Fluoride solid electrolytes

Relations between the structure and electrical properties of fluoride systems with different structures are presented. Physical properties of fluorite-structured (MF 2 -RF 3 , MF 2 -AF, MF 2 -M′F 2 , M = Ba, Pb, R = La-Lu, Sc, Y, A = Li, Na, K, Rb, M′ = Ba, Cd, Mg), orthorhombic (RF 3 , R = Tb-Er, Y), tysonite-structured (RF 3 -MF 2 , R = La-Nd, M = Sr), and monoclinic (BaR 2 F 8 , R = Ho-Yb, Y) fluoride single crystals or ceramics (ErF 3 , MF 2 -RF 3 , M = Ca, Ba, R = La, Gd, Tb, Y), glasses (ZBLAN, PIBAL) and eutectic composites (LiF-RF 3 , R = La-Gd, Y, PbF 2 -RF 3 , R = Ho, Yb, Y, Sc, PbF 2 -AF, A = Li, Na, PbF 2 -MgF 2 ) are compared. Anisotropy of electrical properties is explained. Models of aggregation of defects into clusters are proposed. In fluoritestructured crystals, the highest ionic conductivity was found for PbF 2 : 7 mol % ScF 3 (at 500 K, σ 500 = 0.13 S/cm). In tysonite-structured crystals, the highest ionic conductivity was found for LaF 3: 3 mol % SrF 2 (σ 500 = 2.4 × 10 −2 S/cm). Different types of coordination polyhedrons and their different linking in orthorhombic and tysonite structures explain a large difference between conductivities in both structures.