Superconducting, plastic, and superionic states driven by four-membered lithium rings in a high-pressure lithium-lead compound

Using a combination of crystal structure search methods, first-principles calculations, and machine learning potential based simulations, we explored the lithium-lead system and predicted five phases: ${I}_{4}$/mcm ${\mathrm{LiPb}}_{2}$, Pnma LiPb, ${I}_{4}$/mmm ${\mathrm{Li}}_{4}\mathrm{Pb}, C2/m {\mathrm{Li}}_{5}\mathrm{Pb}$, and Cmcm ${\mathrm{Li}}_{6}\mathrm{Pb}$. Among them, ${I}_{4}$/mmm ${\mathrm{Li}}_{4}\mathrm{Pb}$ displayed remarkable properties, including superconductivity, plasticity, and superionic behavior at varying temperature ranges. At lower temperatures, ${I}_{4}$/mmm ${\mathrm{Li}}_{4}\mathrm{Pb}$ manifests superconductivity with a critical transition temperature of 4--5 K. Its superconducting behavior is attributed to the interplay between the ${B}_{2}$ g vibration mode, which signifies the rotational motion of four-membered lithium rings within the stacking layer, and the participation of $p$-orbital electrons. As temperature rises, ${I}_{4}$/mmm ${\mathrm{Li}}_{4}\mathrm{Pb}$ first transitions into a plastic phase, marked by continuous collective rotation of intralayer four-membered lithium rings, and then shows superionic behavior characterized by the emergence of interlayer lithium atom diffusion. These unique behaviors stem from stronger Li-Li bonds within four-membered lithium rings and a lower energy barrier for collective motion, distinct from interstitial localized electrons in electrides found in other lithium-based systems. This work provides an intriguing platform for exploring distinct states and establishes a correlation between various physical phenomena and the system's structure and bonding.