Pressure dependence of the absorption edge of some divalent rare-earth monochalcogenides

By means of thermodynamic principles, the pressure dependence of the 4f→5d excitation energy is calculated for some divalent rare-earth chalcogenides. The agreement with experiment is rather satisfactory. Implications on the pure metals europium, ytterbium, and cerium are discussed. For the two former elements a valence transition is estimated to take place at a pressure of about 175 and 140 kbar, respectively. Also, ytterbium monoxide is briefly commented on.