Strongly correlated transition metal compounds investigated by soft X-ray spectroscopies and multiplet calculations

Direct probe of Mott–Hubbard (MH) to charge-transfer (CT) insulator transition in the MF₂ (M = Cr–Zn) family of compounds was observed by combining F K and M L X-ray emission spectra (XES). This transition is evident as a crossover of the F-2p and M-3d occupied states. By combining F K XES data with F K edge X-ray absorption (XAS) data we directly obtained values for the M-3d Hubbard energy (U_dd) and the F-2p to M-3d charge-transfer energy (Δ_CT). Our results are in good agreement with the Zaanen–Sawatzky–Allen theory. We also present three examples where X-ray absorption at the transition metal L_2,3 edges is used to study the oxidation state of various strongly correlated transition metal compounds. The metal oxidation state is obtained by direct comparison with crystal field multiplet calculations. The compounds are CrF₂, members of the La_1−xSr_xCoO₃ family, and the MVO₃ (M = La and Y) perovskites