Stabilization of mixed valence by polaronic effects

Polaronic effects in mixed valence systems are studied in a simple model of localized d-electrons for arbitrary values of relevant parameters. Variational approach is used which permits one to interpolate between the limit of average static lattice distortion and that of different local distortions for different valence states of an ion. It is shown that the degree of local electron-lattice correlation and polaron narrowing of virtual f-level changes with a position of f-level. Polaronic effects make a valence transition more gradual and stabilize mixed valence phase. At the end some comments on a possibility of Valence Density Waves in mixed valence compounds are made and certain properties of CeAl₂ are tentatively ascribed to them.