Atomic mechanisms of appearance of intermediate valency states in lanthanides and actinides

Atomic mechanisms of the appearance of intermediate valency states in rare earth and actinide compounds are investigated which may be ascribed to the transition of an electron of the unfilled f-shell from a collapsed to weakly localized state. Self-consistent calculations of the corresponding atomic states are carried out by the Dirac-Fock method and the possibility of such transitions for a number of elements is demonstrated (Ce, Pr, Sm, Eu, Yb, U, Np). Arguments are presented which demonstrate how this atomic phenomenon, which is transformed on the formation of the solid state from such atoms, can explain the appearance of an intermediate valence and heavy fermions in the crystals. The important role is noted of electron-polaron effects related to the comparatively soft intra-atomic excitations existing in this case