DEFECT STRUCTURE OF QUENCHED GAMMA-BICOVOX BY COMBINED X-RAY AND NEUTRON POWDER DIFFRACTION

A detailed investigation of the defect structure of the Co doped BIMEV OX solid electrolyte, Bi2V1-xCoxO5.5-3x-2 (x = 0.1 and x = 0.2), quenc hed from high temperature, has been carried out using X-ray and neutro n powder diffraction data measured at room temperature. The structure is built up from alternating layers of [B2O2](2n+)(2) and [V1-xCoxO3.5 -3x/2](2n-)(n) with disorder limited to the vanadate layer. The ideal V/Co co-ordination is octahedral with corner sharing of equatorial oxy gens. The refinements show that the true structure is distorted, with disorder in both apical and equatorial oxygens and oxygen vacancies co ncentrated in the equatorial positions. Detailed analysis of the oxyge n site occupancies reveals two main types of V/Co co-ordination viz. d istorted octahedral and distorted tetrahedral. The majority of the sit es in both compositions are tetrahedral.