XRD, EXAFS and XANES studies of new bismuth cubic or pseudo-cubic perovskite-type phases

Abstract

New bismuth cubic or pseudo-cubic perovskite type phases have been prepared by solid state reactions. Cubic symmetry (Fm3¯m) was observed for Ba2(AxBi2−x)O6−x/2 (A = Sr2+, Ca2+) compounds, except for Ba2Sr1/3Bi5/3O5.83 compositions where the structure crystallizes with monoclinic symmetry (I2/m). Creation of oxygen vacancies contributing towards a gradual contraction of the unit cell, are competing with Ca2+ or Sr2+ substitutions regarded as the partial replacement of Bi3+ leading to an increase of the unit-cell. The XRD patterns of the Ba2(RuxBi2−x)O6 (x = 1/2, 2/3) pseudo-cubic solid solutions are indexed using a body-centered monoclinic cell (I2/m), the unit cell volume decreasing as the Ru content increases. X-ray absorption spectroscopy measurements of Bi LIII near edge structure and EXAFS oscillations have revealed evidence of mixed valencies in these compounds and have given complementary and new information about the environment of bismuth which can be five or six-fold coordinated to oxygen atoms. The Bi LIII edges exhibit a continuous evolution with increasing Ca or Sr doping level or with lowering Ru doping. The Bi LIII edges are dominated by two main features due to 2p3/2 ↦ 6d and 2p3/2 ↦ 6s transtions due to the unfilled 6s states of Bi5+ only. Such a feature which is indicative of the absence of the lone pair allows an unambiguous determination of the highest valence states of these elements and to discriminate between Bi3+ and Bi5+. Detailed analysis of XAFS reveals that with increasing Ca or Sr doping, or with lowering Ru doping, the average Bi−O distance decreases continuously, in good agreement with XANES and XRD analysis.