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 P2(1)/n, monoclinic in I2/m, rhombohedral in R (3) over bar, and finally cubic in Fm (3) over barm. The low-temperature monoclinic structure has been determined for the first time. The transitions from P2(1)/n to I2/m, at 132K, and R (3) over bar to Fm (3) over barm, at 820K, are tricritical in nature; the transition from I2/m to R (3) over bar 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 (3) over bar and 515K for R (3) over bar to Fm (3) over barm-and the low-temperature structure is not formed at all. The R (3) over bar to Fm (3) over barm transition in this compound is closer to second order in nature, although there is evidence for some contribution from higher-order terms.