Finite temperature effects on the X-ray absorption spectra of crystalline aluminas from first principles

X-ray Absorption spectroscopy (XAS) is a high-resolution probe of the electronic density of states within solid-state materials. By including phonon-assisted transitions within plane-wave density-functional theory methods of calculating the XAS we obtain the Al K-edge XAS at 300 K for two crystalline phases of alumina (Al₂O₃). From these finite-temperature spectra, we reproduce the pre-edge peak for the α-Al₂O₃ phase, which arises from a mixed s-p state that is not visible at the static-lattice level of approximation. In addition, we calculate the finite-temperature XAS for γ-Al₂O₃, which has not previously been reported, and find that the fully ordered spinel-based model of γ-Al₂O₃ describes two out of the three experimental transitions seen in the Al K-edge. We observe that the third peak in the calculated γ-Al₂O₃ spectrum arises from transitions in AlO₆ octahedral coordination environments from Al 1s to delocalized d-like states. Finally, we propose this letter as a basis for future applications of finite-temperature XAS from first principles, as the methods used are fully generalizable to any atom and edge, beyond the Al K-edge