THEORY FOR THE PHONON-DISPERSION OF MIXED-VALENT SM0.75Y0.25S

The phonon dispersion and elastic constants of mixed-valent Sm0.75 Y0.25 S are calculated. The microscopic model takes into account interactions of localized 4f electrons with lattice vibrations and phonon-induced 45d transitions leading to a breathing of the rare-earth ionic radius. The resulting three-parameter model yields a quantitative overall agreement with quite anomalous experimental dispersion curves. As input data the unrenormalized frequencies are obtained from a second-neighbor Born-von Krmn model. © 1979 The American Physical Society.