Hybridization effect on phase transitions in the Falicov-Kimball model

The Falicov-Kimball model extended by the hybridization between the localized electron states and the conduction band states is studied in Hartree-Fock approximation. Taking into account the periodicity of the system, the occupation number of the localized levels always varies continuously with the level energy for non-vanishing hybridization. This result is in contradiction with earlier mean-field calculations which are not self- consistent for the periodic model. Since first order phase transitions in intermediate valence compounds such as SmS have been observed experimentally, it follows that their explanation needs a more general theoretical frame than the model considered here.