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

Multiphase fluoride polycrystalline eutectics pRF3 × qMF2 forming in the MF2–RF3 (M = Ca, Sr, Ba; R = La–Nd) binary systems were synthesized by the directional crystallization technique from a melt. The phase composition, morphology, and temperature dependences of fluorine ionic conductivity in fabricated composites were studied in detail. The pRF3 × qMF2 (p and q are the mole percentages of components) eutectic composites consist of both extremely saturated fluorite-type structure M1−xRxF2+x solid solutions and the tysonite-type R1−yMyF3−y ones. Microsized growth blocks with a fine lamellar structure are typical for synthesized composites. The thinnest (from 3 μm) and longest lamellae are observed in the 68LaF3 × 32BaF2 composition. The ionic conductivity values of pRF3 × qMF2 composites are determined by the phase composition, practically do not depend on their morphological features, and reach 10−3–10−2 S/cm at 500 K (with an ion transport activation enthalpy of about 0.5–0.6 eV). Crystallized eutectics are superior to any single-phase M1−xRxF2+x solid solutions and ball-milling R1−yMyF3−y nanoceramics in terms of ion-conducting properties. These fluoride materials represent an alternative to widely applied tysonite-type ceramic composites in various electrochemical devices and require further in-depth studies.