Compressed Arsenolite As₄O₆ and Its Helium Clathrate As₄O₆·2He

Abstract

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 As 4 O 6 crystal compression exceeds that obtained by various theoretical models within the density functional theory framework. The As 4 O 6 molecules are deformed toward a more tetrahedral shape. A surprising property of arsenolite immersed in helium has been revealed above 3 GPa; the As 4 O 6 ·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.