Nanostructured crystals of fluorite phases Sr1–x R x F2 + x and their ordering: 11. Influence of structural cluster ordering on fluorine ionic conductivity of the Sr1–x Lu x F2 + x Phase

Abstract

Ionic conductivity σ of the ordered phase (binary compound) Sr4Lu3F17 (sp. gr. \(R\overline 3 \), Z = 6) with a fluorite-derivative structure obtained from a melt has been studied for the first time by impedance spectroscopy. The octahedral–cubic clusters {Sr8[Lu6F37]F32} formed in the structure of the disordered (fluorite) phase Sr1–xLuxF2 + x (sp. gr. \(Fm\overline 3 m\), Z = 4) become structural blocks with long-range order in the Sr4Lu3F17 compound. The effect of the clusters on the conductivity of nonstoichiometric phases Sr1–xLuxF2 + x (2–25 mol % LuF3) and Sr4Lu3F17 (42.86 mol % LuF3) has been studied for the first time. Cluster ordering in Sr4Lu3F17 is accompanied by a decrease in the conductivity (σ = 6.3 × 10–7 S/cm at 673 K and ΔHσ = 1.12 eV) by a factor of ~25 with respect to a disordered Sr0.75Lu0.25F2.75 crystal (experiment) and by a factor of 125 with respect to the saturated fluorite phase Sr0.63Lu0.37F2.37 (extrapolation). The unit-cell volume changes per F– ion during ordering are small and should not significantly affect the fluorine ion conductivity.