Universal dynamic response in polycrystals of advanced superionic conductors

Abstract

The work develops the material science of advanced superionic conductors (AdSICs). The AdSICs have been distinguished by the authors as a new class of solid-state ionic conductors due to the best ion-transport properties and their special crystal structure that is close to optimum for fast ion transport (FIT). Structure descriptors, a new geometry of 3D close packing of FIT tunnels inside the unit cell, and the distinctive properties of grain boundaries (GBs) in AdSICs have been revealed. On this structural basis and a recently developed theory of ion transport on a nanoscale (“structure-dynamic approach of nanoionics,” SDA), the absence of Jonsher’s universal dynamic response (UDR) in polycrystalline AdSICs has been explained. In solid-state ionic conductors, UDR holds in a frequency (ω) range depending on heights (η) and heights difference (Δη) of potential barriers which mobile ions overcome in a non-uniform crystal potential landscape. In polycrystalline AdSICs, η is not above 0.2 eV and Δη ≤ ~ 0.1 eV; therefore, the SDA indicates the response to external electric excitations has an ohmic character, i.e., does not depend on ω. The important role of processes of structural self-organization and the formation of low-energy coherent GBs in AdSICs is emphasized. It is concluded that a deeper understanding of the structure and properties of AdSICs can serve as a guide for searching and designing new materials/devices with FIT.