Prediction of Cyclic O6 Molecules Stabilized by Helium under Pressure.

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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.