Oxide Ion Conductivity and Chemical Stability of Lanthanum Fluorides Doped with Oxygen, La(Sr,Na)F3-2XOX

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Oxide ion conductivity in doped LaO X F 3 - 2 X was investigated. It was found that oxygen doped into LaF 3 can be mobile through introduced anion vacancy and LaO 0.6 F 1.8 exhibits the high oxide ion conductivity, which is comparable with those in doped Bi 2 O 3 -based oxides. The oxide ion conductivity in LaO 0.6 F 1.8 was further improved by simultaneous doping of SrO, SrF 2, and NaF. The highest conductivity is achieved with the composition La 0.9 Sr 0.1 Na 0.05 O 0.4 F 2.0 in the present study. La 0.9 Sr 0.1 Na 0.05 O 0.4 F 2.0 is chemically stable in oxygen partial pressure from 1 to 10 - 21 atm, and almost the theoretical electromotive force is exhibited in the oxygen gas-concentration cell. The conductivity of oxygen-doped LaF 3 was further confirmed by the dc polarization method. No polarization in conductivity was observed over 24 h, and also no segregation of F - ion at positive electrode was noticed. Furthermore, desorption of F 2 from lattice was not observed. Therefore, it was confirmed that oxygen-doped LaF 3 exhibits oxide ion conductivity. The chemical stability of La 0.9 Sr 0.1 O 0.4 F 2.0 was further studied, and it was confirmed that F-rich lanthanum oxyfluoride is highly stable against humidity not only at elevated but also at room temperature.