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 met hod. In addition, the Gruneisen parameters of the cubic and hexagonal diamo nd are obtained. It is found that most of the phonon frequencies of the cub ic and hexagonal diamond exhibit blueshift under pressure except for the E- 2u mode of the hexagonal diamond, which is softening above 42 GPa. Therefor e, a phase transformation of the hexagonal diamond at high pressure is expe cted. The Gruneisen parameters of the A(1g) and E-1g modes are quite close to that of the T-2g mode of the cubic diamond. [S0163-1829(99)01226-6].