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A two-dimensional type I superionic conductor
Nature Materials  (IF43.841),  Pub Date : 2021-07-22, DOI: 10.1038/s41563-021-01053-9
Alexander J. E. Rettie, Jingxuan Ding, Xiuquan Zhou, Michael J. Johnson, Christos D. Malliakas, Naresh C. Osti, Duck Young Chung, Raymond Osborn, Olivier Delaire, Stephan Rosenkranz, Mercouri G. Kanatzidis

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.