Ordered helium trapping and bonding in compressed arsenolite: Synthesis of $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6·2\mathrm{He}$

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 $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6$ cages. Our calculations indicate that, at relatively small pressures, helium establishes rather localized structural bonds with arsenic forming a compound with stoichiometry $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6·2\mathrm{He}$. All properties of $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6·2\mathrm{He}$ 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 $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6·2\mathrm{He}$, thus resulting in a mechanical stability of $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6·2\mathrm{He}$ 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.

Figure 1

Detail of the cubic unit cell of arsenolite $\left(\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6\right)$ (left) and $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6·2\mathrm{He}$ (right). $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6·2\mathrm{He}$ is a compound formed above 3 GPa by arsenolite with He atoms inserted into $16d$ Wyckoff sites. Large gray balls, medium-size red balls, and small blue balls represent As, O, and He atoms, respectively.

Figure 2

Evolution of the structural parameters of arsenolite under pressure for different PTM. Gray, blue, purple, and red symbols correspond to experimental data of arsenolite compressed without PTM, with silicone oil, with methanol-ethanol mixture, and with He, respectively. Black (red) solid lines correspond to theoretical data of pure (He inserted in $16d$ sites) arsenolite. Dashed lines correspond to fit of experimental data. (a) EOS of arsenolite compressed with different PTM. (b) Compression of the polyhedral volume around different Wyckoff crystallographic sites: $16d$ (circles), $16c$ (squares), and $8a$ (triangles).

Figure 3

Isosurfaces of the reduced density gradient (revealed by NCI index) evidencing noncovalent interactions in $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6$ (top) and $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6·2\mathrm{He}$ (bottom) near 16 GPa. As expected for a noble gas, van der Waals (green) interactions around the He atoms are revealed. However, unexpectedly localized interactions (in turquoise blue) also make their appearance along the He-As interaction lines.