K-Edge XANES Investigation of Fe-Based Oxides by Density Functional Theory Calculations

X-ray absorption spectroscopy (XAS) is a powerful technique for simultaneously characterizing the oxidation states and local structures of working catalysts and battery materials, among others. However, deciphering the apparent oxidation state through XAS remains challenging because of the high sensitivity of spectra to multiple factors. Here, comprehensive first-principles calculations of X-ray absorption near-edge structure (XANES) spectra of a series of Fe-based catalysts and battery electrodes, including FeO, Fe2O3, Fe(OH)2, FeOOH, FeAl2O4, and MFeO2 (M = Li, Na, or K), were performed to shed light on the issue by dissecting the dependence of XANES line shapes on detailed electronic and geometric structures. We revealed that, in comparison with the composition and factors usually extracted from XAS measurements (i.e., the oxidation state and local structure), nonlocal structures (e.g., crystal structure) that cannot be straightforwardly obtained from XAS experiments are more dominant factors of the XANES line shapes of the main edge, main peak, and postedge. As demonstrated through Fe compounds with the same or similar nonlocal structures, their line shapes are alike and shift in response to a change in the chemical environment. On the other hand, we found that the local coordination (octahedra vs tetrahedra) and the oxidation state are more dominant on the intensity of the pre-edge and the energy splitting between the pre-edge and the main edge, respectively. We demonstrated that clarifying these control factors and origins not only provides valuable insights into reliable assignment of spectra and understanding semiempirical rules but also unravels the structural information that is not directly accessible in XAS measurements.