# Stability of Proton Superoxide and its Superionic Transition Under High Pressure

## TL;DR

Scientists predict a new form of superoxide, proton superoxide (HO2), that can be stable under high pressure and shows unique electrical properties.

## Contribution

The study identifies proton superoxide (HO2) as a stable compound under high pressure and reveals its superionic transition and metallic behavior.

## Key findings

- HO2 is energetically stable at high pressure and temperature conditions.
- HO2 transitions from metallic to insulating behavior as pressure decreases.
- HO2 becomes superionic with high electrical conductivity at elevated temperatures.

## Abstract

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 (Lix,H1‐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.

The study predicts the stable proton superoxide (HO2) under extreme conditions using Density Functional Theory (DFT)‐based crystal structure prediction. HO2 exhibits metallic behavior at high pressure, transitions to an insulator under reduced pressure, and becomes superionic with high electrical conductivity under elevated temperatures. The possibility of creating hydrogen‐lithium mixed superoxides at lower pressures is also explored.