IMF2017

Recently the mixed phase-state of the rhombohedral and the other – presumably the nonpolar orthorhombic – phases, has been shown to exist in unpoled Na_1/2Bi_1/2TiO₃ (NBT) over a wide temperature range including the room temperature instead of the pure rhombohedral phase while only the rhombohedral phase is observed in the poled state. The finding raises the question about the characterisation of the structure of NBT solid solutions approaching the morphotropic phase boundary (MPB) frequently being assumed as rhombohedral even in the unpoled state. CaTiO₃ is found to stabilise the orthorhombic Pnma crystallographic structure. Clear stabilisation of the rhombohedral phase is found in NBT-PbTiO₃ solid solutions upon approaching the MPB. Even if some indications about stabilisation of the rhombohedral phase in NBT-BaTiO₃ solid solutions exist, there is no clearly increasing concentration of the rhombohedral phase found in all the concentration range of BaTiO₃ below the MPB. In difference from CaTiO₃, the NBT solid solutions with structurally similar CdTiO₃ have a complicated phase diagram. The solid solutions at low concentrations of CdTiO₃ have nonpolar symmetry group with a⁺a⁺a⁺ octahedral rotations according to the Glazer notation, transforming to the a⁺b⁺c⁺ polar symmetry group upon increasing the CdTiO₃ concentration. Such kind of tilting so far has never been found in real crystallographic structures.