We present a theoretical description of the Raman-Brillouin spectra from the coupled soft optical phonon and acoustical modes in IV-VI compounds. The spectra are studied above and below the cubic to rhombohedral transition temperature Tc. A mean-field description of the sublattice distortion and static strains below Tc is also given, supposing electrostrictive coupling as well as anharmonic phonon-phonon interactions are present. The theoretical results are compared with data on SnTe. We predict strong interference between the Brillouin and second-order Raman spectra above Tc. A different class of interference phenomena is present below Tc. We illustrate these points through model calculations on SnTe.
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Katayama et al. (1979) studied this question.
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