PHONONS IN MIXED VALENCE COMPOUNDS

Abstract

Several mixed-valence NaCl-structure compounds show strong anomalies in the phonon dispersion curves. These anomalies are related to the interaction of localized f-electrons with delocalized band electrons near the Fermi surface and to the iso-structural phase transitions characterized by a softening of the bulk modulus. The lattice dynamics of these compounds is reviewed and the dispersion curves are analyzed in terms of a breathing shell model including various modifications. The relation between the phonon self-energies and the electronic band structure is discussed. In addition, the polarized Raman spectra are interpreted in terms of local cluster deformabilities of breathing and quadrupolar symmetry. Similarities and differences of phonon anomalies, Raman spectra and electronic band structure in semiconducting, superconducting and mixed valence compounds are pointed out.