Structures and phase transitions in the ordered double perovskites Ba2BiIIIBiVO6 and Ba2BiIIISbVO6

High-resolution neutron powder diffraction has been used to investigate the structures and phase transitions in the double perovskites Ba(2)Bi(3+)Bi(5+)O(6) (dibarium dibismuth hexaoxide) and Ba(2)BiSbO(6) (dibarium bismuth antimony hexaoxide) in the temperature ranges 4.2-973 and 4.2-625 K, respectively. The charge-ordered bismuthate adopts four structures in the temperature range - monoclinic in P2(1)/n, monoclinic in I2/m, rhombohedral in R3, and finally cubic in Fm3m. The low-temperature monoclinic structure has been determined for the first time. The transitions from P2(1)/n to I2/m, at 132 K, and R3 to Fm3m, at 820 K, are tricritical in nature; the transition from I2/m to R3 at ca 430 K is discontinuous. The behaviour of Ba(2)BiSbO(6) is very similar, except that the transition temperatures are lower - 250 K for I2/m to R3 and 515 K for R3 to Fm3m - and the low-temperature structure is not formed at all. The R3 to Fm3m transition in this compound is closer to second order in nature, although there is evidence for some contribution from higher-order terms.