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The displacive ferroelectric

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The displacive ferroelectric BizWO6 [Mr = 697"81, a = 5.4559 (4), b = 5.4360 (4), c = 16.4298 (17) A, Z = 4, Dx=9.512gcm -3, MoKa, A=0.7107A, #= 958-6 cm -l, F(000)= 1151.73], is described at room temperature as a commensurate modulation of an idealized Fmmm parent structure derived from an 14/mmm structure. Transmission electron microscopy clearly showed that there are coherent intergrowths of two distinct modulated variants in BizWO6 crystals. Displacive modes of inherent F2mm and Bmab symmetry are substantial and coherent over a 0108-7681/91/060870-12503.00 large volume. They reduce the space-group symmetry to B2ab. A further substantial displacive mode corresponds to rotation of corner-connected WO6 octahedra about axes parallel to e and has either of two inherent symmetries, Abam or Bbam, the difference being associated with the way this mode propagates along e. The dominant Abam mode reduces the space-group symmetry to P2~ab, while the existence of the Bbam mode reduces the intensity of h + l ---2n + 1 data and acts like a stacking fault. Group theoretical analysis of the problem details how the X-ray data can be classified so as to monitor the refinement. Anomalous dispersion selects the overall

Crystalline (3-methylpentane-3-thiolato)silver is one-dimensionally nonmolecular. Silver and sulfur atoms constitute the cores of chains, which are approximately linear and well separated from each other by the alkyl substituents that radiate from the chains. Each chain contains two separate strands, each with alternating silver atoms and doubly bridging thiolate ligands. All silver atoms are approximately linearly mordimted (SAg-S range 169-178"). A structural feature unprecedented in homoleptic silver thiolate ~t~c t~r e~ is the absence of any seandary silversulfur bonding. Each strand contains approximately planar zigzag S-Ag-S-Ag-S-Ag-S segments, which are connected at their ends by single silver atoms linking segments on one side of the chain to parallel segments on the opposite side of the chain. The two strands of the chain thereby wind around each other at every fourth silver atom (the r? repeat of the crystal) without interstrand bonding. The closest contact between strands is Ag--Ag = 2.886 (4) A. which is interpreted as nonbonding. A straightforward mechanism for interconversion of the nonmolecular crystal structure and the molecular (AgSRh structure in benzene solution is proposed. A zigzag cyclic structure for (AgSR), is postulated on the basis of the crystal structure, Approximately 8% slipfaulting of the chains has been observed and modeled for one of the two crystals investigated. Crystal data: C,H,,Ag,S,, M, 1800.8; a = 12.105 (2). b = 14.289 (2). c = 19.539 (3) A; Z = 2; space group Pi, Cu Ku, 1878 observed reflections, R = 0.061. = 85.77 (I), p = 87.77 (I), y = 86.30