Ordered helium trapping and bonding in compressed arsenolite: Synthesis ofAs4O6·2He

Abstract

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 $\mathrm{A}{\mathrm{s}}_{4}{\mathrm{O}}_{6}$ cages. Our calculations indicate that, at relatively small pressures, helium establishes rather localized structural bonds with arsenic forming a compound with stoichiometry $\mathrm{A}{\mathrm{s}}_{4}{\mathrm{O}}_{6}\ifmmode\cdot\else\textperiodcentered\fi{}2\mathrm{He}$. All properties of $\mathrm{A}{\mathrm{s}}_{4}{\mathrm{O}}_{6}\ifmmode\cdot\else\textperiodcentered\fi{}2\mathrm{He}$ are different from those of parent $\mathrm{A}{\mathrm{s}}_{4}{\mathrm{O}}_{6}$. In particular, pressure-induced amorphization, which occurs in arsenolite above 15 GPa, is impeded in $\mathrm{A}{\mathrm{s}}_{4}{\mathrm{O}}_{6}\ifmmode\cdot\else\textperiodcentered\fi{}2\mathrm{He}$, thus resulting in a mechanical stability of $\mathrm{A}{\mathrm{s}}_{4}{\mathrm{O}}_{6}\ifmmode\cdot\else\textperiodcentered\fi{}2\mathrm{He}$ 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.