Pressure dependence of theflevel in rare-earth systems

Abstract

The pressure dependence of the position of a localized $f$ level relatie to the Fermi level is treated theoretically. For a rare-earth system (say divalent) a model is applied which treats the excited state as a trivalent impurity site in an otherwise divalent rare-earth ionic array. Special attention is paid to the pressure dependence of this excitation energy. In a calculation for metallic europium, the excitation is found to become unstable at 180 kbar, signaling an instability of the system towards an intermediate valence state. Similarly, the high-pressur trivalent europium phase is found unstable upon releasing the pressure below 350 kbar.