Electronic Structure of Mixed Valence State

A model for the mixed valence state is presented. The basic lattice is composed of N trivalent rare earth ions. Extra N electrons enter either the localized f level at each ion site or the conduction band. When an electron occupies the f level of rare earth ion, this ion becomes divalent ion. The electron can jump from the f level to the conduction band and vice versa through the hybridization between these states. The effect of the strong intra-atomic Coulomb and the exchange interactions between the f electrons on the hybridized states is studied by applying the coherent potential approximation. The gap in the density of states and/or the mobility gap appear around the f level, and the Fermi level is located within the gap. The energy of the paramagnetic state is shown to be lower than that of the ferromagnetic state.