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 Grüneisen 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 $E_{2u}$ 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 Grüneisen parameters of the $A_{1g}$ and $E_{1g}$ modes are quite close to that of the $T_{2g}$ mode of the cubic diamond.