Proton or Deuteron Transfer in Phase IV of Solid Hydrogen and Deuterium

The recent discovery of phase IV of solid hydrogen and deuterium consisting of two alternate layers of graphenelike three-molecule rings and unbound ${\mathrm{H}}_2$ molecules have generated great interest. However, the vibrational nature of phase IV remains poorly understood. Here, we report a peculiar proton or deuteron transfer and a simultaneous rotation of three-molecule rings in graphenelike layers predicted by ab initio variable-cell molecular dynamics simulations for phase IV of solid hydrogen and deuterium at pressure ranges of 250–350 GPa and temperature range of 300–500 K. This proton or deuteron transfer is intimately related to the particular elongation of molecules in graphenelike layers, and it becomes more pronounced with increasing pressure at the course of larger elongation of molecules. As the consequence of proton transfer, hydrogen molecules in graphenelike layers are short lived and hydrogen vibration is strongly anharmonic. Our findings provide direct explanations on the observed abrupt increase of Raman width at the formation of phase IV and its large increase with pressure.

Figure 1

(a) Side views of the mixed phase IV structure at 250 GPa. (b) Disordered ${\mathrm{H}}_2$ layer (layer I) and (c) graphenelike layer containing three-molecule rings (layer II).

Figure 2

(a) Mean squared displacements as a function of time at various pressures and temperatures for mixed phase IV are shown. (b) The averaged intramolecular (D1 and D2 are shown in Fig. 1) and intermolecular (D3 and D4 are shown in Fig. 1) bond lengths for mixed phase IV are plotted as a function of pressures. It is found that intramolecular bond lengths (D2) in layer II gradually increase with pressure, while intramolecular bond lengths (D1) in layer I remain nearly invariant with pressure.

Figure 3

The fractional coordinates of hydrogen protons in layers I and II for mixed phase IV at 300 GPa and 300 K as a function of time, respectively.

Figure 4

(a) Mean squared displacements as a function of time at various pressures and temperatures for various phases ($C2/c$ and Cmca-4 phases for hydrogen, mixed phase IV for deuterium) are shown. (b) The total energies of mixed phase I at 250 and 300 GPa as a function of the rotational angle, respectively. The lowest energies are scaled as the zero energy.