Compressed arsenolite As4O6 and its helium clathrate As4O6·2He

Crystal structure of arsenolite, the cubic polymorph of 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. In the structure the most compressed are As···As contacts which contrasts with increased intramolecular As···As distance in the deformed molecule. The ratio between As···As inter- and intramolecular distances decreases from 1.47 at 0.1 MPa 10 to 1.03 at 30 GPa. The As4O6 molecules are deformed to become more tetrahedron-like. Pressure above 3 GPa favours the formation of As4O6·2He inclusion compound in the surface layer increasingly deeper with pressure. The experimental As4O6 crystal compression has been compared with various theoretical models within the DFT framework. According to band-structure calculations the arsenolite band gap falls from 4.2 eV at ambient pressure to 2.7 eV at 27.8 GPa.
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