XRD, EXAFS AND XANES STUDIES OF NEW BISMUTH CUBIC OR PSEUDO-CUBIC PEROVSKITE-TYPE PHASES

New bismuth cubic or pseudo-cubic perovskite type phases have been pre pared by solid state reactions. Cubic symmetry (Fm $($) over bar$$ 3m) was observed for Ba-2(A(x)Bi(2-x))O-6-x/2 (A = Sr2+, Ca2+) compounds, except for Ba2Sr1/3Bi5/3O5.83 compositions where the structure crysta llizes with monoclinic symmetry (I2/m). Creation of oxygen vacancies c ontributing towards a gradual contraction of the unit cell, are compet ing with Ca2+ or Sr2+ substitutions regarded as the partial replacemen t of Bi3+ leading to an increase of the unit-cell. The XRD patterns of the Ba-2(RuxBi2-x)O-6 (x = 1/2, 2/3) pseudo-cubic solid solutions are indexed using a body-centered monoclinic cell (I2/m), the unit cell v olume decreasing as the Ru content increases. X-ray absorption spectro scopy measurements of Bi L(III) near edge structure and EXAFS oscillat ions 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 L(III) edges exhibit a continuous evolution with increasing Ca or Sr doping level or with lowering Ru doping. The Bi L(III) edges are do minated by two main features due to 2p(3/2) --> 6d and 2p(3/2) --> 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 unambigu ous determination of the highest valence states of these elements and to discriminate between Bi3+ and Bi5+. Detailed analysis of XAFS revea ls that with increasing Ca or Sr doping, or with lowering Ru doping, t he average Bi-O distance decreases continuously, in good agreement wit h XANES and XRD analysis.