Gasketing optimized for large sample volume in the diamond anvil cell: First application to MgGeO₃ and implications for structural systematics of the perovskite to post-perovskite transition

Structure models of MgGeO₃ post-perovskite (Cmcm) are presented, along with a structure survey, demonstrating that all perovskite, post-perovskite and CaIrO₃-type structures (ABX₃) have specific ranges of the volume ratio between cation-centered polyhedra (V _A:V_B). The quality of the reported diffraction data and MgGeO₃ structure models is enhanced via implementation of a new graphite gasket for the diamond anvil cell, which stabilizes a larger sample volume, improving powder statistics during X-ray diffraction, and via the thermal insulation required to achieve ultra-high temperatures while laser-heating samples at pressures near 100 GPa. The structure survey supports the theory that the pressure-temperature conditions under which the perovskite/post-perovskite phase transition occurs can be estimated by extrapolating the change in V_A:V_B to a value of 4, which corresponds to a maximum tilt of BX₆ octahedra in the perovskite structure (Pbnm) where inter-octahedral anion-anion distances match the average intra-octahedral anion-anion distance. Once these short inter-octahedral distances between anions are reached in the perovskite structure, further tilting of octahedra and decrease of the V_A:V_B ratio does not occur, driving the transition to post-perovskite structure as pressure is increased.