Postperovskite Phase Transition of ZnGeO_3: Comparative Crystal Chemistry of Postperovskite Phase Transition from Germanate Perovskites

摘要

The postperovskite phase of ZnGeO_3 was confirmed by laser heating experiments of the perovskite phase under 110-130 GPa at high temperature. Ab initio calculations indicated that the phase transition occurs at 133 GPa at 0 K. This postperovskite transition pressure is significantly higher than those reported for other germanates, such as MnGeO_3 and MgGeO_3. The comparative crystal chemistry of the perovskite-to-postperovskite transition suggests that a relatively elongated b-axis in the low-pressure range resulted in the delay in the transition to the postperovskite phase. Similar to most GdFeO_3-type perovskites that transform to the CaIrO_3-type postperovskite phase, ZnGeO_3 perovskite eventually transformed to the CaIrO_3-type postperovskite phase at a critical rotational angle of the GeO_6 octahedron. The formation of the postperovskite structure at a very low critical rotational angle for MnGeO_3 suggests that relatively large divalent cations likely break down the cornersharing GeO_6 frameworks without a large rotation of GeO_6 to form the postperovskite phase.