On the role of phonons in intermediate valence compounds

The influence of phonons on the pressure-temperature (P, T) phase diagram of intermediate valence compounds is investigated. It is shown that due to phonon induced 4f-5d interband transitions, the renormalized value of the hydridization energy V of 4f and conduction electrons may become zero and change its sign and that this is a necessary condition for a discontinuous change of the occupation of the 4f state. The model describes correctly the positive slope (dP/dT > 0) of the phase boundary of the mixed valence state which terminates in a critical point