Abstract
Usually first-principles x-ray absorption near-edge structure (XANES) calculations are performed in the Born-Oppenheimer approximation assuming a static lattice, whereas the nuclear motion undoubtedly impacts XANES spectra notably at the K pre-edge of light elements in oxides. Here, an efficient method based on density-functional theory to account for quantum thermal fluctuations of nuclei is developed and is successfully applied to the K edge of corundum for temperatures up to 930 K. The zero-point motion influence is estimated. Comparison is made with previous theoretical approaches also developed to account for vibrations in XANES.
Highlights
Standard x-ray absorption near-edge structure (XANES) calculations rest upon the BornOppenheimer (BO) approximation, considering nuclei fixed at their equilibrium positions defined by the set of nuclear coordinates Req
An efficient framework to account for temperature in XANES spectra is proposed
The good agreement between calculated and measured XANES spectra demonstrates that this approach successfully explains the temperature effects experimentally observed in XANES
Summary
Standard x-ray absorption near-edge structure (XANES) calculations rest upon the BornOppenheimer (BO) approximation, considering nuclei fixed at their equilibrium positions defined by the set of nuclear coordinates Req. The dynamical matrix is determined to generate temperature-dependent sets of nonequilibrium nuclear configurations R for which individual XANES spectra are calculated. Their average at a given temperature may be compared to the corresponding experimental spectrum. The calculated spectra are compared with two previous theoretical approaches developed to account for thermal motion, either by small absorbingatom displacements (AAD) [2] or by moving the 1s initial wave function in the crude BornOppenheimer (CBO) approximation [3]
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