Synthesis, structures, and phase transitions of barium bismuth iridium oxide perovskites Ba sub(2)BiIrO sub(6) and Ba sub(3)BiIr sub(2)O sub(9)

The Ba-Bi-Ir-O system is found to contain two distinct perovskite-type phases: a rock-salt ordered double perovskite Ba sub(2)BiIrO sub(6); and a 6H-type hexagonal perovskite Ba sub(3)BiIr sub(2)O sub(9). Ba sub(2)BiIrO sub(6) undergoes a series of symmetry-lowering phase transitions on cooling [MathML equation], all of which are second order except the rhombohedral-->monoclinic one, which is first order. The monoclinic phase is only observed in a 2-phase rhombohedral+monoclinic regime. The transition and 2-phase region lie very close to 300 K, making the room-temperature X-ray diffraction patterns extremely complex and potentially explaining why Ba sub(2)BiIrO sub(6) had not previously been identified and reported. A solid solution Ba sub(2)Bi sub(1+) sub(x)r sub(1)-i sub(x) sub(6), analogous to Ba sub(2)Bi sub(1+) sub(x)u sub(1-) sub(x) sub(6), 0