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

Abstract

Investigations into the helium permeation of arsenolite, the cubic, molecular arsenic(III) oxide polymorph As O , were carried out to understand how and why arsenolite helium clathrate As O ⋅2 He 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 O ⋅2 He within 45 h under 5 GPa. After release of the pressure, arsenolite was recovered and a repeated increase in pressure up to 3 GPa led to practically instant As O ⋅2 He formation. However, when a pristine arsenolite single crystal was quickly subjected to a pressure of 13 GPa, 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 As O molecules at the distances observed in the As O ⋅2 He crystal structure. Detailed analysis of As O 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 As O molecules. This effect and the pΔV term, rather than any specific As⋅⋅⋅He binding, are the driving forces for the formation As O ⋅2 He.

Towards a quantitative bond valence description of coordination spheres – the concepts of valence entropy and valence diversity coordination numbers

“…For instance, studying how molecular environment in crystal structures changes as a function of pressure using n VECN would require a whole theory of 'intermolecular interactions valences' and, consequently, ECoN is better suited for this purpose for now. Another example of a problem for which ECoN is more appropriate than n VECN is the illustration of the fact that the As crystallographic orbit in arsenolite approaches the cubic closest packing of spheres as pressure is increased (Guń ka et al, 2018). This can be demonstrated by plotting iterative (see Fig.…”

Crystal Structure and Non‐Hydrostatic Stress‐Induced Phase Transition of Urotropine Under High Pressure

“…Indeed, while the lattice of As 4 O 6 contains small voids of approximately 1.8 Å, [13] a different molecular packing pattern in urotropine results in a lack of such cavities (see Figure S10). This, together with the fact that volume reduction is the main driving force of the As 4 O 6(s) +2He (s) =As 4 O 6 ⋅2 He (s) process, explains why He does not penetrate urotropine [43] …”

Crystal structure and non-hydrostatic stress-induced phase transition of urotropine under high pressure

“…This, together with the fact that volume reduction is the main driving force of the As4O6(s) + 2He(s) = As4O6•2He(s) process explains why He does not penetrate urotropine. 62 ASSOCIATED CONTENT download file view on ChemRxiv manu_ChemRxivV3.pdf (1.28 MiB) Table S1. Structural phase transitions in cage-like compounds in the function of pressure or temperature.…”