THEORY OF CORE-LEVEL SPECTROSCOPY OF RARE-EARTH-OXIDES

A review is presented of the recent development in theoretical studies of X-ray photoemission spectra (XPS) and X-ray absorption spectra (XAS) for rare-earth sesquioxides (R2O3) and rare-earth dioxides (RO2). From the analysis of 3d XPS for R2O3 (R = La, Ce,...,Yb) and RO2 (R = Ce, Pr, Tb) with the impurity Anderson model (without the multiplet coupling effect), we estimate the strength of the covalency hybridization between rare-earth 4f and oxygen 2p states, the charge transfer energy and other physical quantities, and discuss their systematic variation with the change in rare-earth elements. Two possible mechanisms for the splitting of the 3d XPS are pointed out for R2O3. One is the initial-state hybridization for R = La, Ce, Pr and Nd; the other is the final-state hybridization for R = Eu and Yb. For RO2 (R = Ce, Pr, Tb), both of the initial- and final-state hybridizations are essential in explaining the 3d XPS spectra. Then, we study the effect of the intraatomic multiplet coupling on 3d and 4d XPS for La2O3, Ce2O3, Pr2O3, Nd2O3, Yb2O3, and CeO2. The multiplet coupling effect is not very important for the 3d XPS, causing only an additional spectral broadening and some minor spectral structures. In the analysis of 4d XPS, however, the interplay between the multiplet coupling and the hybridization plays an essential role. Finally, we study the multiplet structure in 3d and 4d XAS for CeO2 and PrO2. It is shown that the original atomic multiplet structure in these XAS is strongly modified by the interatomic hybridization effect, and the experimental multiplet structure is explained consistently with the analysis of 3d and 4d XPS, by the mixed-valence ground state with strong covalency hybridization.