Solid-State Structures of XeO3

The solid-state structure of xenon trioxide, XeO₃, was reinvestigated by low-temperature single-crystal X-ray diffraction and shown to exhibit polymorphism that is dependent on the crystallization conditions. The previously reported α-phase (orthorhombic, P2₁2₁2₁) only forms upon evaporation of aqueous HF solutions of XeO₃. In contrast, two new phases, β-XeO₃ (rhombohedral, R3) and γ-XeO₃ (rhombohedral, R3c), have been obtained by slow evaporation of aqueous solutions of XeO₃. The extended structures of all three phases result from Xe═O---Xe bridge interactions among XeO₃ molecules that arise from the amphoteric donor-acceptor nature of XeO₃. The Xe atom of the trigonal-pyramidal XeO₃ unit has three Xe---O secondary bonding interactions. The orthorhombic α-XeO₃ displays the greatest degree of variation among the contact distances and has a significantly higher density than the rhombohedral phases. The ambient-temperature Raman spectra of solid α- and γ-XeO₃ have also been obtained and assigned for the first time.