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Ab initio and experimental pre-edge investigations of the Mn K-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.
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F Farges. Ab initio and experimental pre-edge investigations of the Mn K-edge XANES in oxide-type materials. Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2005, 71 (15), ⟨10.1103/PhysRevB.71.155109⟩. ⟨hal-00693811⟩



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