Superionic conductors possess liquid-like ionic diffusivity in the solid state, finding wide applicability from electrolytes in energy storage to materials for thermoelectric energy conversion. Type I superionic conductors (for example, AgI, Ag₂Se and so on) are defined by a first-order transition to the superionic state and have so far been found exclusively in three-dimensional crystal structures. Here, we reveal a two-dimensional type I superionic conductor, α-KAg₃Se₂, by scattering techniques and complementary simulations. Quasi-elastic neutron scattering and ab initio molecular dynamics simulations confirm that the superionic Ag⁺ ions are confined to subnanometre sheets, with the simulated local structure validated by experimental X-ray powder pair-distribution-function analysis. Finally, we demonstrate that the phase transition temperature can be controlled by chemical substitution of the alkali metal ions that compose the immobile charge-balancing layers. Our work thus extends the known classes of superionic conductors and will facilitate the design of new materials with tailored ionic conductivities and phase transitions.

超离子导体在固态下具有类似液体的离子扩散性，从储能电解质到热电能量转换材料都有广泛的应用。I 型超离子导体（例如 AgI、Ag ₂ Se 等）由到超离子态的一级跃迁定义，迄今为止仅在三维晶体结构中发现。在这里，我们通过散射技术和互补模拟揭示了一种二维 I 型超离子导体 α-KAg ₃ Se _{2 。}准弹性中子散射和从头算分子动力学模拟证实超离子 Ag ⁺离子被限制在亚纳米片中，模拟的局部结构通过实验 X 射线粉末对分布函数分析得到验证。最后，我们证明相变温度可以通过构成固定电荷平衡层的碱金属离子的化学取代来控制。因此，我们的工作扩展了已知的超离子导体类别，并将促进具有定制离子电导率和相变的新材料的设计。