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 A (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.