Abstract
The ternary compound CuGaS2 is a direct energy gap semiconductor ~Eg;2.50 eV at 300 K! that crystallizes in the tetragonal chalcopyrite (I 42d) 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 G5 1 mode ~which corresponds to the edge-of-zone TA in ZnS! is negative throughout the whole temperature range, reaching 21.12 at 77 K. At low temperatures, this provides a possible driving mechanism for the negative values of the thermal expansion coefficient in semiconductors. @S0163-1829~96!07428-0#
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