Structures and phase transitions in the ordered double perovskites Ba 2 Bi III Bi V O 6 and Ba 2 Bi III Sb V O 6

• Published in: Acta crystallographica. Section B, Structural science Structural science, 2006-08, Vol.62 (4), p.537-546
• Main Authors: Kennedy, Brendan J., Howard, Christopher J., Knight, Kevin S., Zhang, Zhaoming, Zhou, Qingdi
• Format: Article
• Language: English
• ISSN: 0108-7681
• DOI: 10.1107/S0108768106018842

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 P 2 1 / n , monoclinic in I 2/ 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 P 2 1 / n to I 2/ m , at 132 K, and R\bar 3 to Fm\bar 3m, at 820 K, are tricritical in nature; the transition from I 2/ m to R\bar 3 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 I 2/ 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.