Temperature effect on the phase stability of hydrogen C2/c phase from first-principles molecular dynamics calculations

The structural stability of hydrogen C2/c phase from 0 K to 300 K is investigated by combining the first-principles molecular dynamics (MD) simulations and density functional perturbation theory. Without considering the temperature effect, the C2/c phase is stable from 150 GPa to 250 GPa based on the harmonic phonon dispersion relations. The hydrogen molecules at the solid lattice sites are sensitive to temperature. The structural stability to instability transition of the C2/c phase upon temperature is successfully captured by the radial distribution function and probability distribution of atomic displacements from first-principles MD simulations, confirmed by the phonon power spectrum analysis in the phase space. The existence of phonon quasiparticle for different normal modes is observed directly. The phonon power spectrum of specific normal modes corresponding to the Raman and infrared (IR) activations are depicted at different temperatures and pressures. The changes of frequency with temperature are in agreement with experimental results, supporting the C2/c as the hydrogen phase III. For the first time, the anharmonic phonon dispersion curves and density of states are predicted based on the phonon quasi-particle approach. Therefore, the temperature dependence of lattice vibrations can be observed directly, providing a more complete physical picture of phonon frequency distribution with respect to the Raman and IR spectra. It is found that the high-frequency regions adopt significant frequency shifts compared to the harmonic case.