Stability of the perovskite structure and possibility of the transition to the post-perovskite structure in CaSiO3, FeSiO3, MnSiO3 and CoSiO3

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.

Phase stability of iron germanate, FeGeO3, to 127 GPa

“…As already mentioned, experimental studies have reported that the Pv and pPv phases are not stable in FeSiO 3 . Instead, a mixture of FeO (B1 phase) and a high-pressure SiO 2 phase was found in experiments covering a wide pressure range up to GPa (Fujino et al 2009). In this study, we have shown that FeGeO 3 behaves similarly and breaks down into GeO 2 + FeO at least up to 1.27 Mbars.…”

“…Further understanding of the behavior of FeGeO3 relative to other analogs can by obtain by consideration of the the Goldschmidt diagram. This type of diagram has been commonly used to predict the stability of the perovskite and post-perovskite structures in ABO 3 compounds (Goldschmidt 1926;Hattori et al 1999;Fujino et al 2009;Tateno et al 2010) (Figure 6). The Goldschmidt tolerance factor, t, can be used to assess the potential stability of the two phases.…”

“…The perovskite phase of FeTiO 3 has been synthesized from the LiNbO 3 -type structure at 16 GPa (Leinenweber et al 1991) and from the ilmenite phase on heating at 20 GPa and 973 K (Ming et al 2006). However, FeSiO 3 appears to be an exception to the Goldschmidt criterion as experimental studies indicate it does not transform to the perovskite structure (Fujino et al 2009).…”

“…and 149 GPa (Fujino et al 2009). It has also been suggested that (Mg Fe)SiO 3 perovskite dissociates at high P-T (95 -101 GPa, 2200 -2400 K) to a Fe-poor perovskite and a hexagonal Fe-rich phase called "H phase" (Zhang et al 2014).. A perovskite of almandine composition (Fe 3 Al 2 Si 3 O 12 or (Fe,Al)(Si, Al)O 3 ) is stabilized above 80 GPa (Dorfman et al 2012).…”

Superior solid solubility of MnSiO3 in CaSiO3 perovskite

“…Some theoretical studies also suggested that the solid solutions of the CaSiO 3 -MgSiO 3 perovskite would be unexpected (e.g., Vitos et al 2006;Jung and Schmidt 2011). Conversely, Fujino et al (2009) suggested that some amount of FeSiO 3 may be incorporated into CaSiO 3 perovskite; however, no quantitative data have been available so far. Thus, CaSiO 3 perovskite has been often treated as a pure phase in the lower mantle.…”