Single-Crystal Growth and Fluorine-Ionic Conductivity of Tysonite-type Ternary (R, R')0.95Sr0.05F2.95 (R, R' = La−Nd) Solid Electrolytes

To search for superionic fluoride materials with improved conductive properties, the qualitative composition of the rare-earth component of crystals with the tysonite-type structure (sp. gr. P3̄c1 ) in the ternary systems SrF2−LaF3−NdF3 and SrF2−CeF3−PrF3 was complicated, and conductometric properties were studied. Multicomponent crystals (R, R')0.95Sr0.05F2.95 (R, R' = La−Nd) were grown from a melt by the vertical directional crystallization. Fluorine-ion conductivity σdc = 8.8 × 10−4 S/cm at 293 K is maximum for the (Ce0.5Pr0.5)0.95Sr0.05F2.95 composition, which is currently the limit value for the class of tysonite-type crystals. A comparative analysis of the ion-conducting properties of ternary (R, R')0.95Sr0.05F2.95 and binary R0.95Sr0.05F2.95 compositions was carried out. The effect of increasing ionic conductivity upon transition to ternary solid solutions (R, R')0.95Sr0.05F2.95 is confirmed and explained within the crystal-chemical model framework. The results demonstrate the potential of quantitatively increasing the complexity of the crystal chemical composition and studying multicomponent solid electrolytes in ternary (and more complex) systems for further optimization and achieving maximum room-temperature fluoride-ion conductivity values.