Pressure dependence of the f level in rare-earth systems

The pressure dependence of the position of a localized f level relative 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-pressure trivalent europium phase is found unstable upon releasing the pressure below 350 kbar