Ordered helium trapping and bonding in compressed arsenolite: Synthesis of A s4O6.2 He

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 A s₄O₆ cages. Our calculations indicate that, at relatively small pressures, helium establishes rather localized structural bonds with arsenic forming a compound with stoichiometry A s₄O₆.2 He . All properties of A s₄O₆.2 He are different from those of parent A s₄O₆ . In particular, pressure-induced amorphization, which occurs in arsenolite above 15 GPa, is impeded in A s₄O₆.2 He , thus resulting in a mechanical stability of A s₄O₆.2 He 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.