Electronic and magnetic structures of the postperovskite-type Fe[subscript 2]O[subscript 3] and implications for planetary magnetic records and deep interiors

Abstract

Recent studies have shown that high pressure (P) induces the metallization of the Fe[superscript 2+]–O bonding, the destruction of magnetic ordering in Fe, and the high-spin (HS) to low-spin (LS) transition of Fe in silicate and oxide phases at the deep planetary interiors. Hematite (Fe[subscript 2]O[subscript 3]) is an important magnetic carrier mineral for deciphering planetary magnetism and a proxy for Fe in the planetary interiors. Here, we present synchrotron Mössbauer spectroscopy and X-ray diffraction combined with ab initio calculations for Fe[subscript 2]O[subscript 3] revealing the destruction of magnetic ordering at the hematite → Rh[subscript 2]O[subscript 3]-II type (RhII) transition at 70 GPa and 300 K, and then the revival of magnetic ordering at the RhII → postperovskite (PPv) transition after laser heating at 73 GPa. At the latter transition, at least half of Fe[subscript 3+] ions transform from LS to HS and Fe[subscript 2]O[subscript 3] changes from a semiconductor to a metal. This result demonstrates that some magnetic carrier minerals may experience a complex sequence of magnetic ordering changes during impact rather than a monotonic demagnetization. Also local Fe enrichment at Earth's core-mantle boundary will lead to changes in the electronic structure and spin state of Fe in silicate PPv. If the ultra-low-velocity zones are composed of Fe-enriched silicate PPv and/or the basaltic materials are accumulated at the lowermost mantle, high electrical conductivity of these regions will play an important role for the electromagnetic coupling between the mantle and the core.