Stability of Ar(H₂)₂ to 358 GPa

• Published in: Proceedings of the National Academy of Sciences - PNAS 2017-04, Vol.114 (14), p.3596-3600
• Main Authors: Ji, Cheng, Goncharov, Alexander F., Shukla, Vivekanand, Jena, Naresh K., Popov, Dmitry, Li, Bing, Wang, Junyue, Meng, Yue, Prakapenka, Vitali B., Smith, Jesse S., Ahuja, Rajeev, Yang, Wenge, Mao, Ho-kwang
• Format: Article
• Language: English
• Subjects: hydrogen-rich compound; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; intermolecular interaction; metallization; Physical Sciences; ultrahigh pressure
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.1700049114

“Chemical precompression” through introducing impurity atoms into hydrogen has been proposed as a method to facilitate metallization of hydrogen under external pressure. Here we selected Ar(H₂)₂, a hydrogen-rich compound with molecular hydrogen, to explore the effect of “doping” on the intermolecular interaction of H₂ molecules and metallization at ultrahigh pressure. Ar(H₂)₂ was studied experimentally by synchrotron X-ray diffraction to 265 GPa, by Raman and optical absorption spectroscopy to 358 GPa, and theoretically using the density-functional theory. Our measurements of the optical bandgap and the vibron frequency show that Ar(H₂)₂ retains 2-eV bandgap and H₂ molecular units up to 358 GPa. This is attributed to reduced intermolecular interactions between H₂ molecules in Ar(H₂)₂ compared with that in solid H₂. A splitting of the molecular vibron mode above 216 GPa suggests an orientational ordering transition, which is not accompanied by a change in lattice symmetry. The experimental and theoretical equations of state of Ar(H₂)₂ provide direct insight into the structure and bonding of this hydrogen-rich system, suggesting a negative chemical pressure on H₂ molecules brought about by doping of Ar.