Crystal structures of (Mg _1-x,Fex)SiO ₃ postperovskite at high pressures

X-ray diffraction experiments on postperovskite (ppv) with compositions (Mg _0.9Fe _0.1)SiO ₃ and (Mg _0.6Fe _0.4)SiO ₃ at Earth core-mantle boundary pressures reveal different crystal structures. The former adopts the CaIrO ₃-type structure with space group Cmcm, whereas the latter crystallizes in a structure with the Pmcm (Pmma) space group. The latter has a significantly higher density (ρ = 6.119(1) g/cm ³) than the former (ρ = 5.694(8) g/cm ³) due to both the larger amount of iron and the smaller ionic radius of Fe ²⁺as a result of an electronic spin transition observed by X-ray emission spectroscopy (XES). The smaller ionic radius for low-spin compared to high-spin Fe ²⁺ also leads to an ordered cation distribution in the M1 and M2 crystallographic sites of the higher density ppv structure. Rietveld structure refinement indicates that approximately 70% of the total Fe ²⁺ in that phase occupies the M2 site. XES results indicate a loss of 70% of the unpaired electronic spins consistent with a low spin M2 site and high spin M1 site. First-principles calculations of the magnetic ordering confirm that Pmcm with a two-site model is energetically more favorable at high pressure, and predict that the ordered structure is anisotropic in its electrical and elastic properties. These results suggest that interpretations of seismic structure in the deep mantle need to treat a broader range of mineral structures than previously considered.