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 Γ1+), dipolar (Γ15−), and quadrupolar (Γ12+) 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.
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