Phonon-promoted superionic conduction in fluorite-structured compounds

Solids with ionic conductivities rivaling those of typical liquids are dictated by a rare family of materials exhibiting certain chemical and structural features for fast diffusion. Compounds with crystalline structures containing distinctly coordinated lattice sites are generally not considered to be in this class. Here, we demonstrate that superionic conduction is indeed possible in a compact lattice containing large enough metastable interstices to frustrate ions from steric confinement. The analysis of conduction in fluorite compounds confirms that the geometrically frustrated configurations give rise to the response of a non-diffusing framework in the form of interactions between ions and phonons. This type of phonon-ion interaction dynamically promotes the formation of mobile defects and, as a result, superionic conduction. The concept of phonon-promoted conduction through geometric frustration opens the door to trigger superionic conduction in compounds that were overlooked in past studies, thereby providing a new strategy for the discovery of novel superionic conductors.