High-pressure and low-temperature structural study of claudetite I, a monoclinic layered As2O3polymorph

Abstract

High-pressure structural studies of arsenic(III) oxide polymorphs are completed by presenting crystal structures of claudetite I determined using synchrotron powder X-ray diffraction up to ~30 GPa. The crystal structure of claudetite I is also redetermined based on synchrotron single-crystal X-ray diffraction measurements at room and low temperatures. Claudetite I undergoes no phase transitions under high pressure nor at low temperature. It is the densest and least compressible ambient-pressure As2O3 polymorph with the following third-order Vinet equation of state parameters: V0 = 312.1(13) Å3, B0 = 21.0(11) GPa, and B'0 = 6.72(17). Fitting of a simple Berman thermal equation of state to the lowtemperature single-crystal data yields V00 = 307.4(4) Å3 and a0 = 9.1(7) × 10-5 K-1 with a reference temperature of 298 K. The coordination sphere of arsenic atoms is thoroughly characterized by the valence entropy coordination number and the description of their lone electron pair stereoactivity using bond-valence vectors. Surprisingly, it is found that although claudetite I is the densest ambient-pressure arsenic(III) oxide polymorph, it does not exhibit the highest arsenic coordination numbers. The pressure dependence of secondary As?O bond lengths follows the same trends for all crystalline As2O3 polymorphs confirming the same nature of the interactions in all forms. It is also found that pressure does not affect the stereoactivity of arsenic lone electron pairs.