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

Abstract

The Ba–Bi–Ir–O system is found to contain two distinct perovskite-type phases: a rock-salt ordered double perovskite Ba 2BiIrO 6; and a 6H-type hexagonal perovskite Ba 3BiIr 2O 9. Ba 2BiIrO 6 undergoes a series of symmetry-lowering phase transitions on cooling Fm 3 ¯ m → R 3 ¯ c → 12 / m ( C 2 / m ) → I 1 ¯ ( P 1 ¯ ) , 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 2BiIrO 6 had not previously been identified and reported. A solid solution Ba 2Bi 1+ x Ir 1− x O 6, analogous to Ba 2Bi 1+ x Ru 1− x O 6, 0≤ x≤2/3, was not observed. The 6H-type phase Ba 3BiIr 2O 9 undergoes a clean second-order phase transition P6 3/ mmc→ C2/ c at 750 K, unlike 6H-type Ba 3LaIr 2O 9, the P6 3/ mmc structure of which is highly strained below ∼750 K but fails to distort coherently to the monoclinic phase.