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 F 3 -y with a tysonite type structure (LaF 3 ) 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, RF 3 (R = La-Nd), in dependence of the radius R 3+ ( ); two component stoichiometric La 1 -y R y F 3 phases (R = Pr, Nd) in dependence on the average cation radius (r cat ); and two component nonstoichiometric phases R 1 -y Ca y F 3 -y (R = La-Lu, Y) in dependence of the CaF 2 content. The optimization of the compo sition with respect to thermal stability is based on studying the phase diagrams of CaF 2 -RF 3 systems and the behavior of R 1 -y Ca y F 3 -y crystals upon heating when measuring temperature dependences σ(T). Single crystal samples of a number of investigated R 1 -y Ca y F 3 -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.

The key role of the composition and structural features in fluoride ion conductivity in tysonite Ce1−xSrxF3−x solid solutions

“…Then, single-crystals of numerous combinations (RE = La-Lu and AE = Ca, Sr, Ba) with various contents of fluorine vacancies were elaborated to study the influence of the substitution rate on the ionic conductivity. [6][7][8][9][10][11] Various synthesis methods and shaping processes (ceramic route, 3 ball milling, 4 precipitation, 12 crystal growth, 7,13 hot pressing 14 and thin films 7,15 ) were developed to obtain the best ceramics, single crystals or thin films in order to evaluate the conduction properties. Considering single-crystal analyses, the alkaline earth Sr 2+ substitution gives the highest ionic conductivity.…”

Synthesis, Microstructure, and Electrical Conductivity of Eutectic Composites in MF2–RF3 (M = Ca, Sr, Ba; R = La–Nd) Systems

“…Only limited regions of SSs M 1−x R x F 2+x (fluorite type) and R 1−y M y F 3−y (tysonite type) are formed in these systems. The M 1−x R x F 2+x and R 1−y M y F 3−y SSs have superionic conductivity and are medium-and low-temperature solid electrolytes, respectively [41][42][43]57,58]. Mobile carriers are interstitial fluorine ions in fluorite-type anion-rich M 1−x R x F 2+x crystals [59] and fluorine vacancies in tysonite-type anion-deficient R 1−y M y F 3−y crystals [60].…”

“…The eutectic composites pRF 3 × qMF 2 are two-phase materials consisting of interpenetrating components-extremely saturated SSs M 1−x R x F 2+x and R 1−y M y F 3−y . In terms of conductivity, the composites are located between the M 1−x R x F 2+x [41][42][43]50] and R 1−y M y F 3−y [41,51,57,58] crystals (see Figure 6) and are superior to ceramics of similar compositions synthesized by mechanochemical (ball milling) synthesis [55,[61][62][63][64]. Between the ionic conductivity characteristics of eutectic composites and single phases of fluorite or tysonite-type in the SrF 2 -RF 3 (R = La-Nd) systems, a "conductivity-composition" correlation was found [65].…”

Role of Ca2+ in the change in the structure type and the fluoride-ion conductivity in crystals upon transition from β-ErF3 to Er0.715Ca0.285F2.715

“…Tysonite phases containing large light REEs (La-Nd) are the best conductors, belonging to the most promising fluoride ion conducting solid electrolytes [4]. The conductiv ity of crystals of the tysonite phases R 1 ⎯ y Ca y F 3 ⎯ y with small R 3+ ions (Tb-Lu, Y) decreases in the series of REEs [3,5]. Single component fluorides RF 3 (R = Tb-Ho) in the whole temperature range and the low temperature modifications β RF 3 (R = Sm-Gd, ErLu, and Y) crystallize in the β YF 3 structure type belonging to the orthorhombic system and are charac terized by lower conductivity [6,7].…”