Raman scattering in beta-ZnS

The first- and second-order Raman spectra of cubic ZnS (beta-ZnS, zinc-blende) are revisited. We consider spectra measured with two laser lines for samples with different isotopic compositions, aiming at a definitive assignment of the observed Raman features and the mechanisms which determine the linewidth of the first order TO and LO Raman phonons. For this purpose, the dependence of the observed spectra on temperature and pressure is investigated. The linewidth of the TO phonons is found to vary strongly with pressure and isotopic masses. Pressure runs, up to 15 GPa, were performed at 16 K and 300 K. Whereas well-defined TO Raman phonons were observed at low temperature in the whole pressure range, at 300 K the TO phonons appear to hybridize strongly with the two-phonon background and lose their identity, especially in the (3-10)-GPa region. The intensity of the TO phonons, which nearly vanishes when measured with a red laser line, is shown to result from a destructive interference of the amplitudes of the band-edge resonance and that of a background of opposite sign. The analysis of these effects is aided by calculations of the densities of one- and two-phonon states performed with the adiabatic bond charge model of the lattice dynamics.