Cascade of High-Pressure Transitions of Claudetite II and the First Polar Phase of Arsenic(III) Oxide

Claudetite II, one of the three known As₂O₃ polymorphs, has been studied by single-crystal synchrotron X-ray diffraction up to 21 GPa. Three new As₂O₃ polymorphs, all structurally related to claudetite II, are formed through a sequence of single-crystal-to-single-crystal phase transitions: the ambient pressure claudetite IIα (monoclinic space group P2₁/n, Z = 4) transforms at 2 GPa continuously to phase IIα′ (P2₁/a, Z = 8), at 6 GPa to phase IIα″ (P2₁/a, Z = 24), and at 11 GPa it transforms discontinuously to phase IIβ (P2₁, Z = 4). The experimentally observed phases α–α″ are centrosymmetric, whereas phase β is polar. A significant hysteresis on decompression skips the α″ phase and only α′ phase is formed before the ambient pressure α polymorph is recovered. The transitions and compression of claudetite II preserve its structure of 2D polymeric As₂O₃ layers; however, the intersheet As···As distances are compressed at 21 GPa to about 2.9 Å, only 0.5 Å more than the As–As single bond length. Previous DFT modeling of claudetite II structure under pressure suggested only one transformation around 15 GPa, but to a different structure (Guńka, P. A.; Dranka, M.; Piechota, J.; Żukowska, G. Z.; Zalewska, A.; Zachara, J. Cryst. Growth Des. 2012, 5663).