Synthesis, Crystal Structure, and Oxide Ion Conductivity in Bi_4.6Ca_1.1VO_10.5

A novel bismuth calcium vanadate, Bi4.6Ca1.1VO10.5, has been synthesized. Its crystal p structure has been determined by synchrotron X-ray diffraction. The compound crystallizes in orthorhombic space group Pnnm, with cell parameters a = 11.930(2) (A) under circle, b = 16.030(3) (A) over circle, and c = 11.500(2) (A) over circle. The crystal structure can be viewed as consisting of fluorite-like slabs of edge-sharing OA(4) (A = Bi, Ca) tetrahedra that extend along the c axis of the crystal and interspersed disordered VO4 groups. Bi and Ca atoms undergo site disorder, with Bi preferentially occupying locations with low site symmetry and irregular coordination environments. Variable temperature powder diffraction measurements reveal a change in the thermal expansion coefficient of Bi4.6Ca1.1VO10.5 above 600 degreesC and this coincides with the change in conductivity as revealed by a.c. impedance measurements.