X-ray absorption studies of ${\mathrm{CeO}}_2$, ${\mathrm{PrO}}_2$, and ${\mathrm{TbO}}_2$. II. Rare-earth valence state by $L_{\mathrm{III}}$ absorption edges

The $L_{\mathrm{III}}$ absorption edges of fluorite-structure-related ${\mathrm{CeO}}_2$, ${\mathrm{PrO}}_2$, and ${\mathrm{TbO}}_2$ show four well-resolved spectral features. The systematic trend in their positions and relative intensities suggests the presence of a specific valence state of the rare-earth atom in these oxides. The present study reveals an appreciable 4f-2p oxygen overlap in the valence band following the complete delocalization of the outermost 4f electron. Of the two main structures, the highest-energy one is interpreted as due to the localized 4f state and the additional presence of a hybridized 4f external state which we denote as 4f¯, is evidenced by the lower-energy peak. The weaker shoulder and preabsorptionlike feature are attributed to a shakedown effect due to the presence of an additional screening electron. This effect occurs both for the localized and for the extended orbitals of f symmetry, and a weaker structure therefore attaches to each of the strong peaks. The coexistence of localized and extended states is interpreted within a double-valley-potential model.