Electronic structure of Fe₂O₃ above 700 GPa

Characterizing chemical bonding and the behavior of valence electrons in solids has historically been difficult at high-energy-density (HED) conditions. X-ray absorption fine structure (XAFS) spectroscopy was performed on Fe₂O₃ (hematite) to determine the interactions between the valence electrons of iron and oxygen under compression. At the OMEGA-60 Laser Facility, Fe₂O₃ was ramp compressed to above 700 GPa and probed with an implosion x-ray source. A new x-ray spectrometer with improved spectral resolution and energy calibration measured the absorption spectrum, allowing x-ray absorption near edge spectroscopy (XANES) features of Fe₂O₃ to be measured at these extreme conditions. Fe₂O₃ undergoes a structural and insulator-to-metal transition when compressed to above 150 GPa, resulting in a negative shift in the K-edge absorption energy of 3 eV. When further compressed to above 700 GPa, the K-edge shifted to higher energy with increasing density. Analysis of the XANES spectrum revealed the iron 3d electron orbitals, which are bonded to the oxygen 2p electron orbitals, spread in energy as the oxygen atoms were compressed closer to the absorbing iron atom. The persistence of this 1s to 3d transition peak is an indication that iron remains bonded to oxygen above 700 GPa demonstrating the ability for iron to capture oxygen deep into the cores of super-Earth planets. In this talk, these techniques for characterizing chemistry at HED conditions will be presented and discussed as well as how to extend these techniques to characterize other complex changes in electron structure at extreme conditions.