Electronic configuration of Yb compounds

The total energy differences between divalent and trivalent configurations of Yb ions in a number of Yb compounds are studied. Two different band theo retical methods, which differ in the treatment of the localized f electrons , are used. The results show that in all Yb compounds the valence energy di fferences are equal to the energy needed to localize an f electron. These v alence energy differences correlate with the number of f electrons hybridiz ing with the conduction bands in the trivalent configuration. For divalent YbS, the pressure induced f-electron delocalization implies an intermediate valency, as also indicated by experiment.