Ab initio and experimental pre-edge investigations of the MnK-edge XANES in oxide-type materials

Abstract

Mn $K$ edge ab initio FEFF8.2 calculations of the pre-edge features of the x-ray-absorption near-edge structure (XANES) region were undertaken for a series of Mn-bearing oxide-type compounds. The aim of the study is to provide a reliable method for determining quantitative and accurate redox and symmetry information for manganese. In agreement with multiplet calculations by Glatzel and co-workers, FEFF8.2 predicts a doublet and a triplet for Mn(II) and Mn(III) in octahedral symmetry, respectively, in agreement with high-resolution XANES experiments. Site distortion increases notably the contribution from dipolar transitions and, consequently, the pre-edge feature integrated area. An even more intense pre-edge feature is calculated and measured for the ${T}_{d}$ symmetry (singletlike). For Mn(IV), a triplet is predicted and measured for the ${O}_{h}$ symmetry. However, additional transitions are found in Mn(IV)-rich compounds, that are related to metal-metal transitions. These transitions overlap strongly with the ``true pre-edge,'' making extraction of redox and symmetry information for Mn(IV) more challenging. However, a model of the pre-edge with pseudo-Voigt functions of fixed calculated width (based on core-hole lifetime and experimental resolution) helps to separate the contributions related to first-neighbor symmetry from those of the metal-metal pairs. Application to multivalent defective manganese oxide materials suggests that the pre-edge information varies linearly as a function of Mn redox state or symmetry but varies nonlinearly as a function of both parameters. Finally, the polymerization of the manganese networks can be estimated from the metal-metal transitions found in the pre-edge region.