Phonon Spectra of Chevrel-Phase Lead and Tin Molybdenum Sulfides: A Molecular-Crystal Model and Its Implications for Superconductivity

Abstract

Heat-capacity (2-400 K) and inelastic-neutron-scattering studies of the phonon spectra are reported for superconducting, high-critical-field, Chevrel-phase, lead and tin molybdenum sulfides (${T}_{c}\ensuremath{\sim}11\ensuremath{-}15$ K). Nine phonon modes per unit cell are identified whose frequencies are in the immediate region Bergmann and Rainer indicate is necessary to optimize ${T}_{c}$. Acoustic-phonon softening on cooling is observed. A molecular-crystal model is proposed for the lattice dynamics of these materials.