Structural transition of solid hydrogen between phases I and II at low temperatures.

The structural transition of solid hydrogen between phases I and II at low temperatures has been investigated to determine the detailed structure of phase II by performing quantum molecular-dynamic simulations. Molecular orientations are revealed to arrange orderly in the [0001] and [112¯0] lattice directions in phase II although it has a lower-symmetry P63/mmc lattice structure than phase I. Consequently, both molecular centers and orientations tend to stack in the ABAB fashion in the [0001] lattice direction when solid hydrogen transits from phase I to II. By monitoring the evolution of infrared frequency with temperature at different pressures, the critical pressure at which solid hydrogen is initiated to transit from phase I to II is determined at different temperatures, especially 55 GPa at 0 K. This finding is expected to resolve the long-term confusion on the structure of phase II and provides new insight into solid hydrogen at low temperature, especially involving the arrangement nature of molecules therein.