293-K conductivity optimization for single crystals of solid electrolytes with tysonite structure (LaF3): I. Nonstoichiometric phases R 1−yCayF3−y (R = La-Lu, Y)

Abstract

The systematic optimization of single-crystal fluoride-conducting solid electrolytes R 1 − y Ca y F3 − y with a tysonite type structure (LaF3) with respect to the conductivity at room temperature, σ(293 K), is based on high-temperature measurements of σ(T) of stoichiometric fluorides of rare earth elements, RF3 (R = La-Nd), in dependence of the radius \(R^{3 + } (r_{R^{3 + } } )\); two-component stoichiometric La1 − y R y F3 phases (R = Pr, Nd) in dependence on the average cation radius (r cat ); and two-component nonstoichiometric phases R 1 − y Ca y F3 − y (R = La-Lu, Y) in dependence of the CaF2 content. The optimization of the composition with respect to thermal stability is based on studying the phase diagrams of CaF2-RF3 systems and the behavior of R 1 − y Ca y F3 − y crystals upon heating when measuring temperature dependences σ(T). Singlecrystal samples of a number of investigated R 1 − y Ca y F3 − y compounds has σ(293 K) values high enough to be applied in solid-state electrochemical devices operating at room temperature (chemical sensors, fluorine-ion batteries, and accumulators) and in devices subjected to thermal cycling.