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

Abstract

The recent discovery of phase IV of solid hydrogen and deuterium consisting of two alternate layers of graphene-like three-molecule rings and unbound H 2 molecules have generated great interests. However, vibrational nature of phase IV remains poorly understood. Here, we report a peculiar proton transfer and a simultaneous rotation of three-molecule rings in graphene-like 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 transfer is intimately related to the particular elongation of molecules in graphene-like 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 graphene-like 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.

Identification of high-pressure phases III and IV in hydrogen: Simulating Raman spectra using molecular dynamics

“…Our 768-and 288-atom results are significantly different from previous MD work 34,35 which has been equivocal about the structure of phase IV, due to reorienting of molecules, transformation of one layer type to another, and "mixed" phases of apparently random B-and G-layer stacking. In our own calculations with 24 or 48 molecules per unit cell we find the same behavior as described in previous work.…”

“…This metastable structure illustrates the hierarchical natural of the bonding: a primary tendency to form layers, a secondary tendency to order as B or G within the layers, and a tertiary effect of interlayer interactions. A similar transformation, between po-hcp (phase IV) and C2/c (phase III) has been seen by Liu et al 34 Figure 6 also shows that the phase IVb structure with rotating trimers does correctly reproduce the experimental frequencies, while the phase IVa and P c structures do not. The phase IVb lower peak seems to be composed of two overlapping peaks as shown in Fig.…”

“…We note the similarity between this BBBB phase V and the molecular phases I and II; however, Mulliken bond-charge analysis suggests that the bond charge in phase V is much reduced. The transition to Cmca-4 was also found by Liu et al 34 in their metadynamics calculations. We conclude that the already-achievable experimental conditions 17 are already close to the transition to a Cmca-4 "phase V" state.…”

Graphene physics and insulator-metal transition in compressed hydrogen

“…A new phase (phase IV) has been discovered above 220 GPa in the higher temperature regime 4,5,8 . Though several different structures have been proposed for phase IV from first-principles calculations, 11,12,15 there is agreement that this phase can be viewed as a mixed-layer structure, where layers of weakly interacting H 2 molecules are sandwiched between graphene-like layers.…”

“…Recently, solid hydrogen has been intensively investigated in a new pressure-temperature domain (200-360 GPa, and just above 300 K), both experimentally [4][5][6][7][8][9] and theoretically [10][11][12][13][14][15] . A new phase (phase IV) has been discovered above 220 GPa in the higher temperature regime 4,5,8 .…”