Abstract
Oxygen usually exists in the form of diatomic molecules at ambient conditions. At high pressure, it undergoes a series of phase transitions from diatomic O(2) to O(8) cluster and ultimately dissociates into a polymeric O(4) spiral chain structure. Intriguingly, the commonly found cyclic hexameric molecules in other group VIA elements (e.g., S(6) and Se(6)) are never reported in the bulk oxygen. Through extensive computational crystal structure search, herein it is reported that such hexameric O(6) molecules can exist in a stable compound HeO(3) 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 O(2), and thus significantly promoting the formation of cyclic O(6) molecules. Furthermore, the transition pathway calculations demonstrate that molecular O(2) is dissociated first, and then six oxygen atoms form a polymeric digital 2-shaped intermediate O(6). Subsequently, each unstable intermediate O(6) decomposes into two intermedia O(3) trimers. Finally, O(3) trimers transform into cyclic O(6) molecules at high pressure. This study expands the known molecular forms of oxygen and suggests a route to the synthesis of intriguing cyclic O(6) molecules.