Valence state of Mn in Ca-doped LaMnO3 studied by high-resolution Mn K ß emission spectroscopy

Abstract

Mn K ß x-ray emission spectra provide a direct method to probe the effective spin state and charge density on the Mn atom and is used in an experimental study of a class of Mn oxides. Specifically, the Mn K ß line positions and detailed spectral shapes depend on the oxidation and the spin state of the Mn sites as well as the degree of d covalency/itinerancy. Theoretical calculations including atomic charge and multiplet effects, as well as crystal-field splittings and covalency effects, are used as a guide to the experimental results. Direct comparison of the ionic system MnF2 and the covalent system MnO reveals significant changes due to the degree of covalency of Mn within atomic-type Mn K ß simulations. Moreover, comparisons of measurement with calculations support the assumed high spin state of Mn in all of the systems studied. The detailed shape and energy shift of the spectra for the perovskite compounds, LaMnO3 and CaMnO3, are, respectively, found to be very similar to the covalent Mn^(3+)-Mn2O3 and Mn^(4+)-MnO2 compounds thereby supporting the identical Mn-state assignments. Comparison to the theoretical modeling emphasizes the strong covalency in these materials. Detailed Mn K b x-ray emission results on the La1_xCaxMnO3 system can be well fit by linear superpositions of the end member spectra, consistent with a mixed-valent character for the intermediate compositions. However, an arrested Mn-valence response to the doping in the x<0.3 range is found. No evidence for Mn^2+ is observed at any x values seemingly ruling out proposals regarding Mn^3+ disproportionation.