Inorder to provide anaccurate basis forthe interpretationofthe physical properties of this complex incommensurate system, the structure of the room temperature quasicommensurate phase of barium sodium niobate is determined by taking into account. for the first time, the satellite reflections present in the diffraction spectrum of this material. The quasi-commensurate phase considered is assimilated to a strictly commensurate phase, the satellite reflections being considered as superlattice reflections and referred to a crystalline supercell with a = 35.187 A. b = 17.620 A and c = 7.987 A. This approximation gives an excellent agreement index R = 0.031 for the fit to the intensities of the 2432 basic reflections andthe 1691satellitereflectionscollectedat294 K(AMo Ka = 0.71069A,p, = 111.47cm.'. space group : Bbm2). The structural resultsshow that the commensurate modulation consists of a tilting and a shearing of all the NbO, octahedra in the structure in contrast to previous inferences which had only suggested the occurrence of a shear. The framework of the structure may be described on the basis of the existence of two different Nb50?4 units of five corner-sharing octahedra each. The structure obtained by spatially averaging the modulation is consistent with previous structural studies of the basic reflections of this material. Physical characteristics pertaining to the polar character, or the domain structure, are also deduced.

Phosphate tungsten bronzes have been shown to be conductors of low dimensionality. A review of the crystallographic and structural properties of this huge series of compounds is given here, corresponding to the present knowledge of the different X-ray studies and electron microscopy investigations. Three main families are described, monophosphate tungsten bronzes, A x (PO 2 ) 4 (WO 3 ) 2m , either with pentagonal tunnels (MPTBp) or with hexagonal tunnels (MPTBh), and diphosphate tungsten bronzes, A x (P 2 O 4 ) 2 (WO 3 ) 2m , mainly with hexagonal tunnels (DPTBh). The general aspect of these crystal structures may be described as a building of polyhedra sharing oxygen corners made of regular stacking of WO 3 -type slabs with a thickness function of m, joined by slices of tetrahedral PO 4 phosphate or P 2 O 7 diphosphate groups. The relations of the different slabs with respect to the basic perovskite structure are mentioned. The structural description is focused on the tilt phenomenon of the WO 6 octahedra inside a slab of WO 3 -type. In this respect, a comparison with the different phases of the WO 3 crystal structures is established. The various modes of tilting and the different possible connections between two adjacent WO 3 -type slabs involve a great variety of structures with different symmetries, as well as the existence of numerous twins in MPTBp's. Several phase transitions, with the appearance of diffuse scattering and modulation phenomena, were analysed by Xray scattering measurements and through the temperature dependence of various physical properties for the MPTBp's. The role of the W displacements within the WO 3 -type slabs, in two modulated structures (m = 4 and m = 10), already solved, is discussed. Finally, the complexity of the structural aspects of DPTBh's is explained on the basis of the average structures which are the only ones solved.

The incommensurate modulated structure of the high Tc superconductor of nominal composition Bi2Sr2CaCu2O8 has been refined using an optimized data set of 1530 reflections (496 main reflections, 753 first-order and 281 second-order satellites) in superspace group Bbmb(Oβ1) using a Fourier expansion of the modulation functions up to second order. A large improvement of the R (wR) factor [global: 0.056 (0.051); main reflections: 0.049 (0.043); first-order satellites: 0.056 (0.058); second-order satellites: 0.078 (0.079)] is obtained by the introduction of a static modulated disorder on the Bi site without any thermal factor modulation and by considering occupational modulation functions. A detailed comparison with previous studies in modulated structure models or in supercell models is given, in particular for the description of the BiO layers.

ExperimentalThe crystal structure of P4WsO32 has been solved by three-dimensional single-crystal X-ray analysis. The refinements have been made in the space group P212121 with a = 5.285 (2), b = 6.569 (1), c = 17.351 (3)A, leading to R --0.058 and R w --0.071 for 967 independent reflections. The framework of this structure, built up of WO 6 octahedra and PO4 tetrahedra, can be described as ReOa-type slabs connected through PO 4 tetrahedra. This type of connection creates pentagonal tunnels in the [100] direction. Two sorts of empty cages are formed, limited by twelve or eighteen O atoms. The possibility of insertion of cations in the cavities of the structure and the existence of microphases P4W4nO 12n÷ 8 are discussed.