Relationship between the defect structure and fluorine-ion conductivity of La1 − ySryF3 − y (0 ⩽ y ⩽ 0.15) crystals according to X-ray and neutron diffraction data

The defect structure of crystals of as-grown nonstoichiometric tysonite (LaF3 type) La1 − ySryF3 − 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-La1 − ySryF3 − y modification to the α-La1 − ySryF3 − 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-LaF3 and β-La0.95Sr0.05F2.95 crystals belong to the trigonal system (sp. gr. \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$P\bar 3c1$$\end{document}, Z = 6) and are twinned according to the merohedral twin law. Fine changes in the fluorine sublattice of β-La0.95Sr0.05F2.95 increase conductivity σ293K 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-La0.85Sr0.15F2.85 crystal the symmetry group changes to P63/mmc, Z = 2; it is not twinned and its fluorine-ion conductivity is reduced.