Stability of Proton Superoxide and its Superionic Transition Under High Pressure

Under extreme conditions, condensed matters are subject to undergo a phase transition and there have been many attempts to find another form of hydroxide stabilized over H₂O. Here, using Density Functional Theory (DFT)-based crystal structure prediction including zero-point energy, it is that proton superoxide (HO₂), the lightest superoxide, can be stabilized energetically at high pressure and temperature conditions. HO₂ is metallic at high pressure, which originates from the 𝜋^* orbitals overlap between adjacent superoxide anions (O₂ ^-). By lowering pressure, it undergoes a metal-to-insulator transition similar to LiO₂. Ab initio molecular dynamics (AIMD) calculations reveal that HO₂ becomes superionic with high electrical conductivity. The possibility of creating hydrogen-mixed superoxide at lower pressure using a (Li_x,H_1-x)O₂ hypothetical structure is also proposed. This discovery bridges gaps in superoxide and superionicity, guiding the design of various H-O compounds under high pressure.