The Intrinsic Fluorine-Ion Conductivity of Crystalline Matrices of Fluoride Superionics: BaF2 (Fluorite Type) and LaF3 (Tysonite Type)

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The intrinsic fluorine-ion conductivity σ_lat of BaF₂ (CaF₂ fluorite type) and LaF₃ (tysonite type) crystals is studied by the impedance spectroscopy method. These compounds represent two major structural types taken as the basis to form the best nonstoichiometric fluorine-conducting solid electrolytes. The conductivity σ_lat caused by thermally activated defects is manifested in the field of high temperatures, where conductometric measurements are complicated by pyrohydrolysis. The experiments carried out in inert atmosphere with application of the impedance method have for the first time produced the reliable values of σ_lat of fluoride crystals in conditions of suppression of pyrohydrolysis (BaF₂) or partial pyrohydrolysis (LaF₃). Values of the σ_lat at 773 K for BaF₂ and LaF₃ crystals grown from melt by the Bridgman method using the vacuum technology are 2.2 × 10^–5 and 8.5 × 10^–3 S/cm differing by a factor of ~400. The tysonite structural type has been proved feasible for making high-conductivity solid fluoride electrolytes based on the analysis of energy characteristics of formation and migration of anionic defects.