Raman spectroscopy of intermediate valent (IV) systems: electron-phonon coupling and charge fluctuation

Abstract

Raman scattering measurements on intermediate valent rare earth chalcogenides crystallizing in the NaCl structure are reviewed. The experimentally obtained Raman intensities are compared with the theoretically calculated intensities, using a lattice dynamical model in which the electron lattice interaction has been expressed in terms of local intraionic charge deformabilities, with monopolar (breathing Γ⁺ ₁), dipolar (Γ^- ₁₅), and quadrupolar (Γ⁺ ₁₂) symmetry. Phonon anomalies in SmS, Sm0·75 Y0·25S and YS are described and discussed. Finally, evidence for a new excitation involving 4f charge fluctuations is presented, which is observed as a dispersionless excitation in the acoustic-optic phonon gap in (IV) materials.