Mixed valence as a polaronic effect - Lagos, M.; Kiwi, M. (Elsevier)
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.
http://linkinghub.elsevier.com/retrieve/pii/0022-3697(87)90088-6
Keywords: V; U