogie. Bd.8. Auflösung von Kristallen by R. B. Heimann (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 253-253 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    URL: http://dx.doi.org/10.1107/S0567739477000606
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  • 15
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    Crystal growth and characterization. (Proceedings of the Second International Spring School on Crystal Growth, Japan, 1974) edited by R. Ueda and J. B. Mullin (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 254-254 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S056773947700062X
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  • 16
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    Improvement of alignment in diffuse fibre diffraction patterns (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 390-395 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Molecular alignment of crystalline polymers leads to 'fibre type' X-ray diffraction patterns which are a rich source of structural information. For aligned glassy polymers, however, it is only possible to obtain a very diffuse fibre pattern. This paper describes a technique developed to improve the alignment of such patterns by azimuthal sharpening. The technique is based on consideration of the contribution to the diffraction pattern due to scattering from a fibre entity oriented at an angle to the specimen's extension axis. The distribution of these entities about the specimen axis is determined from the azimuthal profile of a diffuse diffraction halo which has concentrated on the equator of the pattern. Integral equations are developed which describe the appearance of a smeared reflexion on the diffraction pattern in terms of the entity distribution. Solution of these equations by an iterative procedure enables the diffraction pattern to be azimuthally sharpened. The result is a diffuse version of a typical fibre pattern, which is useful in the prediction of chain conformation and packing as well as levels of disorder present.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739477001041
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  • 17
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    Long-period superstructure in sodium–lithium metasilicates (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 408-411 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The solid-solution series, high-(Na2 - x, Lix)SiO3, 0.86 ≲ x ≲ 1.02, has an orthorhombic subcell similar to the unit cells of Na2SiO3 and Li2SiO3 but with a one-dimensional long-period superstructure. The ratio κ = asupercell: asubcell varies linearly with composition and integral κvalues occur at simple Na: Li ratios: κ = 6 for 1Na: 1Li (x = 1.00) and κ = 5 for 5Na: 4Li (x = 0.89). Possible origins of the superstructure are discussed.
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    URL: http://dx.doi.org/10.1107/S0567739477001077
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  • 18
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    Ferroelastic phase transitions in lead phosphate–vanadate Pb3(PxV1−xO4)2 (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 399-408 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The 'pure' ferroelastic behaviour of P-rich Pb3(PxV1 - xO4)2 is of the improper type. The ferroelastic phase transition is antiferrodistortive and accompanied by a second one at a lower temperature. The temperature range of the intermediate phase is approximately 20°C. At the transition points singularities of phonon spectra, dielectric and optical behaviour, and lattice constants are found. The Raman frequencies, the infrared absorption and reflexion spectra are given and interpreted on the basis of a mechanical and a rigid-ion lattice-dynamical model. From the results a critical mode is predicted at the boundary of the Brillouin zone (L in R{\bar 3}m). Further, the Γ phonon becomes weak at the lower transition point. The normal coordinates of both modes give parallel and antiparallel deviations of Pb positions from the [111] axis of the rhombohedral R{\bar 3}m high-temperature phase.
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    URL: http://dx.doi.org/10.1107/S0567739477001065
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  • 19
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    On simple and double diffraction enhancement of symmetry (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 466-474 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Under special circumstances, the diffraction symmetry of a crystal may be higher than that corresponding to the crystal Laue class. The phenomenon has been termed 'diffraction enhancement of symmetry'. Iwasaki [Acta Cryst. (1972), A28, 253-260] is responsible for the first systematic attempt to develop a comprehensive theory of this phenomenon. In this paper, a more general formulation has been developed which leads to enhancement conditions more compact and easier to use than Iwasaki's. By use of these conditions, several new cases of enhancement have been found, in addition to all the cases previously published. Several theorems are enunciated and proved which strongly limit the possible cases of enhancement. The formulation includes also the so-called 'double enhancement'. A set of double-enhancement cases has been found and tabulated. The extension of some of the theorems proved for simple enhancement provide limitations to the possible cases of double enhancement.
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    URL: http://dx.doi.org/10.1107/S0567739477001181
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  • 20
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    The differential geometry of two-phase interfaces (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 480-486 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Coherent as well as incoherent interfaces associated with simple two-phase boundaries have been analyzed in terms of the concepts of differential geometry. Such tensor quantities as distortion, torsion and curvature have been developed and are all shown to give significant insight in describing the Burgers circuits and dislocation content of such boundaries.
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  • 21
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    Temperature factors for internuclear density units. II. Considerations with respect to experimental accuracy (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 430-433 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A simple two-parameter approximation for the vibration tensors of internuclear density units is introduced and numerical values are calculated. The approximation is used to estimate the errors in electron density distributions which are due to incorrect vibration tensors of the internuclear density units. It is concluded that, at the present state of experimental accuracy, the errors due to simple approximations for the temperature factors are hardly significant.
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    URL: http://dx.doi.org/10.1107/S0567739477001120
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  • 22
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    An experimental investigation of the truncation errors incurred in single-crystal diffractometry (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 438-440 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Integrated intensity measurements depend on the wavelength range of integration. In conventional X- ray measurement procedures this range decreases with increasing diffraction angle, which results in a systematic intensity error for which normally no correction is applied. This paper presents an experimental investigation of this error for standard integration scans. A correction formula is derived and tested, and it is shown that failure to take account of this error has led to spuriously high temperature factors in most X-ray structure analyses.
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    URL: http://dx.doi.org/10.1107/S0567739477001144
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  • 23
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    Test of the behaviour of the MDKS formula (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 441-444 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A study of the efficiency of functions related to the MDKS formula in sorting the triplet invariants was performed on eight solved structures. Both D and S terms were checked separately. A new mixed function D + S is described. The K scale factor departs from the commonly used values and depends on the nature of the structure.
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    URL: http://dx.doi.org/10.1107/S0567739477001156
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  • 24
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    An improved method for combining isomorphous replacement and anomalous scattering diffraction data for macromolecular crystals (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 75-85 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The coefficients for the 'best' Fourier synthesis for electron density have been shown by Blow and Crick to be the centroids of probability distributions (over the complex plane) for the structure factors. In defining probabilities for the case in which Bijvoet differences are ignored or averaged, Blow and Crick utilized simplifying assumptions not in accord with probability theory. Raiz and Andreeva defined probabilities for the same case in a theoretically correct manner. The present treatment provides somewhat better approximations for that case, and an extension to the case in which Bijvoet differences are utilized. The variances arising from experimental errors are maintained separately from those due to uncertainties in the heavy-atom parameters. The treatment is restricted here to derivatives in any one of which the scattering factors for heavy-atom anomalous scatterers have the same ratio of real and imaginary components.
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    URL: http://dx.doi.org/10.1107/S0567739477000175
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  • 25
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    Some information-theory aspects of structure determination (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 474-479 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The iterative procedure of phase determination in direct methods is considered as a filtering process. The use of different filter transforms for more effective filtering is proposed. The eigenvector transform for optimal extraction of reliable structure information from approximate densities is introduced.
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    Magnetic structure of Fe2WO6, a neutron diffraction study (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 663-667 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The structure of the compound Fe2WO6 is of the tri-α-PbO2 type, and it belongs to the crystallographic space group Pbcn (D142h). The lattice constants are a = 4.576, b = 16.766 and c = 4.967 Å, with four formula units per lattice point. Neutron (λ ̃2.4 Å) diffraction patterns were taken at 300, 170, 77 and 4.2 K. Some intensity-temperature curves were also taken. The results may be summarized as follows. (1) The room- temperature pattern agrees with the reported X-ray determination of the crystallographic structure and ion positions. (2) Reflexions with h + k odd not allowed by Pbcn, {010}, {030}, {100}, {031}, appear, on cooling, at T1 ̃240 K and increase in intensity down to liquid-helium temperature. (3) Reflexions with h + k even, allowed by Pbcn: {020}, {021}, {040}, {110}, {111}, {150}, show, on cooling, an increase in intensity starting at T2 ̃150 K. The first set of reflexions is consistent with Cz+, i.e. ferromagnetic (100) planes coupled antiferromagnetically with spin along [001]. This structure belongs to Pbc'n'. The space group Pbc'n', however, allows the modes Cz+ and Fx+ or Cz+ and Fx- which is equivalent to a spin component along [100]. The second set of reflexions is consistent with Fx+ and/or changes in the ion positions.
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    Electron-optical imaging of the hollandite structure at 3 Å resolution (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 672-676 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: This paper shows that it is possible markedly to improve the resolution obtainable in electron micrographs by deliberately using low beam divergence (2 × 10-4 rad) and by minimizing thermal spread in the electron gun. Point-to-point resolutions in the sub-3 Å range will be illustrated for the hollandite structure. Image matching, including the effects of both beam divergence and chromatic aberration, showed that the experimental images could be fitted using 2 Å objective aperture, with no divergence and defocus ripple of only 50 Å. In fact, these stringent experimental requirements must be met if useful structural information is to be obtained in the 2-3 Å range.
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    Polytypisme du carbure de silicium: localisation et structure de polytype 102R basé sur la séquence (34) (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 676-677 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Reflexion electron diffraction (RHEED) patterns have been made on a SiC crystal fracture, showing a 102R polytypic modification on a 6H matrix. Its Zhdanov sequence can be written as |(34)433]3. The transition between 6H and 102R polytypes appears rather complex and can be explained with some typical diagrams.
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    Structural phase transition in the spinel MgA12O4 (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 678-678 
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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 crystallographic phase transformation in MgAl2O4 is reported in which the low-temperature low-symmetry space group of MgAl2O4 transforms to the 'spinel' space group Fd3m at ̃450 °C. The in situ electron diffraction experiments suggest that this is a second-order phase transformation.
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    Establishment of a President's Fund (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 685-685 
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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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    Verification of crystal elastic anisotropy theory by ultrasonic diffraction experiments (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 704-705 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: This paper points out an indirect but convincing experimental verification of certain theories concerning elastic-wave propagation in anisotropic media. Relations between the Poynting vector and the propagation vector for longitudinal wave propagation near, but not exactly parallel to, certain pure-mode axes permit the calculation of the ultrasonic diffraction from large single apertures (transducers) oriented for propagation exactly along the pure-mode axes. Then the diffraction loss versus distance, measured in oriented single crystals, permits the verification of the theories concerning these relations and concerning ultrasonic diffraction. It is shown that five items of theory have been verified by the ultrasonic diffraction measurements.
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    Continuous distribution of dislocations in intercrystalline boundaries (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 730-739 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The structure of a dislocation network in a crystal boundary depends, among other parameters, on the two crystal structures and their relation, i.e. the linear transformation leading from one crystal lattice to the other. For the same transformation, the structure of the network is related to the crystal structure. The link between the transformation and the many possible dislocation networks is described by a theorem which states that the displacement field of the linear transformation in the boundary can be described in an infinite number of ways by continuous dislocation distributions. The discrete dislocations are then obtained by grouping the continuous dislocations. The O-lattice theory is discussed in relation to these new aspects, particularly with respect to the features which tend to be conserved in the boundary. A special discussion is given of the case where a common crystallographic axis, without the relaxation pattern being periodic, represents the preferred state.
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    Electron diffraction study of short-range-order diffuse scattering from disordered binary alloys (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 784-788 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A qualitative study was made by electron diffraction of the short-range-order diffuse scattering from disordered Au-Pd, α phase Au-Zn, and α phase Ag-Mg alloys. Twofold and fourfold splittings of diffuse maxima are observed at 100, 110 and equivalent positions on the diffraction patterns with the incident beam parallel to [001], in the composition ranges from about 28 to 62 at.% Pd for Au-Pd, from about 7 to 24 at.% Zn for Au-Zn and from about 11 to 28 at.% Mg for Ag-Mg systems. The separation of the split diffuse maxima increases monotonically with Pd, Zn or Mg content. The results can be interpreted with the Fermi-surface-imaging concept proposed by Krivoglaz. The variation with composition of the Fermi wave vector parallel to the [110] directions is discussed with the nearly-free-electron and the rigid-band models. The latter model is also applied to the experimental results for disordered Cu-Pd and Cu-Pt alloys reported by the present authors [Ohshima & Watanabe (1973). Acta Cryst. A29, 520-526].
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    Multilayer neutron monochromators (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 805-813 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Thin-film multilayer monochromators have been fabricated and studied by neutron diffraction. Alternating thin films of two materials, such as Mn and Ge, constitute a one-dimensional crystal owing to the contrast in their scattering amplitude densities. The following are novel features of such a monochromator: (1) large and adjustable d spacing, (2) very high reflectivity, (3) adjustable bandwidth, Δλ/λ. Both kinematical and dynamical theories have been developed to give a proper understanding of the properties of a multilayer. Since a deviation from perfect periodicity can have a substantial effect on diffraction properties of a multilayer, a 'disorder factor' has been introduced in the dynamical theory. Finite resolution of the spectrometer has been taken into consideration when the experimental data are compared with the theoretical expressions. These multilayers can also be used as excellent polarizers and filters for neutrons.
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    The calculation of electron diffraction intensities by the multislice method (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 844-846 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The calculation of wave functions of scattered electrons by the multislice method of Cowley and Moodie with a finite number of beams is shown to lead to the solution of a finite, closed set of differential equations in the limit that the slice thickness approaches zero. The solution is normalized but differs from the exact wave function unless sufficient beams are included in the calculation. Hence, normalization is not sufficient to ensure that the computed wave function equals the exact wave function. The implications of this result for numerical work are discussed.
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    On the conformational varieties of 5-methoxy-N,N-dimethyltryptamine and serotonin in crystalline environment (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 885-889 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Conformational energy maps obtained from quantum-mechanical computations for two indolalkylamines (5-methylbufotenine and serotonin) indicate preferred conformers which do not correspond to those observed in some crystals containing these molecules. Moreover, two different conformations are observed for serotonin in two different mixed crystals [serotonin creatinine sulphate (hydrated) and serotonin picrate]. These facts suggest an influence of the crystalline environment upon intramolecular conformation. The intermolecular lattice energy has been computed for the experimental crystals and a number of hypothetical ones, obtained by varying the conformation of the indolalkylamine molecule. The results obtained establish complete agreement with the experimental observations: for the three crystals considered, the lattice energy is much lower when use is made of the corresponding observed conformation and these differences between the lattice energies largely overcompensate the variations of the intramolecular conformational energy, thus explaining the role of the lattice energy in the determination of the conformations observed in the crystals.
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    The early history of the International Union of Crystallography (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 1-3 
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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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    Computer science and crystallography (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 4-4 
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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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    Algorithms and computational complexity (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 5-12 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Computer programming was and, in many cases, still is an art rather than a science. Programs are often written without the benefit of any design theory, analysis techniques, or awareness of what others in the field have done. Recently, however, a systematic body of knowledge concerning the design, analysis, and implementation of computer algorithms has begun to emerge. This paper highlights some current developments in this field and shows how proper design techniques can lead to order-of-magnitude improvements in program performance.
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    A correlation between crystallographic computing and artificial intelligence research (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 13-18 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Artificial intelligence research, as a part of computer science, has produced a variety of programs of experimental and applications interest: programs for scientific inference, chemical synthesis, planning robot control, extraction of meaning from English sentences, speech understanding, interpretation of visual images, and so on. The symbolic manipulation techniques used in artificial intelligence provide a framework for analyzing and coding the knowledge base of a problem independently of an algorithmic implementation. A possible application of artificial intelligence methodology to protein crystallography is described.
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    Criteria for selecting and interconnecting micro, mini, midi and maxi computers (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 20-24 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Four commonly accepted computer classes today are: micro, mini, midi and maxi. The dominant parameter which gives rise to this categorization is price, which varies from a ten dollar microprocessor chip to a ten million dollar maxi. This paper attempts to identify other parameters associated with each class in order to provide a mechanism for determining optimization of computer use. Trends towards increased parallelism at both ends of the spectrum are discussed. Finally, a method is described for interconnecting all classes of computers and peripherals at a site to form an installation network.
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    Automated diffractometry (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 18-20 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A review of the status of automated crystallographic measurement has shown that, other than in the mechanics of data measurement for structure determination, little research use is being made of the power of algorithmic, programmed control diffractometers. Rather, existing computer-based systems are being used largely as non-interactive sequencers. This appears to be due to a lack of a strongly perceived need coupled with frustrations over the awkwardness of the man-machine interface, weaknesses in the programming languages, and doubts concerning the basic utility of automated diffractometry. However, possibly valid applications are outlined for such studies in crystal physics as those dealing with radiation damage, handling measurements near phase transitions, studying extinction in small crystals, sensing the unexpected, and detecting and eliminating artifacts. The lowered cost of computing equipment and more widespread use of higher-level languages are suggested as improving the future prospects for automated diffractometry.
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    Study of a mixed-polytype crystal of silver iodide containing a new 12H polytype (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 25-28 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A mixed-polytype crystal of AgI is studied by X-ray diffraction. The constituent polytypes are found to be the common 2H and 4H varieties plus a new 12H polytype, and their proportions are determined. The stacking sequence of the 12H polytype is 1211111121 (Zhdanov notation). Disorder-diffuse scattering observed in the diffraction pattern is attributed to intrinsic-2h stacking faults in 2H regions of the crystal. The degree of disorder corresponds to a fault probability α = 0.15.
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    Ordering of hydrogen in niobium hydride phases (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 208-211 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The ordering of H in α, β, γ and δ-niobium hydride phases can be classified on the basis of electrostatic forces. In α and α'-NbH the H atoms should not be distributed statistically on all tetrahedral sites of the b.c.c. Nb lattice, but should prefer short-range ordering. In agreement with the structure determination, the Madelung energy of orthorhombic β-NbH increases at a positive deviation from the 90° Nb lattice, whereas in γ-NbH the maximum energy is at 90°. NbH2 has a higher Madelung energy in the fluorite structure than in structural models based on the b.c.c. Nb lattice.
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    A structure factor equation for structures consisting of point scatterers with known minimum distance (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 365-367 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: For unitary, normalized and neutron structure factors the equation 0 = \sum_{\bf m} A(h, m)F(m)F(h - m) with A(h, m) = F(m, β11)F(h - m, β12)F(h, β21β22) - F(m, β21)F(h - m, β22)F(h, β11β12) is valid, where the F(m, βμv) are the form factors of arbitrary artificial density functions βμv, which are not more extensive than a sphere of diameter equal to the minimum atomic distance of the structure.
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    Evidence for the Fankuchen effect in neutron diffraction by curved crystals (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 360-365 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Experimental evidence of the Fankuchen effect for neutron diffraction by curved crystals is presented and discussed. The Fankuchen effect consists of a space condensation of neutrons (or X-rays) diffracted by an asymmetrically cut crystal. Theoretical considerations are presented on the Fankuchen effect in the case of ideal mosaic crystals, perfect crystals and ideal curved crystals. This analysis shows that mosaic crystals behave quite differently from perfect crystals. Whereas for perfect crystals the asymmetric cut is expected to induce a real gain in neutron current density, for mosaic crystals no gain is expected. Curved crystals are expected to behave in a similar way to perfect crystals. The experiments were carried out on chemically curved Si crystals. The neutron-diffraction characteristics of a symmetrically cut Si crystal and an asymmetrically cut Si crystal were compared. A gain in the neutron current density of a factor of 4 was observed in the case of the asymmetric crystal. This value is in good agreement with the estimated theoretical value of 4.2. This fact could lead to interesting applications in the selection of monochromatic beams to be used in neutron diffractometry of small biological crystals.
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    Theory of neutron diffraction in mosaic crystals (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 373-381 
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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 the Darwin equations, which describe the effect of multiple Bragg reflexion in mosaic crystals and have previously been obtained by heuristic arguments, arise naturally in a systematic solution of the neutron transport equation. With a method similar to that developed for calculating multiple-scattering corrections for neutron inelastic scattering in liquids, an approximate expression for the secondary extinction factor is obtained, which is applicable to a crystal of arbitrary size and shape. The results are compared with exact results for a plane slab, with Hamilton's [Acta Cryst. (1963), 16, 609-611] results for a cylinder and with Becker & Coppens's [Acta Cryst. (1974), A30, 129-147] results for a sphere.
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    Probability distribution of anomalous phase angles in a centrosymmetric crystal (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 298-300 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Theoretical expressions for the complementary cumulative function of the anomalous phase angle αA are worked out for three cases: a centrosymmetric crystal containing 1, 2 and 4 anomalous scatterers in the unit cell besides a large number of similar normal scatterers. The results are used to study the influence of the number of anomalous scatterers on the distribution of αA. It is found that neglect of αA in the computation of the final difference map will lead to the largest spurious effects when the number of anomalous scatterers in the unit cell is least, namely, the P = 1 case (the other conditions such as k and σ21 being similar).
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    Theoretical approach to the derivation of condensed models of crystal structures based on square-type layers (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 395-398 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Condensed models of crystal structures based on the stacking of equal square-type layers corresponding to the three possible symmetries (plane groups p4m, p4g and p4) are studied in a general way for the regular stacking modes. The minimum set of standard sheets required to represent any structure based on each layer symmetry is derived by considering transparent sheets, either square or standard rectangular in shape. In the latter case four sheets are necessary for p4m and p4g patterns, and eight sheets for p4 patterns. An example of a p4g layer occurring in the CuAl2 and TISe structures is presented.
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    On centrosymmetric space groups in close-packed MX2-type structures (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 255-256 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Shortcomings in the conditions imposed on Zhdanov sequences for close-packed MX2-type structures to be centrosymmetric have been pointed out and the correct conditions derived. Accordingly, the space groups of six CdI2 and two PbI2 polytypes, wrongly reported earlier, have been rectified.
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    Relations entre la symétrie moléculaire et le groupe spatial de cristallisation: cas des molécules possedant un axe binaire (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 307-310 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Among the chiral molecules which crystallize as conglomerates, 25% are twofold molecules. Some simple packing hypotheses bounded by molecular symmetry permit one to select, with Belov's plane groups, some probable space groups. Only the cases of the primitive lattice and monoclinic and orthorhombic systems are treated.
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    Elimination of the finite-range effect on the block-size distribution from the Fourier transform (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 323-326 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A diffraction profile can be recorded for only a finite range around the peak. This leads to spurious oscillations in the block-size distribution determined by means of the Fourier transform. The Bertaut [Acta Cryst. (1952), 5, 117-121] correction gives an approximate distribution g1(m) related to the real one g(m) by a convolution product with a function d(m), of the type (sin x)/x. A modified variant of the successive-convolution unfolding method is proposed by considering the convolution product only over a finite size range where the distribution could be non-zero. The method was tested for two hypothetical distributions. A procedure to set the limits of the size interval is suggested.
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    A general dynamical theory of the X-ray Laue diffraction from a homogeneously bent crystal (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 311-319 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: General properties of the rigorous solution of the problem of the X-ray dynamical Laue diffraction from a bent crystal are discussed. The solution based on the Takagi equations is formulated in the form of the Huygens-Fresnel principle. Special attention is paid to the behaviour of the quasi-classical asymptotic expansion of the exact dynamical wave fields and to the comparison of the results with those obtained by the eikonal theory. The eikonal theory is developed as a result of the application of the stationary-phase method for the evaluation of the Kirchhoff integral in accordance with the Huygens-Fresnel principle. The focusing of X-rays (the intersection of the ray trajectories and the formation of a caustic) is analysed. The generalized eikonal theory when elaborated yields that the integrated diffracted intensity from a bent crystal tends to the correct kinematical value as the strain gradient increases. For practical purposes, a simple numerical method of calculation of the integrated intensity applied in a general case of asymmetric Laue diffraction is proposed.
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    Elastic, thermoelastic, and dynamic piezoelectric properties of trigonal potassium bromate (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 320-322 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The elastic moduli, their temperature derivatives, and the dynamic piezoelectric constants of trigonal potassium bromate, point group 3m, were measured by ultrasonic methods. The coupling coefficient k, for longitudinal waves which propagate parallel to the threefold axes is 0.58. As the damping coefficient for these waves is extremely small, even at high frequencies up to 100 MHz, trigonal potassium bromate is preferentially suitable for highly efficient generators and sensors of ultrasonic waves. The dielectric constants are relatively small and no disturbing electric conductivity occurs as, for example, in lithium iodate. Therefore this material is superior to most other crystals in its use for such devices. All thermoelastic constants are negative, and it is not possible to cut a crystal with temperature-independent properties.
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    Utilisation de méthodes d'approximation pour l'étude des coefficients du développement en série de Fourier des raies de diffraction X (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 327-333 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: An expression for the A(L) Fourier coefficients of an X-ray diffraction line is developed assuming a Gaussian function for the distribution of the chords, i.e. A(L) ∼ ( 1 - {{L}\over{D}} + {{L^{2}}\over{2\sigma \radic 2\pi D}} + \dots) exp (- 2π2L2Sb2{\bar \epsilon}^{2}_{L} + π4L4Sb4{\bar \epsilon}^{4}_{L} + \dots) = Ap(L)Aε(L). Various applications are then performed in order to give the relative magnitude of each of the terms appearing in A(L). The L2 quadratic term of the Ap(L) that has been neglected in previous works is numerically obtained in the special case of elastically isotropic W. Moreover, the anharmonic {\bar \epsilon}^{4}_{L} term is shown to be 105 less than the m.s. (mean square) strain {\bar \epsilon}^{2}_{L} for the same metal. Hyperbolic behaviour as a function of L is found for these terms. An approximate law for the expression of A(L) is then deduced from calculation of the relative magnitude of each term in its expansion. Such a law may be used in order to calculate the particle size and m.s. strains from one single line profile: A(L) ∼ ( 1 - {{L}\over{D}}) exp (- 2π2L2Sb2(〈{\epsilon}^{2}_{L}〉 - 〈{\epsilon}_{L}〉2) . The results are then compared, by the polynomial expansion approximate method, with the values previously obtained. This relation allows us to calculate the particle size and m.s. strain values in each crystallographic direction that may be experimentally reached. The influence of the elastic anisotropy on the behaviour of m.s. strains for the Cu-Sn and W-Th alloys is then deduced.
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    Identification of the rhombohedral lattice in CdI2 crystals (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 341-341 
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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 method is described by which a rhombohedral lattice may be distinguished from a hexagonal lattice in CdI2 crystals. Two rows of spots, 10. l and 20. l, from the same crystal of a polytype are recorded side by side on the same photograph, are compared and the lattice identified.
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    Magnetic properties of crystals: an alternative method (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 338-340 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A method, based on the group-theoretical concept of the factor groups contained in a composition series, of obtaining the number of the constants required to describe a magnetic property in respect of the 90 magnetic point groups is described. The advantage of the method presented here is that one can enumerate simultaneously the constants needed for the description of the magnetic property for all the point groups involved in a composition series and their magnetic variants. Piezomagnetiism is worked out in detail for one composition series.
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    The molecular replacement method. II. The translation function problem; a new translation function (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 62-70 
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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 translation function, a generalization of the translation function introduced by Rossmann, Blow, Harding, and Coller, is defined. Peaks of this new translation function are related to the orientation and point group of and translation vectors between molecules in a crystal. A method is formulated to determine all peaks of this new translation function corresponding to a given crystal. The applicability of this method is shown in the analysis of translation function data to determine the translation vectors between molecules, and to determine the point group of the molecules when the solution of the rotation function problem is not unique. This new translation function has an advantage over the translation function introduced by Rossmann and co-workers, in that it may contain peaks which are not streaked. It is proposed that this new translation function be used in the structure determination of bovine liver catalase. For trigonal crystalline bovine liver catalase it is shown how this new translation function can be used to determine the point group of and the translation vectors between the catalase molecules.
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    X-ray diffraction study of the structure of 1-propanol at –25°C (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 433-437 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Molybdenum Kα X-radiation, monochromated by an NaCl single crystal, was scattered in 1-propanol at -25°C. The angular distribution of the scattered X-ray intensity was determined and used for the calculation of the experimental pair function. On the basis of some models of the structure of liquid 1-propanol, the theoretical curves of pair functions were calculated and then compared to the experimental curve. The most probable model of the liquid is that of statistically distributed chains of 1-propanol molecules. Such chains are created by intermolecular bonds of the O-H. . .O type, the length of which is 2.65 Å. The angles between the O. . .O bonds have a value between 100 and 110°. In this model the oxygen and equivalent carbon atoms all lie in the same plane, and the carbon atoms of the individual molecules are in parallel planes, inclined at an angle of 80° to the oxygen-containing plane. The inhibited rotation of the (CH2)2CH3 group around the O-C(1) bond and the free rotation of the CH3 group around the C(2)-C(3) bond are possible.
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    The closest packing of equal spheres on a spherical surface (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 98-100 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A table is given of putative solutions to the Fejes problem: to find the maximum value of the smallest angular distance between any two of N points movable on the surface of a sphere. Values of N run without omission up to 27 with six sporadic cases thereafter. Some applications of this system as a model are discussed.
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    Approximations for the calculation of high-resolution electron-microscope images of thin films (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 109-113 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Phase-grating calculations were carried out to calculate lattice images from thin crystals. A method for including the effect of chromatic aberration in these calculations is shown and the results are compared with experimental images. The improved agreement of calculations with experiment when chromatic aberration is taken into account is shown.
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    Une nouvelle approche du problème de la phase (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 520-521 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: From the Patterson function Ui vectors can be found, with P(Ui) ̃0. This information with Q(x) ≥ 0 can be used to derive a structure-factor equation of the type: {\bf \sum_{h}}FhFk - h exp ( - 2/πh. Ui) ≡ 0. For a simple model, the Sayre model, we compare the phase information deduced from probability formulae, Sayre's equations and the new relations.
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    Crystal structure and chemical bonding in inorganic chemistry edited by C. J. Rooymans and A. Rabenan (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 349-349 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Photographic neutron diffraction study of L-histidine.HCl.H2O by the modified Laue method (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 649-654 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Film-recorded neutron intensities from modified Laue measurements (a convergent-beam technique) were evaluated quantitatively and a crystal structure was refined. About 4000 reflexions were collected on 60 films in altogether 30 hours exposure of a 0.3 mm3 crystal of L-histidine. HCl. H2O, space group P212121 with a = 15.36, b = 8.92 and c = 6.88 Å. The corrections necessary for this technique are described. The structure factors were derived from the optical densities scanned by an automatic microdensitometer and processed by computer programs. Refinement on 741 symmetry-independent reflexions yielded R(F2) = 0.121. The parameters obtained are compared with those from conventional four-circle work. The mean differences in the positional parameters are 0.009 Å for C, O and Cl and 0.022 Å for H. For the anisotropic temperature factors Uij differences of 0.0054 and 0.0105 Å2 for heavy atoms and hydrogens respectively were observed between the results of the two techniques. A hydrogen atom misplaced in earlier X-ray work was located from film data collected in only 15 hours.
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    Generalized X-ray scattering factors. Simple closed-form expressions for the one-centre case with Slater-type orbitals (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 679-680 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A method for evaluating in closed form the Fourier transform of a product of two atomic orbitals, i.e. the so-called generalized scattering factor, is described for the case where both atomic orbitals are centred on the same atom.
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    Notes and News (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 685-685 
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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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    Transformation orientée de l'hydrate 2H2O β en hydrate 2H2O α du nitrate de calcium. Interprétation structurale des formes cristallines des deux hydrates (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 717-722 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The observed habits of crystals of the compounds α-Ca(NO3)2.2H2O and β-Ca(NO3)2.2H2O can be explained in terms of the crystal structure with the Hartman theory. A structural interpretation is given of the growth of α-phase crystals from a β-phase crystal with the [110] directions of the two phases parallel.
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    A study of the electron-density distribution in sodium azide, NaN3 (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 723-729 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The crystal structure of sodium azide, NAN3, has been reinvestigated by X-ray diffraction methods. The electron density of the azide ion as determined using a high-order X-ray refinement is compared with the theoretical density determined in an ab initio molecular-orbital calculation. The wavefunction of the azide ion has been used to calculate improved atomic scattering factors which result in a significant improvement in the refinement. Molecular scattering factors calculated from the wavefunction have been used in rigid-body refinements including librational thermal motion. The molecular refinement results in a significantly better fit than do the atomic refinements. When standard ellipsoidal temperature factors are used false detail is indicated in deformation-density maps. This problem is eliminated when a third-cumulant expansion is used.
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    Expansion of the Madelung parameters of non-cubic structures as a function of the ratio of two lattice parameters (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 819-822 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A new method is described to express the Madelung parameter of a structure with a non-cubic Bravais lattice as a function of a lattice parameter by means of a power series expansion. The method is derived for the case of a cubic lattice which is tetragonally distorted. The generalization to other crystal structures is shown. For illustration the method is applied to the rutile and cadmium iodide structures.
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    Quintets in P\overline{1}: probabilistic theory of the five-phase structure invariant in the space group P\overline{1} (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 829-833 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: It is assumed that a crystal structure in P{\bar 1} is fixed and that the 15 non-negative numbers R1, R2, R3, R4, R5; R12, R13, R14, R15, R23, R24, R25, R34, R35, R45 are also specified. The random vector (h, k, l, m, n) is assumed to be uniformly distributed over the subset of the fivefold Cartesian product W × W × W × W × W of reciprocal space W defined by |Eh| = R1, |Ek| = R2, |El| = R3, |Em| = R4, |En| = R5; (1) |Eh + k| = R12, |Eh + l| = R13, |Eh + m| = R14, |Eh + n| = R15, |Ek + l| = R23 |Ek + m| = R24, |Ek + n| = R25, |El + m| = R34, |El + m| = R35, |Em + n| = R45; (2) and h + k + l + m + n = 0. (3) Then the structure invariant (φ = φh + φk + φl + φm + φn, (4) as a function of the primitive random variables h, k, l, m, n, is itself a random variable, and its conditional probability distribution, given (1) and (2), is derived. The distribution yields reliable estimates for large numbers of quintets φ in terms of the 15 magnitudes (1) and (2).
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    Lattice energy calculations for (C6H5)5M0.5C6H12, M = P, As and Sb: towards an understanding of crystal packing in the pentaphenyl group V compounds (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 898-902 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Semi-empirical calculations of the energy due to intermolecular interactions have been made for crystals of (C6H5)5M. ½C6H12, M= P, As, and Sb. This energy has been calculated as a pairwise sum over nonbonded atoms; energy minimizations have been performed with respect to the unit-cell parameters and molecular positions and orientations. Two sets of potential functions including repulsive and van der Waals terms have been employed; one of the sets also contains r-1 (Coulombic) contributions to the energy. The experimentally determined structures of the arsenic and antimony compounds have been well reproduced, as has the disorder observed for the solvent molecule in the former; it is suggested that this disorder is also present in the latter crystal. A comparison of the energies calculated for these solvates with those computed previously for unsolvated species predicts the stability of (C6H5)5P.½C6H12, for which no crystallographic data have been previously reported. Crystals of this compound have now been identified. These comparisons also show the relative lattice energies of the three possible crystal structures seen in this system (Cc, P{\bar 1}, and the P{\bar 1} cyclohexane solvate considered here) to be the same for all three molecules, suggesting that the molecular packing is not greatly affected by a change in the central group V atom.
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    Structural mechanism of pyroelectricity in tourmaline (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 927-932 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Pyroelectricity in tourmaline, known since antiquity, was ascribed by S. von Boguslawski to a charged, asymmetric, anharmonic oscillator based on the Einstein model of a crystal. His predicted values of the pyroelectric coefficient k were in good agreement with Ackermann's measurements in the range 20-400 K. We have tested Boguslawski's model by refining the structure, at 193 and 293 K, on a sphere of gem-quality elbaite. The pyroelectric effect is due primarily to the asymmetric anharmonic vibrations of O(1), the oxygen atom of point symmetry 3m which has a polar environment. Its centre of gravity moves 0.005 Å from 193 to 293 K. It is the only atom with a displacement well above experimental uncertainty. Its large thermal parameters, which are ten times their standard deviation at both temperatures, clearly invalidate the assumption of an ellipsoidal thermal movement. This probably holds for Na and 0(2), which also have abnormally large temperature factors, but show no significant displacement.
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    Quintets in P1 and related phase relationships: a probabilistic approach (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 944-948 
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    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A probabilistic theory is described which is able to estimate in P{\bar 1} the signs of the quintet invariants. An investigation is carried out on the use of special quintets in order to estimate one and two-phase seminvariants by means of the complementary-invariants method.
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    A general approach to phase relationships: the method of representations (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 933-944 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A theory is described which, for any universal structure invariant or structure seminvariant, Φ, is able to arrange the set of reflexions in shells, each shell a subset of the succeeding one, with the property that Φ may be estimated in terms of the magnitudes constituting any shell. The theory uses the idea that numberless representations exist of any universal structure invariant or seminvariant, each of them able to contribute to the estimate of Φ.
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    An optimized Monte Carlo simulation for diffraction experiments using pulsed neutron sources (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 962-967 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The method of determining corrections to neutron time-of-flight diffraction data by Monte Carlo techniques is costly since a large range of neutron wavelengths must be considered. An optimized simulation is presented in which many wavelengths are considered simultaneously, while paths are traced only once for all wavelengths. Collision positions and scattering angles are selected from cumulative distribution functions representing all neutron wavelengths simulated. The scattered intensity is computed by weighting the simulated paths to account for the probability of their occurrence for each wavelength. The results of a calculation for a vanadium slab are given as an example.
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    About sequences of maximal subgroups: a few answers to a question from Bertaut (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 1007-1008 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: To each subgroup gi of the space group G does not necessarily correspond a derivative structure si (space group gi) of the structure S (space group G). This paper discusses some symmetrical, geometrical and thermodynamical aspects which occur in the real existence of a derivative structure.
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    Further notes on the generalized inverse (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 1010-1011 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Fast algorithms for 'inverting' singular matrices by modified Cholesky procedures produce a non-unique Ar2 generalized reciprocal matrix rather than the generalized inverse A+.
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    Symmetrical cases of simultaneous X-ray diffraction and the Borrmann effect. I. The analysis of photoelectric absorption (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 178-184 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The strong enhancement of the anomalous transmission (Borrmann) effect in the symmetrical cases of simultaneous diffraction of three, four and six waves is considered. The analytical solution of the diffraction problem has been obtained when the Bragg conditions are satisfied exactly. The effect of the suppression of photoelectric absorption is investigated with consideration of both dipole and quadrupole interactions.
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    Radiation damage of an organic crystal (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 198-200 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A study has been made of the kinetics of radiation damage of copper hexadecachlorophthalocyanine with an electron microscope. It is considered that cross-linking between neighbouring molecules along [100] or [110] is an initial process for radiation damage. Optical diffraction from the high-resolution micrograph revealed a dynamical deformation of the crystal structure.
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    Advances in structure research by diffraction methods edited by W. Hoppe and R. Mason (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 254-254 
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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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    Solution growth and some physical properties of a new lithium iodate–iodic acid solution (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 261-264 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A method is described for the growth from aqueous solution of Li1 - xHxIO3 single crystals. Unit-cell parameter, low-frequency resistivity and the temperature dependence of the α - β transition for several x values were determined. With an increase in x there is an increase in a0 and a decrease in C0 compared with the lattice constants of LiIO3. The maximum increase in a0 is 0.082 Å and the maximum decrease of C0 is 0.219 Å.
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    A definition of coordination and its relevance in the structure types AlB2 and NiAs (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 226-227 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The largest gap in the list of reciprocal interatomic distances is used to limit the coordination polyhedra. For the structure types AIB2 and NiAs, the polyhedra are evaluated according to c/a. In plots of c/a versus rA/rB, the various coordination types correspond to actually observed families of representatives. These results as well as others from more than 50 structure types support the view that coordination as limited by the largest gap is a realistic factor.
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    Values of ε for obtaining normalized structure factors (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 227-229 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The quantity ε, which is necessary to calculate the normalized structure factor E(h) for a reflexion, is equal to the ratio of the number of symmetry-equivalent positions in a unit cell to the number of point-group equivalent (not Laue-group equivalent) reflexions. Values of ε are tabulated for all point groups.
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    Dispersion corrections for X-ray atomic scattering factors (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 229-230 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Recently tabulated values of dispersion corrections for X-ray atomic scattering factors are misleading since they imply an unrealistically high reliability and do not take into account their variation with sin θ/λ, which can in general be quite significant.
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    International Union of Crystallography. Commission on Crystallographic Apparatus and Commission on Charge, Spin and Momentum Densities. A critique of Zachariasen's theory of extinction (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 232-234 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A description and assessment of the theory and application of Zachariasen's theory of extinction [Zachariasen (1973). Acta Cryst. 23, 558-564] is given.
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    International Union of Crystallography. Commission on Crystallographic Apparatus and Commission on Charge, Spin and Momentum Densities. Extinction corrections: theory and experiment. Results of γ-ray diffractometry (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 235-243 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The difficulties in relating any diffraction theory to the defect structure of 'real crystals' used in crystallography are discussed qualitatively and it is shown that most of these problems become less severe in structure factor measurements by means of γ-ray diffractometry. Probably more direct information on the defect structure of the samples used in accurate diffraction experiments is needed in order to improve the situation. At present and probably for the near future it seems best to design experiments where extinction is only of the order of 10% or less.
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    World Directory of Crystallograhers (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 250-250 
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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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    Notes and News (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 251-251 
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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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    Dynamical contrast of the topographic image of a crystal with continuous X-ray radiation. II. A theoretical study of the polychromatic interference fringes (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 418-423 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A theoretical analysis is given of the contrast of the topographic image of a perfect crystal with continuous radiation. Conditions for observation of polychromatic interference fringes in this diffraction scheme are analysed on the basis of Kato's dynamical theory of spherical-wave diffraction. The effect of polarization and azimuthal divergence of X-rays on the contrast of topographs is also discussed.
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    Dynamical contrast of the topographic image of a crystal with continuous X-ray radiation. I. An experimental observation of polychromatic interference fringes and their application for the investigation of the anomalous scattering of X-rays by perfect crystals (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 412-418 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Contrast is investigated of topographic images obtained on perfect plane-parallel and wedge-shaped crystals with continuous X-ray radiation. Quasiperiodic variation of intensity was observed on the topographs of plane-parallel wafers in the direction of the change of wavelength. It is experimentally shown that these oscillations of intensity are interference fringes. An analysis is given of the formation of polychromatic interference fringes from wedge-shaped wafers. A method is suggested on the basis of this analysis for determining the atomic scattering factor over a wide range of wavelengths. The method was used for the determination of atomic scattering factors of Si and Ge. The phenomenon of anomalous scattering at the K absorption edge of Ge was experimentally observed.
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    The use of structure factors to find the origin of an oriented molecular fragment (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 423-425 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: When a molecular fragment has a known orientation, it is possible test rapidly all possible positions in the unit cell for this fragment by calculating structure factors at each position for a moderately large number of reflexions. A program, LOCOFOROM, has been written for this purpose and application to two structures is discussed.
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    Temperature factors for internuclear density units. I. Theory in the harmonic approximation (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 426-429 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Expressions for the temperature factors of internuclear density units are derived in the harmonic approximation of lattice dynamics. The vibration tensors of internuclear density units are a linear combination of the vibration tensors of the adjacent nuclei and the corresponding coupling tensors. The coupling tensors cannot be determined by diffraction experiments but must be determined by other means. For molecular crystals, the coupling tensors for the internal modes can be calculated from a complete force-constant matrix, and the coupling tensors for the external modes can be determined by applying the model of rigid-body motions.
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    An interactive model-building program for macromolecules (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 497-499 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A computer model-building program (suitable for interactive use on a small machine) is described. The program fits a polymer chain with idealized stereochemistry to a number of guide coordinates, by rotation about single bonds. It has been used to record accurate atomic coordinates from a skeletal wire model of a protein in a fraction of the time which manual measurement would have taken.
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    Efficient structure-factor calculation for large molecules by the fast Fourier transform (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 486-492 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A method is presented for calculating structure factors by Fourier inversion of a model electron density map. The cost of this method and of the standard methods are analyzed as a function of number of atoms, resolution, and complexity of space group. The cost functions were scaled together by timing both methods on the same problem, with the same computer. The FFT method is 3½ to 7 times less expensive than conventional methods for non-centrosymmetric space groups.
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    Symmetry operations for displacively modulated structures (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 493-497 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Displacive modulation is defined as periodic distortion with an incommensurable k vector. If the phase of the distortion is t, a symmetry operation can be expressed as a normal space-group operation combined with a sign reversal and/or a shift in the variable t. Earlier results obtained by de Wolff [Acta Cryst. (1974), A30, 777-785] are reformulated in terms of these operations. The periodic functions defining the displacements of two symmetry-related atoms are shown to be related by a simple equation. Applications to published structures are given. The validity of this equation depends on a symmetry-adapted choice of the vector k. It is shown by a two-dimensional example that there are cases for which that choice of k requires the introduction of 'improper symmetry translations' with an extra t shift of ½ or ½. The ensuing Bravais lattice types are similar to those used for the description of magnetic symmetry.
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    Error analysis in the biophysical applications of a flatbed autodensitometer (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 500-508 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The positional, electronic, and density accuracy of the Syntex AD-1 flatbed autodensitometer is discussed. The standard deviation of optical density measurements due to electronic noise is ± 0.001 optical density units at an optical density of 0, rising to ± 0.04 at an optical density of 2. No deviation from linearity in optical density measurements could be measured. During scanning, the standard deviation in position is less than 1.5 μm. Film grain noise for various films is compared and the effect of signal averaging on this noise is studied. This scanner is compared with other scanners available for accuracy of data collection from precession films.
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    Remarques sur les coreprésentations des groupes ponctuels magnétiques (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 509-511 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The derivation of the representations of a simple or double point group G+ is compared with that of the corepresentations of a non-trivial magnetic group G+m, isomorphous with G+, starting from the representations of their common invariant subgroup H+ of index two.
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    Two papers on the calculation of ε for obtaining normalized structure factors (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 519-519 
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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 pointed out that εII, as defined by Iwasaki & Ito [Acta Cryst. (1977), A33, 227-229] is equal to εSK, as defined by Stewart & Karle [Acta Cryst. (1976), A32, 1005-1007], multiplied by L, the multiplicity of the chosen unit cell relative to a primitive one, i.e. εII = εSKL.
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    The use of structural information in the phase probability of a triple product (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 100-106 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A derivation is given of the probability distribution of the phase of a triple product φ = φh1 + φ = φh2 + φ = φh3 with h1 + h2 + h3 = 0, employing a priori structural information. This derivation is valid if normalized group scattering factors are small and certain conditions for h1, h2, h3 are fulfilled. To derive this distribution it is necessary to regard the atomic position vectors as primitive random variables, not all independent in view of the structural information. It is also shown that if no structural information is available the expression for the probability distribution of the phase of a triple product, where the atomic position vectors are regarded as the primitive random variables, is identical to the one where h1, h2, h3 are regarded as the primitive random variables. In the first case certain conditions for h1, h2, h3 must be fulfilled; in the second the atomic position vectors are subject to certain conditions.
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    New algebraic and probabilistic relations between structure factors and direct space information (1977)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 33 (1977), S. 119-125 
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    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Further investigation of the Karle-Hauptman determinants leads to several new relations: the DN determinant is simply related to interatomic vectors; also obtained is a probabilistic restriction on the atomic positions inside the algebraically allowed regions, as defined by von Eller. On the other hand new Gram determinants are defined and shown to be useful in relating moduli, phases and interatomic vectors. A practical application is given of fitting a stereochemically known fragment in the unit cell.
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