Effect of valence fluctuations on lattice vibrations

The effect of valence fluctuations on various phonon modes are studied theoretically. It is shown that for a system like SmS the valence fluctuations may cause a softening of the longitudinal optical phonons which should be observable. We also obtain for Smsub(1-x)Ysub(x)S that the atomic displacements of only S ions, not of the Sm ions increase upon approaching the transition temperature at which the smooth transition from the golden metallic phase to the black semiconducting-like phase occurs. Furthermore, we discuss how acoustic phonons are affected by the valence transition. (author)