Raman scattering and phonon dispersion in Si and GaP at very high pressure

One- and two-phonon Raman spectra of Si and GaP were measured at room temperature for pressures up to 135 kbar. An opposed diamond-anvil high-pressure cell was employed in the experiments, and its design and use for Raman scattering are described in detail. Mode Gr\"uneisen parameters and quadratic pressure coefficients were measured for phonons at several zone-boundary critical points as well as at q0. In addition the general effect of pressure on large portions of the phonon dispersion near the zone edge could be inferred. In both materials zone-boundary TA modes "softened" with increasing pressure, while optical phonons shifted to higher energy. Using the high-pressure Raman data a calculation of the thermal-expansion coefficient of Si as a function of temperature (negative at low temperature) achieved fair agreement with experiment. Measured and theoretically calculated mode Gr\"uneisen parameters are compared for several tetrahedral semiconductors. The Raman spectrum of Si was measured up to the metallic (-Sn structure) tansformation at 1255 kbar. This transition is discussed within the context of the bond-charge model.