Stability of Ar(H ₂) ₂ to 358 GPa

"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.