Ab initio molecular dynamics investigation of the elastic properties of superionic Li2O under high temperature and pressure

Abstract

Theoretical prediction on the elastic properties of superionic material is challenging due to the fast diffusion of cation/anion in the lattice. Here, we investigated the elastic properties of ${\mathrm{Li}}_{2}\mathrm{O}$ at elevated temperature and pressure using ab initio molecular dynamics (AIMD). We observed the superionic transition above 1300 K, and the highly diffusive ${\mathrm{Li}}^{+}$ leads to local structure change with significant influence on the elasticity of ${\mathrm{Li}}_{2}\mathrm{O}$. We successfully predicted the significant ${C}_{11}$ softening above 1300 K, and the calculated elastic constants fit the previous experimental results very well. It suggests the anharmonic lattice vibration before superionic transition and the diffusion of ${\mathrm{Li}}^{+}$ after superionic transition are very important for the prediction on the elastic properties, and the AIMD method is able to describe the superionic behavior accurately. In addition, we calculated the bulk and shear moduli, sound velocities, as well as elastic and sound velocity anisotropies. We found that the superionic state transition also leads to the weakening of the elastic and sound velocity anisotropies in ${\mathrm{Li}}_{2}\mathrm{O}$. Pressure has negative effect on the mobility of ${\mathrm{Li}}^{+}$, which strengthens the elastic stiffening effect of superionic ${\mathrm{Li}}_{2}\mathrm{O}$ with increasing pressure.