Origin of the chemical shift in X-ray absorption near-edge spectroscopy at the Mn K-Edge in manganese oxide compounds

The absorption edge in Mn K-edge X-ray absorption spectra of manganese oxide compounds shows a shift of several electronvolts in going from MnO through LaMnO3 to CaMnO3. On the other hand, in X-ray photoelectron spectra much smaller shifts are observed. To identify the mechanisms that cause the observed chemical shifts, Is ionization as well as Is --> "4p" transition energies have been determined by electronic structure calculations on embedded Mn ions and embedded MnO6 clusters. Systematic variation of the cluster geometry and the cluster embedding showed that the chemical shifts are predominantly determined by two effects: the changes in the Mn 3d occupation and the changes in the Madelung potential. The large chemical shift in the Is --> 4p transition energies between different materials occurs because the two effects do not compensate each other. The chemical shifts obtained for the embedded MnO6 clusters agree reasonably with the experimental shifts. The small sensitivity to the material observed for the Mn Is ionization energies is explained by the near cancellation of the effects of the Madelung potential and the 3d occupation of the Mn ion. (C) 2002 Wiley Periodicals, Inc.