Tunable Phase Transitions in a Two-Dimensional Superionic Conductor

Superionic conductors (SICs) 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 SICs, where high ionic diffusivity is linked to the crystal structure <i>via</i> a first-order transition and is therefore either on or off, are attractive for energy storage and conversion applications providing the transition temperature can be controlled. However, such SICs (e.g., AgI, Ag₂Se, etc.) have been found exclusively in 3D crystal structures so far, limiting existing approaches to applied pressure and nanoscale size effects – which affect the transition temperature in one direction – while the effects of chemical substitution cannot be easily decoupled from 3D ion-conducting pathways. Layered materials have spatially separated mobile and immobile sublattices, potentially facilitating wide tunability and functioning as platforms to study two-dimensional superionic conduction.<br/><br/>We previously identified a first-order, order-disorder phase transition in 2D KAg₃Se₂ – a dimensionally-reduced derivative of 3D Ag₂Se – at ~695 K using in-situ XRD and DSC [1]. Here, we use quasi-elastic neutron scattering and AIMD simulations reveal that a-KAg₃Se₂ is a type I SIC with a highly disordered Ag sublattice restricted to 4 Å thick layers [2]. The superionic local structure was probed by in-situ XPDF analysis, confirming the defect dynamics from AIMD simulations (Fig. 1b). Thermal analyses of cation-substituted <i>A</i>Ag₃Se₂ (<i>A</i> = Li-Cs) compounds indicate that the superionic transition temperature can be tuned by the composition of the immobile charge-balancing layers (Fig. 1c). Our work extends the known classes of superionic conductors and points the way to the design of new materials, in bulk, single-/few-layer and thin film forms, with tailored ionic conductivities and phase transitions.<br/><br/>1. A.J.E. Rettie et al., <i>J. Am. Chem. Soc.</i>,<b> 2018</b>, 140, 9193−9202.<br/>2. A.J.E. Rettie et al., <i>Nature Mater.</i>, <b>2021</b>, 20 (12), 1683-1688.