Crystalline polymeric phases of sulphur dioxide

SO₂ is a molecule of significant industrial and geochemical importance, known for its role in sulphuric acid production and its natural occurrence in volcanic processes. Recent studies have revealed pressure-induced amorphisation and the formation of polymeric amorphous phases in SO₂, behaviours analogous to those observed in other fundamental molecular systems such as CO₂, N₂, and CS₂. Here, we identify a mixture of polymeric SO₂ phases, with space groups Ama2 (Z = 2) and Pmc2₁ (Z = 8), the latter a homologue of γ-SeO₂, as the crystalline parents of the previously reported threefold-coordinated amorphous SO₂ observed above 25 GPa. These phases were characterized using a combination of advanced synthesis and refined high-pressure loading techniques, alongside x-ray diffraction, Raman, and infrared spectroscopy. Structural assignments were further supported by numerical predictions of candidate crystal structures. Notably, the Ama2 and γ-SeO₂-like phases exhibit in the pressure region 20-60 GPa the lowest and near-degenerate enthalpies, Ama2 being stable below 25 GPa and γ-SeO₂-like above 25 GPa. Both phases feature distinctive W-shaped polymeric units, a structural motif identified long ago at ambient pressure in the rare-mineral Downeyite (SeO₂), but the stacking of chains is different and pressure-dependent.