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 Fm3¯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.

Graphical abstractStructure of Ba3BiIr2O9 at 300 K. BiO6 octahedra are purple, IrO6 octahedra are gold, and Ba atoms are green. Thermal ellipsoids at 90% probability.Figure optionsDownload full-size imageDownload as PowerPoint slide