THEORY OF LIGHT-SCATTERING FROM SOFT MODES IN IV-VI COMPOUNDS

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. © 1979 The American Physical Society.