Oxide Ion Conductivity and Chemical Stability of Lanthanum Fluorides Doped with Oxygen, La(Sr,Na)F32XOX

Oxide ion conductivity in doped LaOXF3-2X was investigated. It was found that oxygen doped into LaF3 can be mobile through introduced anion vacancy and LaO0.6F1.8 exhibits the high oxide ion conductivity, which is comparable with those in doped Bi2O3-based oxides. The oxide ion conductivity in LaO0.6F1.8 was further improved by simultaneous doping of SrO, SrF2, and NaF. The highest conductivity is achieved with the composition La0.9Sr0.1Na0.05O0.4F2.0 in the present study. La0.9Sr0.1Na0.05O0.4F2.0 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 LaF3 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 F2 from lattice was not observed. Therefore, it was confirmed that oxygen-doped LaF3 exhibits oxide ion conductivity. The chemical stability of La0.9Sr0.1O0.4F2.0 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.