Compressed Arsenolite As4O6 and Its Helium Clathrate As4O6•2He

The crystal structure of arsenolite, the cubic polymorph of molecular arsenic(III) oxide, has been determined by single-crystal X-ray diffraction up to 30 GPa. The bulk of the crystal is monotonically compressed with no detectable anomalies to 60% of the initial volume at 30 GPa. The experimental As4O6 crystal compression exceeds that obtained by various theoretical models within the density functional theory framework. The As4O6 molecules ate deformed toward, a more tetrahedral shape, A surprising property of argenolite immersed in helium has been revealed above 3 GPa; the As4O6 center dot 2He clathrate is formed in the surface layer increasingly deeper with pressure. Interestingly, this is the first example of helium clathrate formed in situ with a solid molecular oxide and proof that helium may permeate, even nonporous single crystals in high-pressure diffraction studies. This indicates it is an important and general phenomenon that needs to be taken into account when conducting high-pressure diffraction studies in helium.