Understanding MgSiO3 Post-Perovskite via NaMgF3 and CaIrO3 Analog Materials

Knowledge of the structure and properties of MgSiO3 post-perovskite (ppv) is paramount in the study of Earth`s D" layer and core-mantle boundary. However, the extreme pressures and temperatures required to stabilize this phase make in-situ characterization difficult. Rietveld refinement of structure models obtained from x-ray and neutron diffraction data of analog materials including NaMgF3 and CaIrO3, the structure model of MgSiO3 ppv, reveal structural changes accompanying the perovskite (pv)/ppv phase transition. Refined models of NaMgF3 pv just before ppv phase transition indicate some inter-octahedral F-F distances rival the average intra-octahedral distance, which may cause instability in the pv structure and drive the transformation to the ppv phase. The dataset suggests it may be possible to predict the pressure of a ppv phase transition in other pv- (or A2X3-) type materials by extrapolating the cation polyhedra volume ratio with pressure. Diffraction data from CaIrO3 indicate significant disparity between axial compression and thermal expansion schemes (Ka > Kc &αa < αc) and have implications for MgSiO3 ppv and transverse isotropy of seismic waves within Earth`s D" layer. Rietveld models of CaIrO3 at high temperature detail structure changes expected of MgSiO3 ppv considering the thermal gradient above Earth`s core-mantle boundary. In sum, this work assists an increasingly comprehensive understanding of the structure and properties expected of MgSiO3 ppv above the core-mantle boundary.