VIBRATIONAL PROPERTIES OF INSE UNDER PRESSURE - EXPERIMENT AND THEORY

The pressure dependence of the phonon modes in the layered semiconduct or gamma-InSe has been investigated experimentally and theoretically f or pressures up to II GPa. The mode Gruneisen parameters of all Raman- active zone-center phonons have been determined by Raman scattering un der pressure. In addition, features corresponding to second and third- order scattering processes are apparent in the Raman spectra under res onance conditions, from which information about zone-edge modes can be obtained. For the assignment of the observed Raman features to vibrat ional modes we have calculated the phonon dispersion curves using a ri gid-ion model including couplings to first-nearest neighbors and long- range Coulomb interaction. At about 7 GPa the sample turns from transp arent to opaque and a new Raman mode appears in the spectra at around 165 cm(-1). This is evidence of a pressure-induced structural instabil ity of gamma-InSe, which is optically detected but not by X-ray diffra ction.