Vibrons and Phonons in Solid Hydrogen at Megabar Pressures

Raman and infrared active vibrons and phonons in the solid molecular hydrogen at megabar pressures have been studied by the first-principles band theoretical calculations assuming the hcp lattice of the centers of the molecules. The frequencies of the out-of-phase stretching motion of the two molecules in the unit cell are higher than those of the in-phase stretching motion. Pressure dependence of the frequency of a phonon-like mode, which lies in 1300 cm ^-1 ∼ 1900 cm ^-1 at pressures 70 GPa ∼ 200 GPa, is in good agreement with the recent experiments. Our results suggest that the lattice of the solid hydrogen remains hcp beyond the phase transition at 150 GPa.