VALENCE TRANSITIONS AND INTERMEDIATE VALENCE STATES IN RARE-EARTH AND ACTINIDE MATERIALS

The current status of theoretical and experimental research concerning valence change transitions and the intermediate or mixed valence state in rare earth and actinide crystalline solid compounds is reviewed. First, the results of X-ray, optical, electrical, thermodunamic, neutron diffraction, magnetic susceptibility, and Mössbauer resonance experiments near the critical values of pressure and temperature requires for valence changes are discussed. The important similarities and contrasts between rare earth and actinide materials are pointed out. Next, the essential ideas of the numerous theoretical models of mixed valence phenomena are presented. It is found that the theories of discontinous or first-order valence changes involve one of two mechanisms: either the Coulomb repulsion between localized f electrons and itinerant band electrons, or the electron-lattice interaction. The theories are also sharply divided as to their treatment of the electronic f-states, and it is concluded that the configuration-based treatments offer more hope of a detailed quantum-mechanical description of the electronic states of mixed valent materials than do those models using the single-particle approximation. © 1979.