Valence fluctuation phenomena

Valence fluctuation phenomena occur in rare-earth compounds in which the proximity of the 4f level to the Fermi energy leads to instabilities of the charge configuration (valence) and/or of the magnetic moment. We review the experimental results observed in the subset of such systems for which the 4f ions form a lattice with identical valence on each site. The discussion includes key thermodynamic experiments, such as susceptibility and lattice constant, and spectroscopic experiments such as XPS and neutron scattering. This is followed by a review of existing theoretical work concerning both the ground states and the isomorphic phase transitions which occur in such compounds ; the emphasis is on those aspects which make valence fluctuation phenomena such a challenging manybody problem. This review was received in October 1980. 3: Supported in part by the National Science Foundation. 11 Supported in part by the National Science Foundation and the Office of Naval Research. 0034-4885/81/010001 f84 $06.50