Synthesis, structures, and phase transitions of barium bismuth iridium oxide perovskites Ba2BiIrO6 and Ba3BiIr2O9

The Ba-Bi-Ir-O system is found to contain two distinct perovskite-type phases: a rock-salt ordered double perovskite Ba₂BiIrO₆; and a 6H-type hexagonal perovskite Ba₃BiIr₂O₉. Ba₂BiIrO₆ undergoes a series of symmetry-lowering phase transitions on cooling , 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 Ba₂BiIrO₆ had not previously been identified and reported. A solid solution Ba₂Bi_1+xIr_1−xO₆, analogous to Ba₂Bi_1+xRu_1−xO₆, 0≤x≤2/3, was not observed. The 6H-type phase Ba₃BiIr₂O₉ undergoes a clean second-order phase transition P6₃/mmc→C2/c at 750 K, unlike 6H-type Ba₃LaIr₂O₉, the P6₃/mmc structure of which is highly strained below ∼750 K but fails to distort coherently to the monoclinic phase.