PRESSURE AND TEMPERATURE DEPENDENCES OF THE RAMAN-ACTIVE 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 the 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-the-zone Gamma(5)(1) mode (which corresponds to the edge-of-the-zone TA in ZnS) is negative throughout the whole temperature range, reaching -0.64 at 77 K. At low temperatures, this provides a possible driving mechanism for the negative values of the thermal expansion coefficient in semiconductors.