Pressure dependence of sound velocities in 3C-SiC and their relation to the high-pressure phase transition

The pressure dependences of the elastic constants and the sound velocities are calculated from first principles for 3C-SiC in the range 0–100 GPa. The sound velocities are found to decrease with increasing pressure for the lowest transverse acoustic ~TA! branches for the @110# and @111# sound wave propagation directions. For the @100# direction they first increase and then decrease at higher pressures. The role of anharmonicity is discussed. In particular, the importance of relaxing the relative position of the two sublattices during the acoustic deformation is emphasized. The relation of these negative TA mode Grüneisen parameters to the strains involved in the high-pressure phase transition from zinc blende and wurtzite to rocksalt are discussed. The phonon modes at X and their mode Grüneisen parameters are also calculated, and the correlation of the gTA(X) with the high-pressure transition proposed by Weinstein is discussed.