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

Rare-earth tungsten bronzes, RExWO3, 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 ? (subcell), while the electron microscopy study indicated a lowering of the subcell symmetry and a complex superstructure. The upper phase composition limit, x< or =0.25, for the RExWO3 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 WO3 structure with the rareearth ions. A hypothetical model of the superstructure based on the contrast features in the HRTEM images has been deduced. The relationships between the RExWO3 bronzes formed by solid-state synthesis under high- and ambient-pressure conditions are presented.