Theoretical predictions for low-temperature phases, softening of phonons and elastic stiffnesses, and electronic properties of sodium peroxide under high pressure

Abstract

High-pressure phase stabilities up to 600 K and the related properties of Na₂O₂ under pressures up to 300 GPa were investigated using first-principles calculations and the quasi-harmonic approximation. Two high-pressure phases of Na₂O₂ that are thermodynamically and dynamically stable were predicted consisting of the Amm2 (distorted P2m) and the P2₁/c structures, which are stable at low temperature in the pressure range of 0–22 GPa and 22–28 GPa, respectively. However, the P2m and Pbam structures become the most stable instead of the Amm2 and P2₁/c structures at the elevated temperatures, respectively. Interestingly, the softening of some phonon modes and the decreasing of some elastic stiffnesses in the Amm2 structure were also predicted in the pressure ranges of 2–3 GPa and 9–10 GPa. This leads to the decreasing of phonon free energy and the increasing of the ELF value in the same pressure ranges. The HSE06 band gaps suggest that all phases are insulators, and they increase with increasing pressure. Our findings provide the P–T phase diagram of Na₂O₂, which may be useful for investigating the thermodynamic properties and experimental verification.