Fluoride ion conducting behavior in nonstoichiometric lanthanum oxyfluoride

Abstract

Nonstoichiometric lanthanum oxyfluorides, LaO 1− x F 1+2 x (0 ≤ x ≤ 0.5) and Sr 2+ -doped La 1− y Sr y O 0.6− x −0.5 y F 1.8+2 x (0 ≤ x ≤ 0.2, 0 ≤ y ≤ 0.4), were investigated as potential candidates for fluoride ion (F − ) conductors. Although stoichiometric LaOF has been reported to exhibit a phase transition between the tetragonal α-phase (space group: P 4/ nmm ) and trigonal β-phase ( R -3 m ), such a phase transition was successfully suppressed by introducing excess F − ions into the lattice of lanthanum oxyfluoride. Furthermore, the introduction of excess F − ions, also improved the-conducting properties of the oxyflouride lattice. Among the various samples investigated, it was observed that, the F − ion conductivity of La 0.7 Sr 0.3 O 0.4 F 1.9 ( x = 0.05, y = 0.3) was the highest, with a value of 2.9 × 10 −4 S cm −1 at 300 °C, approximately 930 times higher than that of stoichiometric LaOF.