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Intermolecular coupling and fluxional behavior of hydrogen in phase IV.

ABSTRACT: We performed Raman and infrared (IR) spectroscopy measurements of hydrogen at 295 K up to 280 GPa at an IR synchrotron facility of the Shanghai Synchrotron Radiation Facility (SSRF). To reach the highest pressure, hydrogen was loaded into toroidal diamond anvils with 30-?m central culet. The intermolecular coupling has been determined by concomitant measurements of the IR and Raman vibron modes. In phase IV, we find that the intermolecular coupling is much stronger in the graphenelike layer (G layer) of elongated molecules compared to the Br₂-like layer (B layer) of shortened molecules and it increases with pressure much faster in the G layer compared to the B layer. These heterogeneous lattice dynamical properties are unique features of highly fluxional hydrogen phase IV.

SUBMITTER: Goncharov AF

PROVIDER: S-EPMC6926044 | biostudies-literature | 2019 Dec

REPOSITORIES: biostudies-literature

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Intermolecular coupling and fluxional behavior of hydrogen in phase IV.

Goncharov Alexander F AF Chuvashova Irina I Ji Cheng C Mao Ho-Kwang HK

Proceedings of the National Academy of Sciences of the United States of America 20191203 51

We performed Raman and infrared (IR) spectroscopy measurements of hydrogen at 295 K up to 280 GPa at an IR synchrotron facility of the Shanghai Synchrotron Radiation Facility (SSRF). To reach the highest pressure, hydrogen was loaded into toroidal diamond anvils with 30-μm central culet. The intermolecular coupling has been determined by concomitant measurements of the IR and Raman vibron modes. In phase IV, we find that the intermolecular coupling is much stronger in the graphenelike layer (G l ...[more]

PMID: 31796597

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Project description:It has been proved in experiments that there are at least five phases of solid hydrogen at high pressure, however, only the structure of phase I has been absolutely determined. We revisited the phase space of solid hydrogen in the pressure range of 200-500 GPa using the particle swarm optimization technique combined with first-principles simulations. A novel orthorhombic structure named Ama2 is proposed as a possible candidate structure for phase IV. The Ama2 structure is a 'mixed structure' with two different types of layers and is distinctly different from the previously reported Pc structure. Enthalpies and Gibbs free energies show that Ama2 and Pc are competitive in the pressure region of phase IV. Nevertheless, the Raman and infrared vibron frequencies of Ama2 calculated by using density functional perturbation theory based on first-principles lattice dynamics show a better agreement with the experimental measurements than those of the Pc structure. And the pressure dependence of these low-frequency Raman vibrons of Ama2 obtained from the first-principles molecular dynamics simulation shows a steeper slope, which resolves the long-standing issue of large discrepancies between the calculated Raman frequencies and the experimental ν 1 [P. Loubeyre, F. Occelli and P. Dumas, Phys. Rev. B: Condens. Matter Mater. Phys., 2013, 87, 134101 and C. S. Zha, R. E. Cohen, H. K. Mao and R. J. Hemley, Proc. Natl. Acad. Sci. U.S.A., 2014, 111, 4792]. Structural and vibrational analyses show that the hydrogen molecules in the weakly bonded molecular layer of Ama2 form distorted hexagonal patterns, and their vibration can be used to explain the experimental ν 1 vibron. It is found that the weakly bonded layer is almost the same as the layers in the C2/c structure. This confirms the experimental conclusion [P. Loubeyre, F. Occelli and P. Dumas, Phys. Rev. B: Condens. Matter Mater. Phys., 2013, 87, 134101] that the ordering of hydrogen molecules in the weakly bonded molecular layers of the 'mixed structure' for phase IV is similar to that in the layers of the C2/c structure.

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Intermolecular coupling and fluxional behavior of hydrogen in phase IV.

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Intermolecular coupling and fluxional behavior of hydrogen in phase IV.

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