Effect of pressure and composition on lattice parameters and unit-cell volume of (Fe,Mg)SiO3 post-perovskite

We obtained unit-cell volumes (V) for (Fe0.2Mg0.8)SiO3 and (Fe0.4Mg0.6)SiO3 post-perovskite (PPv) over a pressure (P) range of 80–145 GPa by in-situ X-ray diffraction (XRD) measurements in laser-heated diamond anvil cells (DACs). Our study suggests that the PPv structure is highly resistant to back transformation to Pv and (Fe0.4Mg0.6)SiO3 PPv was preserved down to 80 GPa upon laser heating during decompression before transforming to pure perovskite (Pv). The bulk modulus (K0) of PPv was determined to be 215(1) and 198(1) GPa for (Fe0.2Mg0.8)SiO3 and (Fe0.4Mg0.6)SiO3 PPv respectively, with a fixed K0ʹ = 4. The substitution of Fe for Mg in PPv results in the largest expansion along the b-axis among all three axes and the expansion of the b-axis is nearly independent of pressure whereas the expansion along the a and c directions slightly decreases with increasing pressure. Combining our results with previous studies on (Fex,Mg1-x)SiO3 PPv (x = 0, 0.09, 0.1) shows that the bulk modulus of PPv decreases with increasing Fe content over a composition range x = 0 to 0.4, varying from 231 GPa for x = 0 to 198 GPa for x = 0.4 with zero pressure volumes scaled to composition, V0 (Å3) = 162.2 + 19.5x and fixedK0ʹ = 4.0. The increase of Fe content in PPv from x = 0 to 0.4 results in a 6.3% increase in density and a 5.3% decrease in bulk sound velocity at 130 GPa.