RARE-EARTH VALENCE IN ROX (R=CE, PR, TB 1.5-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-2) STUDIED BY X-RAY ABSORPTION-SPECTROSCOPY

The 4f and sd conduction states of the rare earth atoms in the fluorite-related intermediate rare earth oxides RO(x) (R = Ce, Pr, Tb; 1.5 less-than-or-equal-to x less-than-or-equal-to 2) were studied by X-ray absorption spectroscopy. The spectra thus obtained reveal the presence of only R3+ and R4+ ions. It is deduced from this study that the valence of the rare earth atoms changes abruptly with the increase in oxygen coordination to eight in dioxides. The L(III) and M(IV-V) edges of these oxides reveal the presence of localized and extended 4f states. These results are analysed in the light of a double-valley potential model which takes into account the presence of a repulsive centrifugal barrier for the 4f electron.