Phosphate tungsten bronze series: crystallographic and structural properties of low-dimensional conductors

Phosphate tungsten bronzes have been shown to be conductors of low dimensio nality. A review of the crystallographic and structural properties of this huge series of compounds is given here, corresponding to the present knowle dge of the different X-ray studies and electron microscopy investigations. Three main families are described, monophosphate tungsten bronzes, A(x)(PO2 )(4) (WO3)(2m), either with pentagonal tunnels (MPTBp) or with hexagonal tu nnels (MPTBh), and diphosphate tungsten bronzes, A(x)(P2O4)(2) (WO3)(2m), m ainly with hexagonal tunnels (DPTBh). The general aspect of these crystal s tructures may be described as a building of polyhedra sharing oxygen corner s made of regular stacking of WO3-type slabs with a thickness function of m , joined by slices of tetrahedral PO4 phosphate or P2O7 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 WO6 octahedra inside a slab of WO3-type. In this respect, a comparison with the different phases of the WO3 crystal structures is es tablished. The various modes of tilting and the different possible connecti ons between two adjacent WO3-type slabs involve a great variety of structur es with different symmetries, as well as the existence of numerous twins in MPTBp's. Several phase transitions, with the appearance of diffuse scatter ing and modulation phenomena, were analysed by X-ray scattering measurement s and through the temperature dependence of various physical properties for the MPTBp's. The role of the W displacements within the WO3-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.