Possible structural polymorphism in Al-bearing magnesiumsilicate post-perovskite

In the present study, we summarize indications for the existence of kinked post-perovskite structures in the MAS system. X-ray diffraction data and Raman spectra of aluminous magnesium metasilicate post-perovskite are inconsistent with the CaIrO3 structure. Instead the observations are consistent with structures intermediate between the perovskite and the CaIrO3 structure. Ab initio calculations show that the enthalpies of the kinked structures are slightly higher than the CaIrO_3 structure at 0 K. Finite temperature, minor element chemistry, kinetics of phase transformation, and actual stress regime are plausible reasons for the observed differences between the present and the previously reported post-perovskite phases.