Fe K pre-edge of Fe$_{2}$O$_{3}$ at High Pressure

Hematite ($\alpha$-Fe$_{2}$O$_{3}$), as an archetypal 3d transition metal oxide and important earth mineral, undergoes a series of electronic transitions and structural changes at high pressure. At ambient conditions, Fe$_{2}$O$_{3}$ adopts the $\alpha$-Al$_{2}$O$_{3}$ structure and is an antiferromagnetic Mott insulator, with five 3d electrons in the high-spin state. Upon increasing pressure, it transforms from a high-spin state to a low-spin state in the 40-70 GPa range. Here we report experimental results for the Fe K-edge spectra of Fe$_{2}$O$_{3}$ collected in-situ at high pressure using synchrotron x-ray absorption spectroscopy in partial fluorescence yield geometry. The pre-edge features give explicit information about the crystal field splitting energy (CFSE) of octahedrally coordinated Fe$^{3+}$ in Fe$_{2}$O$_{3}$ as a function of pressure, mapping the electronic structure (high-spin to low-spin) transition. The K-$\alpha$ emission spectra at high pressure are also presented.