Estimating Hybridization of Transition Metal and Oxygen States in Perovskites from O K‑edge X‑ray Absorption Spectroscopy

The interaction between the transition metal 3d and the oxygen 2p states via hybridization underpins many of the phenomena in transition metal oxide materials. We report the empirical trend of this interaction using the pre-edge feature of the O K-edge X-ray absorption spectrum. Our assessment method is built on the dipole approximation and the configuration interaction between the transition metal 3d and the oxygen 2p states. We found that hybridization increases with the number of 3d electrons, consistent with the expected electronegativity trend. We support this analysis with density functional calculations, which reveal a systematic increase in the transition metal 3d and the oxygen 2p state mixing with increasing 3d-electron number. Oxidation of the transition metal was also found to increase hybridization, which we believe reflects the reduced transition metal 3d and oxygen 2p energy difference, causing increased covalency. We compare the analysis from the surface-sensitive electron-yield and the bulk-sensitive fluorescence-yield spectra, revealing that either method can be used to study the hybridization trend. We finally compare and discuss the influence of the lanthanide ions and the influence of the covalency on oxygen electrocatalysis. Our study describes an efficient and simple approach to understand the hybridization trend in transition metal oxides, which has considerable implications for electrochemical energy conversion processes.