Mixed valence as a polaronic effect

A theory of intermediate valence, based on a reformulation of the adiabatic approximation which treats the electron-phonon interaction nonperturbatively, is presented. Without assuming a specific model we show that a narrow quasi-degenerate f-band above the Fermi level becomes unstable. Due to electron-phonon coupling it collapses giving rise to mixed valent configurations, depending on the location of the f-level relative to the Fermi sea. Phenomenological implications are illustrated by a simple model.