Intermediate phases and pseudophases in the system WO 3Nb 2O 5: Tetragonal tungsten bronze phases

Abstract

Ordered intermediate phases with finely graded compositions are found in the Nb 2O 5WO 3 system: homologous series of crystallographic shear phases for x in MO x ( M = Nb + W) > 2.9; a succession of block structures and intergrowth structures for 2.5 ⩽ x ⩽ 2.65. The intermediate range has pentagonal tunnel structures partly based on the tetragonal tungsten bronze network. Ordered filling of some rational but variable, fraction of tunnel sites, in principle, could generate a new closely graded series of phases. Synthetic experiments and lattice imaging electron microscopy have been applied to this system. At least for the experimental conditions employed, the structure of the congruently melting 4Nb 2O 5 · 9WO 3, with 4 12 of pentagonal tunnels occupied, dominates the stability field. However, structures with 6 16 sites in a 2 × 2 TTB superlattice, and with 7 20 sites in a 5 × 1 TTB superlattice occupied form quite extended domains in a 4Nb 2O 5 · 9WO 3 matrix. In addition, the structure with 5 16 sites in a 2 × 2 TTB superlattice forms domains in preparations richer in WO 3 than 4Nb 2O 5 · 9WO 3, and this 2Nb 2O 5 · 5WO 3 structure itself shows evidence of ordered intergrowth with the 2Nb 2O 5 · 7WO 3 characterized electron microscopically by Iijima. The status of these ordered domain structures is discussed. Metastable structures may be effectively stabilized by coherent intergrowth with structures that are absolutely stable.