Stability of the perovskite structure and possibility of the transition to the post-perovskite structure in CaSiO 3, FeSiO 3, MnSiO 3 and CoSiO 3

Abstract

High pressure and high temperature experiments on CaSiO 3, FeSiO 3, MnSiO 3 and CoSiO 3 using a laser-heated diamond anvil cell combined with synchrotron X-ray diffraction were conducted to explore the perovskite structure of these compounds and the transition to the post-perovskite structure. The experimental results revealed that MnSiO 3 has a perovskite structure from relatively low pressure (ca. 20 GPa) similarly to CaSiO 3, while the stable forms of FeSiO 3 and CoSiO 3 are mixtures of mono-oxide (NaCl structure) + high pressure polymorph of SiO 2 even at very high pressure and temperature (149 GPa and 1800 K for FeSiO 3 and 79 GPa and 2000 K for CoSiO 3). This strongly suggests that the crystal field stabilization energy (CFSE) of Fe 2+ with six 3d electrons and Co 2+ with seven 3d electrons at the octahedral site of mono-oxides favors a mixture of mono-oxide + SiO 2 over perovskite where Fe 2+ and Co 2+ would occupy the distorted dodecahedral sites having a smaller CFSE (Mn 2+ has five 3d electrons and has no CFSE). The structural characteristics that the orthorhombic distortion of MnSiO 3 perovskite decreases with pressure and the tolerance factor of CaSiO 3 perovskite (0.99) is far from the orthorhombic range suggest that both MnSiO 3 and CaSiO 3 perovskites will not transform to the CaIrO 3-type post-perovskite structure even at the Earth's core–mantle boundary conditions, although CaSiO 3 perovskite has a potentiality to transform to the CaIrO 3-type post-perovskite structure at still higher pressure as long as another type of transformation does not occur.