Nature of the Superionic Phase Transition of Lithium Nitride from Machine Learning Force Fields

Superionic conductors have great potential as solid-state electrolytes, but the physics of type-II superionic transitions remains elusive. In this study, we employed molecular dynamics simulations, using machine learning force fields, to investigate the type-II superionic phase transition in α-Li ₃N. We characterized Li ₃N above and below the superionic phase transition by calculating the heat capacity, Li ⁺ ion self-diffusion coefficient, and Li defect concentrations as functions of temperature. Our findings indicate that both the Li ⁺ self-diffusion coefficient and Li vacancy concentration follow distinct Arrhenius relationships in the normal and superionic regimes. The activation energies for self-diffusion and Li vacancy formation decrease by a similar proportion across the superionic phase transition. This result suggests that the superionic transition may be driven by a decrease in defect formation energetics rather than changes in Li transport mechanism. This insight may have implications for other type-II superionic materials.