Intermediate valence states in the Falicov-Kimball model

The Falicov-Kimball model for an electronically driven semiconductor to metal transition is treated within an approximation which includes intra-atomic excitonic correlations. Discontinuous transitions from the semiconducting to a metallic phase with intermediate valence are found in contrast with the mean-field treatments where the transition is always to a pure valence metallic state when discontinuous for simple band densities of states. The results are applied in a description of the phase transitions of samaruim monochalcogenides under pressure. It is demonstrated that this simple model provides a resonable theoretical description for intermediate valence phenomenon provided excitonic correlations are taken into account.