Ordered helium trapping and bonding in compressed arsenolite: Synthesis of As4O6 2He

Compression of arsenolite has been studied from a joint experimental and theoretical point of view. Experiments on this molecular solid at high pressures with different pressure-transmitting media have been interpreted thanks to state-of-the-art ab initio calculations. Our results confirm arsenolite as one of the most compressible minerals and provide evidence for ordered helium trapping above 3 GPa between adamantane-type As4O6 cages. Our calculations indicate that, at relatively small pressures, helium establishes rather localized structural bonds with arsenic forming a compound with stoichiometry As4O6 center dot 2He. All properties of As4O6 center dot 2He are different from those of parent As4O6. In particular, pressure-induced amorphization, which occurs in arsenolite above 15 GPa, is impeded in As4O6 center dot 2He, thus resulting in a mechanical stability of As4O6 center dot 2He beyond 30 GPa. Our work paves the way to explore the formation of alternative compounds by pressure-induced trapping and reaction of gases, small atomic or molecular species, in the voids of molecular solids containing active lone electron pairs.