Anharmonic effects in light scattering due to optical phonons in CuGaS2

The ternary compound CuGaS2 is a direct energy gap semiconductor (E(g) similar to 2.50 eV at 300 K) that crystallizes in the tetragonal chalcopyrite (<I(4)over bar 2d>) structure and its isoelectronic cubic analog is ZnS. The temperature dependence (10-300 K) of the Raman active phonons has been studied up to 18 GPa in a membrane diamond anvil cell, using helium gas as a pressure transmitting medium. These measurements allowed us to determine the temperature dependence of the mode Gruneisen parameters and to separate the isobaric temperature dependence of the optical modes into pure-volume and pure-temperature contributions. By this procedure, the cubic and quartic anharmonicities responsible for the pure-temperature contributions to the mode frequencies were determined. The mode Gruneisen parameters of the center-of-zone Gamma(5)(1) mode (which corresponds to the edge-of-zone TA in ZnS) is negative throughout the whole temperature range, reaching - 1.12 at 77 K. At low temperatures, this provides a possible driving mechanism for the negative values of the thermal expansion coefficient in semiconductors.