Relationship between the defect structure and fluorine-ion conductivity of La1 − y Sr y F3 − y (0 ⩽ y ⩽ 0.15) crystals according to X-ray and neutron diffraction data

Abstract

The defect structure of crystals of as-grown nonstoichiometric tysonite (LaF 3 type) La 1 -y Sr y F 3 -y phase (y = 0, 0.05, 0.07, and 0.15) has been investigated by X-ray diffraction and neutron diffraction. A concentration-driven structural transition from the β-La 1 -y Sr y F 3 -y modification to the α-La 1 -y Sr y F 3 -y modification in the range y = 0.05-0.10 has been observed for the first time. This transition correlates with the maximum of fluorine-ion conductivity (y = 0.05 ± 0.02) and the maximum in the melting curve (y = 0.07 ± 0.03). β-LaF 3 and β-La 0.95 Sr 0.05 F 2.95 crystals belong to the trigonal system (sp. gr. P c1, Z = 6) and are twinned according to the merohedral twin law. Fine changes in the fluorine sublattice of β-La 0.95 Sr 0.05 F 2.95 increase conductivity σ 293К to 3 × 10 -4 S/cm; hence, these crystals can be used as fluorine-conducting solid electrolytes (FCSEs) in solid-state ionics devices. Additional possibilities of ion transport over twin joints are discussed. In α-La 0.85 Sr 0.15 F 2.85 crystal the symmetry group changes to P6 3 /mmc, Z = 2; it is not twinned and its fluorine-ion conductivity is reduced.

Low‐temperature phase formation in the SrF2–LaF3 system

“…The crystal structures of both fluorite 25 and tysonite‐type 26 solid solutions were studied. The study of both the structures and properties of solid solutions 27‐30 is aimed at establishing the connection of the defect structure with high fluoride‐ion conductivity.…”