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 View the MathML sourceFm3¯m→R3¯c→12/m(C2/m)→I1¯(P1¯), 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 Ba2Bi1+xIr1−xO6, analogous to Ba2Bi1+xRu1−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.