Vibrational properties of Si and Ge under high pressures

Using a first-principles method, the vibrational properties of silicon and germanium under both ambient conditions and high pressure have been studied. The vibrational properties of silicon and germanium are very similar. The phonon frequencies of the first Brillouin zone center and zone boundary under ambient conditions agree very well with the previous experimental results. The frequencies of all modes exhibit blueshifts under high pressure except for the TA mode. The frequency of the TA mode exhibits a redshift under compression. The mode softening of the TA mode is related to the phase transformation of silicon and germanium from the cubic diamond phase to the β-Sn phase. The anharmonicity of the TA mode becomes more significant as the applied pressure increases, and it help to advance the phase transformation of silicon and germanium from the cubic diamond phase to the β-Sn phase.