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X-ray diffraction experiments on postperovskite (ppv) with compositions (Mg0.9 Fe0.1 )SiO3 and (Mg0.6 Fe0.4 )SiO3 at Earth core-mantle boundary pressures reveal different crystal structures. The former adopts the CaIrO3 -type structure with space groupCmcm , whereas the latter crystallizes in a structure with thePmcm (Pmma ) space group. The latter has a significantly higher density (ρ  = 6.119(1) g/cm3 ) than the former (ρ  = 5.694(8) g/cm3 ) due to both the larger amount of iron and the smaller ionic radius of Fe2+ 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 Fe2+ 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 Fe2+ 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 thatPmcm 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.

Crystal structures of (Mg 1- x ,Fe x )SiO 3 postperovskite at high pressures

Crystal structures of (Mg _{1- x} ,Fe _x )SiO ₃ postperovskite at high pressures