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).