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.