Acta Crystallographica Section B

Structural Science

Volume 54, Part 4 (August 1998)

Cover illustration: Space filling representation of the hydrated anionic framework formed by trimesic acid around piles of cobalticinium cations in crystalline [(C5H5)2Co][(H3TMA)(H2TMA)].2H2O. Graphics (picture or drawing) obtained using SCHAKAL by E. Keller, University of Freiburg, Germany. Courtesy of D. Braga and F. Grepioni

• research papers
• short communications

research papers

Acta Cryst. (1998). B54, 351-357    [doi:10.1107/S0108768197017047]

-Al_4.5FeSi: A Combined Synchrotron Powder Diffraction, Electron Diffraction, High-Resolution Electron Microscopy and Single-Crystal X-ray Diffraction Study of a Faulted Structure

V. Hansen, B. Hauback, M. Sundberg, Chr. Rømming and J. Gjønnes

Synopsis: A previous single-crystal X-ray and electron diffraction structure study of the heavily faulted monoclinic alloy phase -Al_4.5FeSi has been further refined by synchrotron powder data. Based on high-resolution electron microscopy and electron diffraction the basic structure and superstructures are described in terms of faulted stackings of a tetragonal sub-unit.

Acta Cryst. (1998). B54, 358-364    [doi:10.1107/S0108768197017886]

X-ray Diffraction Study of the Crystal Structure of the Tl Ternary Chalcogenides Tl_2xIn_2(1-x)Se₂, x = 0.2, 0.3,0.9

K. G. Hatzisymeon, S. C. Kokkou, A. N. Anagnostopoulos and P. I. Rentzeperis

Synopsis: Single crystal and powder methods have been used to study a series of thallium ternary chalcogenides which are isostructural with the binary semiconductor TlSe. The unit-cell parameters, cell volume, and In-Se and Tl-Se bond lengths vary with x in the series according to Kurnakov's law.

Acta Cryst. (1998). B54, 365-375    [doi:10.1107/S0108768197013785]

Structural Investigation of P₄W₂₄O₈₀: A New Monophosphate Tungsten Bronze

P. Roussel, G. Mather, B. Domengès, D. Groult and P. Labbé

Synopsis: P₄W₂₄O₈₀, the m = 12 member of the monophosphate tungsten bronze family of the general formula (PO₂)₄(WO₃)_2m, has been synthesized. Resolution of its crystal structure from X-ray diffraction data shows that it crystallizes in the orthorhombic system with unit-cell dimensions a = 5.312(1), b = 6.5557(8), c = 42.196(8)Å and space group P2₁2₁2₁ (final reliability factor R = 0.0284; wR = 0.0272).

Acta Cryst. (1998). B54, 376-383    [doi:10.1107/S0108768197019654]

Structures and Phase Transitions of the A₇PSe₆ (A = Ag, Cu) Argyrodite-Type Ionic Conductors. I. Ag₇PSe₆

M. Evain, E. Gaudin, F. Boucher, V. Petricek and F. Taulelle

Synopsis: The crystal structures of -Ag₇PSe₆ and -Ag₇PSe₆, two polymorphic forms of the argyrodite Ag₇PSe₆ compound, are determined from single-crystal X-ray diffraction data. The silver ionic diffusion paths in the high-temperature form are studied by means of a Gram-Charlier non-harmonic development of the atomic displacement factor. The analysis reveals for silver a preference for various low coordination selenium environments, a key feature for ionic conductivity.

Acta Cryst. (1998). B54, 384-390    [doi:10.1107/S0108768198002614]

Prediction of Early- and Late-Growth Morphologies of Ionic Crystals

L. J. Soltzberg, O. Carneiro, G. S. Joseph, Z. Khan, T. D. Meretsky, M. M. Ng and S. A. Ofek

Synopsis: A systematic variation in morphology is observed between early- and late-growth crystals of six water-soluble ionic compounds. These morphologies have been successfully modeled using Coulomb-only potentials.

Acta Cryst. (1998). B54, 391-398    [doi:10.1107/S0108768198002833]

The Yttrium Oxide Fluoride Solid Solution Described as a Composite Modulated System

S. Schmid

Synopsis: The refinement results for one composition within the solid solution demonstrate that reducing the number of refinable parameters leads to lower R values.

Acta Cryst. (1998). B54, 399-416    [doi:10.1107/S0108768197019642]

Centrosymmetric or Noncentrosymmetric? Case Study, Generalization and Structural Redetermination of Sr₅Nb₅O₁₇

S. C. Abrahams, H. W. Schmalle, T. Williams, A. Reller, F. Lichtenberg, D. Widmer, J. G. Bednorz, R. Spreiter, Ch. Bosshard and P. Günter

Synopsis: Fifteen tests based upon X-ray diffraction data and seven tests based on physical measurement are presented for resolving centrosymmetric/noncentrosymmetric space-group ambiguities. The importance of including all reflections in association with their combined standard uncertainties is demonstrated in the elimination of systematic bias before the tests are applied.

Acta Cryst. (1998). B54, 417-423    [doi:10.1107/S0108768197020090]

Aminoguanidinium(2+) Hexafluorozirconate Monohydrate: A Co-Product of Preparing the Ferroelectric Anhydrous Salt

C. R. Ross II, B. L. Paulsen, R. M. Nielson and S. C. Abrahams

Synopsis: The preparation of anhydrous ferroelectric aminoguani- dinium(2+) hexafluorozirconate is often accompanied by several related materials including the nonpolar monohydrate. The structure of CN₄H₈ZrF₆.H₂O has been redetermined to improve the understanding of the preparation pathway, locate all H atoms and compare the remaining structure with that reported previously.

Acta Cryst. (1998). B54, 424-430    [doi:10.1107/S010876819701776X]

Multicomponent Polyanions. 53. Structure of Tetrakis(trimethylammonium) Tetra--oxo-bis(triaquahexadecaoxo(trioxophenylphosphato)hexamolybdate) Dihydrate, [NH(CH₃)₃]₄[{(C₆H₅P)Mo₆O₂₁(H₂O)₃}₂].2H₂O

D.-G. Lyxell, D. Boström, M. Hashimoto and L. Pettersson

Synopsis: A crystal structure is described in which phosphorus coordinates to a six-membered ring arrangement of MoO₆ octahedra, forming a new type of heteropolymolybdate. The monomeric unit was used to deduce the structures of the (C₆H₅P)Mo₆^2- and (C₆H₅P)Mo₇^4- species found in aqueous solution.

Acta Cryst. (1998). B54, 431-437    [doi:10.1107/S0108768197017771]

Structures of Homoleptic Benzyl Derviatives of Zirconium

C. Tedesco, A. Immirzi and A. Proto

Synopsis: Distortions of homoleptic benzyl derivatives of zirconium from tetrahedral geometry have been noticed and discussed along with crystal packing.

Acta Cryst. (1998). B54, 438-442    [doi:10.1107/S0108768197017862]

Disordered Structure of Polymeric Bismuth Tribenzoate, [Bi(C₆H₅COO)₃]_n

A. D. Rae, G. J. Gainsford and T. Kemmitt

Synopsis: The 1:1 disordered structure of P2₁/m symmetry consists of polymeric chains parallel to the a axis and can be related to an ideally ordered structure of B2₁/a symmetry, with a^' = 2 a, b^' = b and c^' = 2 c. This B-centred structure can itself be described as a modulation of an idealized parent structure of Pnma symmetry. The disorder is rationalized in terms of the pseudo-symmetry that relates the two formula units of Bi(C₆H₅COO)₃ in the asymmetric unit of B2₁/a. Each Bi is nominally nine-coordinate.

Acta Cryst. (1998). B54, 443-449    [doi:10.1107/S0108768197018454]

Refinement of Twinned Structures with SHELXL97

R. Herbst-Irmer and G. M. Sheldrick

Synopsis: Four examples of the refinement of twinned structures are discussed. These structures illustrate three different types of twins: twinning by merohedry, pseudo-merohedry and non-merohedral twins.

Acta Cryst. (1998). B54, 450-455    [doi:10.1107/S0108768197014547]

Channel-Type Crystal Packing in the Very Rare Space Group P42₁2 with Z' = 3/4: Crystal Structure of the Complex -Cyclodextrin-Methanol-n-Hydrate

T. Steiner and W. Saenger

Synopsis: Inclusion complexes of the macrocyclic host molecule -cyclo- dextrin crystallize preferably in the extremely rare space group P42₁2. The packing characteristics of these compelxes are described.

Acta Cryst. (1998). B54, 456-463    [doi:10.1107/S0108768197014821]

Hydrogen-Bond Distances to Halide Ions in Organic and Organometallic Crystal Structures: Up-to-date Database Study

T. Steiner

Synopsis: Geometrical data on hydrogen bonds to halide ions are compiled for 25 donor types. Information on hydrogen-bond directionality is also given.

Acta Cryst. (1998). B54, 464-470    [doi:10.1107/S0108768197018193]

Opening and Narrowing of the Water H-O-H Angle by Hydrogen-Bonding Effects: Re-inspection of Neutron Diffraction Data

T. Steiner

Synopsis: For water molecules donating two Ow-HO hydrogen bonds, the correlation of the covalent H-O-H angle with the OOwO angle is inspected from 49 neutron crystal structures. Compared to the sample average, the water angle is opened for large and narrowed for small OOwO angles.

Acta Cryst. (1998). B54, 471-477    [doi:10.1107/S0108768197014596]

Charge Density Studies Using CCD Detectors: Oxalic Acid at 100 K Revisited

A. Martin and A. A. Pinkerton

Synopsis: The suitability of CCD data for charge density studies has been established by comparison of the results of a new study of oxalic acid dihydrate with those of previous studies.

Acta Cryst. (1998). B54, 478-484    [doi:10.1107/S0108768197015589]

Ab Initio Structure Determination of Ammonium Hydrogen Alkyl Phosphates from X-ray Powder Diffraction Data

A. Neels, H. Stoeckli-Evans, J. Neels, A. Clearfield and D. Poojary

Synopsis: The structures of the ammonium hydrogen alkyl phosphates NH₄PO₂(OH)(OR) [R = Me (1), Et (2), Prⁱ (3)] are characterized by an extended hydrogen-bonding network involving the hydrogen phosphate units and the ammonium ions, forming double layers. These layers are separated from each other by van der Waals contacts between the hydrophobic alkyl groups.

Acta Cryst. (1998). B54, 485-493    [doi:10.1107/S0108768197017953]

Experimental Charge Density and Electrostatic Potential of Triglycine

V. Pichon-Pesme and C. Lecomte

Synopsis: Low-temperature experimental electron density and electrostatic potential of triglycine are described. Charge transfer between the two molecules of the asymmetric unit is discussed in relation to the multipolar refinement strategy.

Acta Cryst. (1998). B54, 494-502    [doi:10.1107/S0108768197018351]

Structural Investigations of the Phase Transitions of Tetramethylammonium Sulfate

M. Malchus and M. Jansen

Synopsis: Two reversible first-order phase transitions of [N(CH₃)₄]₂SO₄ at 263 and 462 K have been established by differential thermal analysis and temperature-dependent X-ray powder diffraction. The crystal structures of the three phases show increasing disorder of the sulfate sublattice with increasing temperature. An examination of group-subgroup relations is made.

short communications

Acta Cryst. (1998). B54, 503-504    [doi:10.1107/S0108768198003401]

Is There Evidence for Non-Linear Thiocyanate or Selenocyanate Ions?

R. E. Marsh

Synopsis: An earlier description, in space group Cc, of the structure of a mixed Cu^I-Cu^II salt led to non-linear conformations for the mixed anions (Se,S)CN. Re-refinement in space group C2/c shows that these anions can be described as linear, as expected.

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