ELASTIC-MODULI AND MODE GAMMAS OF GAP - THEIR RELATIONSHIP TO THOSE OF OTHER ISOMORPHIC CRYSTALS AND THE HIGH-PRESSURE STRUCTURAL-ELECTRICAL TRANSITION

The transit times of ultrasonic waves associated with piezoelectrically inactive modes have been measured in sulfur-dope, n-type GaP using hydrostatic pressures up to 5 kbar. The velocities and elastic constants of most modes increase linearly with pressure but CS = 1 2(C11-C12) decreases slightly. The behavior of the elastic constants and their associated mode gammas are compared to those of other III-V compounds and of Si and Ge. We suggest that the behavior of CS is related to the occurrence of the structural-electrical transition which occurs at high pressure. A modified Born criterion for lattice stability developed by Demarest et al. is used to explain this relationship. The contribution of thermal dilatation to the temperature dependence of each elastic constant is deduced. Using it and data from the literature we find that the explicit thermal contribution to each elastic constant is significant and in the case of C12 has an anomalous sign. © 1979.