Pressure dependence of Raman phonons of Ge and3C-SiC

The pressure dependence of several Raman lines of Ge and $3C$-SiC has been studied up to 11 GPa with a gasketed diamond anvil cell. A nonlinear dependence of the $\mathrm{TO}\left(\Gamma \right)$ frequencies on pressure and a softening of the $\mathrm{TA}\mathrm{}\left(X\right)\mathrm{}$ modes of Ge have been observed. The Grüneisen parameters of several optical and acoustical phonons of $3C$-SiC corresponding to critical points at the edges of the zone have been determined. The linewidths of the long-wavelength optical phonons of $3C$-SiC increase for pressures above 10 GPa. This observation is interpreted in terms of an increase in the decay rates of the $\Gamma$ optical phonons into two acoustical modes.