Synthesis and oxide ion conductivity of lanthanum–europium oxide fluoride, La2Eu2O3F6

La2Eu2O3F6 was prepared by solid–solid reaction between 1-mol La2O3 and 2-mol EuF3 at 1473 K. The crystal structure of La2Eu2O3F6 was analyzed to be the monoclinic structure (a0=0.403 nm, b0=1.14 nm, c0=0.572 nm, β=135.2°) derived from the fluorite structure. The ionic arrangement was suggested to be somewhat disordered in the La2Eu2O3F6 in contrast to that in the Nd2Eu2O3F6 with highly ordered ionic arrangement. It was assumed that the disorder in La2Eu2O3F6 resulted in the lower oxide conductivity, La2Eu2O3F6 (σ=0.8 S m−1, τO2−=0.7 at 773 K), than that of Nd2Eu2O3F6 (σ=2.0 S m−1, τO2−=0.9 at 773 K). La2Eu2O3F6 was stable up to ca. 970 K in air, and converted into LaEuO3 (monoclinic) at 1623 K through the metastable state of La2EuO2F2 (rhombohedral) at ca. 1190 K by the pyrohydrolysis. The electrical conductivity declined due to pyrohydrolysis; LaEuO2F2, σ≈2.0×10−2 S m−1, LaEuO3, σ≈7.0×10−5 S m−1 at 873 K.