How and Why Does Helium Permeate Nonporous Arsenolite Under High Pressure?

Investigations into the helium permeation of arsenolite, the cubic, molecular arsenic(III) oxide polymorph As4O6, were carried out to understand how and why arsenolite helium clathrate As(4)O(6)2He is formed. High-pressure synchrotron X-ray diffraction experiments on arsenolite single crystals revealed that the permeation of helium into nonporous arsenolite depends on the time for which the crystal is subjected to high pressure and on the crystal history. The single crystal was totally transformed into As(4)O(6)2He within 45h under 5GPa. After release of the pressure, arsenolite was recovered and a repeated increase in pressure up to 3GPa led to practically instant As(4)O(6)2He formation. However, when a pristine arsenolite single crystal was quickly subjected to a pressure of 13GPa, no helium permeation was observed at all. No neon permeation was observed in analogous experiments. Quantum mechanical computations indicate that there are no specific attractive interactions between He atoms and As4O6 molecules at the distances observed in the As(4)O(6)2He crystal structure. Detailed analysis of As4O6 molecular structure changes has shown that the introduction of He into the arsenolite crystal lattice significantly reduces molecular deformations by decreasing the anisotropy of stress exerted on the As4O6 molecules. This effect and the pV term, rather than any specific AsHe binding, are the driving forces for the formation As(4)O(6)2He.