α-${\mathrm{Li}}_3\mathrm{N}$ is a promising $\mathrm{L}{\mathrm{i}}^{+}$ superionic conductor for battery applications due to its high ionic conductivity. The superionic behavior emerges with the breakdown of anisotropy in $\mathrm{L}{\mathrm{i}}^{+}$ diffusion. This study investigates the role of different Li sites in the hexagonal layered structure in influencing the ionic transport using machine-learned molecular dynamics simulation. We found that below 600 K, the Li at $2c$ Wyckoff site contribute dominantly to diffusion in the $\mathit{ab}$ plane. However, at elevated temperatures, Li at both the Wyckoff sites ($2c$ and $1b$) becomes equally mobile, leading to three-dimensional superionic behavior. Further, we identified that the superionic transition is associated with the strong anharmonicity in low-energy phonon modes dominated by $\mathrm{L}{\mathrm{i}}^{+}$ dynamics. We find that this transition leads to a marked reduction in lattice thermal conductivity, highlighting the role of strongly anharmonic low-energy phonons in suppressing thermal transport.

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• Diffusion
• Ionic conductivity
• Phonons
• Ab initio molecular dynamics
• Machine learning
• Molecular dynamics