The ternary compound ${\mathrm{CuGaS}}_{2}$ is a direct energy gap semiconductor (${\mathit{E}}_{\mathit{g}}$\ensuremath{\sim}2.50 eV at 300 K) that crystallizes in the tetragonal chalcopyrite (I4\ifmmode\bar\else\textasciimacron\fi{}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 Gr\"uneisen 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 Gr\"uneisen parameters of the center-of-zone ${\mathrm{\ensuremath{\Gamma}}}_{5}^{1}$ 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. \textcopyright{} 1996 The American Physical Society.