Quaternary lead-niobium-tungsten oxides based on the tetragonal tungsten bronze structure

High-resolution electron microscopy and diffraction studies of fully oxidiz ed and reduced Pb-Nb-W-O oxides are presented. The phases observed were sup erstructures of tetragonal tungsten bronze (TTB), formed by ordering of pen tagonal columns (PCs) and pentagonal tunnels (PTs). In fully oxidized prepa rations, the structure progressed from a quaternary Nb8W9O47 analogue to th e K5Nb9W2O31 structure, with an orthorhombic similar to root 2a(TTB) x root 2a(TTB) x c(TTB) TTB superstructure having two PCs per cell. Electron diff raction determined the true structure as a 2a(TTB) x 2a(TTB) x 2c(TTB) supe rcell. With further lead incorporation, lead-filled PTs replaced PCs, formi ng a quaternary TTB analogue. Reduced quaternary phases were prepared by re acting lead with Nb8W9O47. The structure was initially maintained in the qu aternary phase, but converted at high-temperature to the K5Nb9W2O31 type. E xcess lead caused the formation of lead-rich interfaces between domains. Re action of lead with oxides in the ratio 4Nb(2)O(5):9WO(3) formed quaternary phases possessing the Nb8W9O47 structure, demonstrating the direct synthes is of mixed PT/PC quaternary "bronze" phases which retained the structure o f the oxidized ternary parent. (C) 2001 Academic Press.