New possible candidate structure for phase IV of solid hydrogen

• Published in: RSC advances Vol. 1; no. 44; pp. 26443 - 2645
• Main Authors: Li, Guo-Jun, Gu, Yun-Jun, Li, Zhi-Guo, Chen, Qi-Feng, Chen, Xiang-Rong
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 15.07.2020; The Royal Society of Chemistry
• Subjects: Bonding strength; Chemical bonds; Chemistry; Enthalpy; First principles; Hydrogen; Mathematical analysis; Molecular dynamics; Molecular structure; Optimization techniques; Particle swarm optimization; Perturbation methods; Perturbation theory; Pressure dependence
• ISSN: 2046-2069; 2046-2069
• DOI: 10.1039/d0ra03295f

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 Ama 2 is proposed as a possible candidate structure for phase IV. The Ama 2 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 Ama 2 and Pc are competitive in the pressure region of phase IV. Nevertheless, the Raman and infrared vibron frequencies of Ama 2 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 Ama 2 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 Ama 2 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 C 2/ 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 C 2/ c structure. As a whole, the vibron frequencies of the Ama 2 structure agree better with the experimental results compared with the Pc structure.