Discovery of a Two-Dimensional Type I Superionic Conductor

Type I superionic conductors (e.g., AgI, Ag₂Se, etc.) are defined by an abrupt transition to the superionic state and have so far been found exclusively in 3D crystal structures. Here, we reveal a new 2D type I superionic conductor, α-KAg₃Se₂ by total scattering techniques and complementary simulations. Quasi-elastic neutron scattering (QENS) from the high temperature superionic phase match a simple Fickian diffusion mechanism with a diffusion coefficient of ~10^-5 cm² s^-1 between 710 and 740 K. Ab initio molecular dynamics simulations confirm that the mobile Ag⁺ ions are confined to 4 Å thick layers, in addition to reproducing the experimental diffusion coefficient from QENS and the local structure obtained from X-ray powder pair-distribution-function analysis. Finally, chemical substitutions suggest that the nature of alkali metal ions comprising the charge-balancing layers can facilitate or inhibit the phase transition temperature.