Zone-center modes of cubic and hexagonal diamond under high pressure: A first-principles study

The high-pressure behavior of the optical modes of the cubic and hexagonal diamond at the Brillouin-zone center is studied with a first-principles method. In addition, the Gr\"uneisen parameters of the cubic and hexagonal diamond are obtained. It is found that most of the phonon frequencies of the cubic and hexagonal diamond exhibit blueshift under pressure except for the E₂ᵤ mode of the hexagonal diamond, which is softening above 42 GPa. Therefore, a phase transformation of the hexagonal diamond at high pressure is expected. The Gr\"uneisen parameters of the A1g and E1g modes are quite close to that of the T2g mode of the cubic diamond.