THE CRYSTAL-CHEMISTRY OF THE HIGHER TUNGSTEN-OXIDES

The crystal chemistry of binary and ternary tungsten oxides with overall compositions between MO(3) and approximately MO(2) is reviewed. The chapter focuses primarily upon the reduced oxides and their fully oxidised analogues and descriptions of the structures have been employed which stress the linkage of component metal oxygen polyhedra. The stable form of the parent oxide WO3 consists of corner-linked WO6 octahedra. Reduction by way of the addition of metal atoms which can occupy the cage sites between these octahedra leads to the formation of the perovskite structure tungsten bronzes. Slight reduction of the binary oxide with medium sized metal atoms produces compounds containing units of edge-shared octahedra. These materials form the crystallographic shear (CS) phases. At greater degrees of reduction with similar metals pentagonal bipyramidal WO7 polyhedra are utilised to form a variety of pentagonal column (PC) phases. This group includes the tetragonal tungsten bronze (TTB) structure and derivatives thereof. Low temperature preparations have lead to the formation of two other WO3 structure types based upon rings of corner-sharing octahedra, one of which is the hexagonal tungsten bronze framework and the other the pyrochlore framework. These can be considered as parent structures for the high temperature hexagonal tungsten bronze (HTB) phases, the intergrowth tungsten bronze (ITB) phases and a number of ternary phases with structures related to that of pyrochlore. An extensive group of phases, the so-called 'phosphate tungsten bronzes', are ternary oxides containing phosphate groups, rather than bronzes. They are described in terms of the octahedral and tetrahedral polyhedra from which they are built up and their relationship to Mo4O11 structure indicated. A concluding section considers some aspects of the stability and reactivity of these phases.