X-Ray and Electron Microscopy Studies of Rare-Earth Tungsten Bronzes

Abstract

Rare-earth tungsten bronzes, RE x WO 3, of perovskite tungsten-bronze (PTB) type, formed by conventional solid-state synthesis, have been studied by X-ray powder diffraction, electron diffraction in combination with microanalysis, and high-resolution transmission electron microscopy (HRTEM). The X-ray patterns indicated cubic PTB type structure with a≈3.83 Å (subcell), while the electron microscopy study indicated a lowering of the subcell symmetry and a complex superstructure. The upper phase composition limit, x≤0.25, for the RE x WO 3 bronzes with PTB-related structures was established from the microanalysis study. Ordered, disordered and microtwinned superstructures were revealed by electron diffraction patterns and HRTEM images taken along 〈110〉 p. The superstructure is due to filling of the interstices in the WO 3 structure with the rare-earth ions. A hypothetical model of the superstructure based on the contrast features in the HRTEM images has been deduced. The relationships between the RE x WO 3 bronzes formed by solid-state synthesis under high- and ambient-pressure conditions are presented.