Quaternary Lead–Niobium–Tungsten Oxides Based on the Tetragonal Tungsten Bronze Structure

Abstract

High-resolution electron microscopy and diffraction studies of fully oxidized and reduced Pb–Nb–W–O oxides are presented. The phases observed were superstructures of tetragonal tungsten bronze (TTB), formed by ordering of pentagonal columns (PCs) and pentagonal tunnels (PTs). In fully oxidized preparations, the structure progressed from a quaternary Nb8W9O47 analogue to the K5Nb9W2O31 structure, with an orthorhombic ∼√2aTTB×√2aTTB×cTTB TTB superstructure having two PCs per cell. Electron diffraction determined the true structure as a 2aTTB×2aTTB×2cTTB supercell. With further lead incorporation, lead-filled PTs replaced PCs, forming a quaternary TTB analogue. Reduced quaternary phases were prepared by reacting lead with Nb8W9O47. The structure was initially maintained in the quaternary phase, but converted at high-temperature to the K5Nb9W2O31 type. Excess lead caused the formation of lead-rich interfaces between domains. Reaction of lead with oxides in the ratio 4Nb2O5:9WO3 formed quaternary phases possessing the Nb8W9O47 structure, demonstrating the direct synthesis of mixed PT/PC quaternary “bronze” phases which retained the structure of the oxidized ternary parent.