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 Ba2Bi3+Bi5+O6 (dibarium dibismuth hexaoxide) and Ba2BiSbO6 (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 P21/n, monoclinic in I2/m, rhombohedral in R\bar 3, and finally cubic in Fm\bar 3 m. The low-temperature monoclinic structure has been determined for the first time. The transitions from P21/n to I2/m, at 132 K, and R\bar 3 to Fm\bar 3m, at 820 K, are tricritical in nature; the transition from I2/m to R\bar 3 at ca 430 K is discontinuous. The behaviour of Ba2BiSbO6 is very similar, except that the transition temperatures are lower – 250 K for I2/m to R\bar 3 and 515 K for R\bar 3 to Fm\bar 3m – and the low-temperature structure is not formed at all. The R\bar 3 to Fm\bar 3 m transition in this compound is closer to second order in nature, although there is evidence for some contribution from higher-order terms.