Prediction of Cyclic O₆ Molecules Stabilized by Helium under Pressure

Oxygen usually exists in the form of diatomic molecules at ambient conditions. At high pressure, it undergoes a series of phase transitions from diatomic O₂ to O₈ cluster and ultimately dissociates into a polymeric O₄ spiral chain structure. Intriguingly, the commonly found cyclic hexameric molecules in other group VIA elements (e.g., S₆ and Se₆) are never reported in the bulk oxygen. Through extensive computational crystal structure search, herein it is reported that such hexameric O₆ molecules can exist in a stable compound HeO₃ above 1.9 TPa. The first-principles calculations reveal that, during the reaction by mixing oxygen with helium, the insertion of helium does not only expand the lattice volume, but also relieves the electron lone pair repulsion among diatomic O₂, and thus significantly promoting the formation of cyclic O₆ molecules. Furthermore, the transition pathway calculations demonstrate that molecular O₂ is dissociated first, and then six oxygen atoms form a polymeric digital 2-shaped intermediate O₆. Subsequently, each unstable intermediate O₆ decomposes into two intermedia O₃ trimers. Finally, O₃ trimers transform into cyclic O₆ molecules at high pressure. This study expands the known molecular forms of oxygen and suggests a route to the synthesis of intriguing cyclic O₆ molecules.