BONDING AND ELECTRONIC CHANGES ASSOCIATED WITH SPIN TRANSITION IN IRON-CONTAINING MINERALS

Spin transition is common with iron-containing minerals at the pressure condition of lower mantle. In this paper we present Raman spectroscopy study and x-ray absorption study of siderite and hematite respectively. For siderite, we observed the appearance of a new CO₃ symmetric stretching mode at 20 cm⁻¹ lower frequency beginning at approximately 46 GPa. This softening is due to the lengthening of the C-O bonds as a result of a combination of rotation and volume shrinkage of the FeO₆ octahedra while siderite undergoes the isostructural volume collapse and spin transition. For hematite, the pressure-induced evolution of the electronic structure as Fe₂O₃ transforms from a high-spin insulator to a low-spin metal is reflected in the x-ray absorption pre-edge. The crystal-field splitting energy was found to increase monotonically with pressure up to 48 GPa, above which a series of phase transitions occur. Atomic multiplet, cluster diagonalization, and density-functional calculations were performed to simulate the pre-edge absorption spectra, showing good qualitative agreement with the measurements. The mechanism for the pressure-induced electronic phase transitions of Fe₂O₃ is discussed and it is shown that ligand hybridization significantly reduces the critical high-spin/low-spin transition pressure.