Electronic and magnetic structure of the postperovskite phase in Fe2O3

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Shim, Sang-Heon۱; Sturhahn, Wolfgang۲; Catalli, Krystle۱; Zhao, Jiyong۲; Lerche, Michael۲; Kubo, Atsushi۳; Prakapenka, Vitali۳
Holding Date 03 September 2008

Fe is the most abundant transition metal in the Earth’s interior. Recent experiments have shown that the degree of spin pairing in Fe increases with pressure in mantle silicate perovskite, ferropericlase, and Fe minerals. Pressure-induced spin pairing is important in geophysics as it influences the physical properties of the mantle and core phases. Furthermore, spin-pairing during impact processes will change the paleomagnetic records in Fe minerals. Fe2O3 is a good proxy for studying the state of Fe in the mantle and the core. For example, it has a stability field for the CaIrO3 type which is isostructural to postperovskite (PPv). Also, hematite (Fe2O3) is an impotant magnetic carrier mineral. We have conducted synchrotron Mössbauer spectroscopy (SMS) and X-ray diffraction (XRD) on Fe2O3 up to 71 GPa in the laser-heated diamond cell at Secor 3 and 13, respectively, of APS. Our XRD shows that hematite transforms to an orthorhombic phase at 60 GPa and 300 K and then subsequently to the PPv phase at 71 GPa and 1250 K, consistent with previous studies. Our SMS confirms that hematite undergoes a transition from canted ferromagnetic (FM) to antiferromagnetic (AFM) phases below 10 GPa and that the magnetic ordering in hematite is sustained up to 55 GPa where it disappears. The Mössbauer spectrum of the orthorhombic phase can be best fit by a nonmagnetic single site model, consistent with previous studies. For the PPv phase, we found two distinct sites for Fe with almost equal weightings, consistent with the predicted crystal structure of PPv. Both Fe sites in Fe2O3-PPv at 71 GPa show magnetic hyperfine fields, indicating that the PPv phase in Fe2O3 is magnetic (possibly ferrimagnetic). Combined with the finite magnetic moments, the observed quadrupole splittings indicate that ferric ions in both sites are in high-spin states. We postulate that the spin ordering in PPv is due to enhanced interactions between cations in the adjacent polyhedra through edge and face sharing. Our study shows that Fe2O3 undergoes a complex sequence of changes in the spin and magnetic states at high pressure: high spin in a canted FM hematite phase → high spin in an AFM hematite phase → low spin in a para- or non-magnetic orthorhombic phase → high spin in a magnetic (likely ferrimagnetic) PPv phase. This demonstrates that phase transitions can have profound influences on the spin structure of Fe at high pressure. The existence of a stability field for another magnetic phase at high pressure would make the effect of impacts on planetary magnetic records much more complicated than previously thought where impacts have been thought to demagnetize minerals. Also the strong influence of phase transitions in the planetary interiors could result in a complex sequence of changes in the spin state of Fe, instead of a monotonic increase in the degree of spin pairing with depth.