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The co-doped lanthanum silicate oxyapatites, La 9.5 A 0.5 Si 5.5 Fe 0.5 O 26.5 (A ¼ Ba, Sr, and Ca), are synthesized by the high-temperature solid state reaction process. The phase formation and structure properties of undoped lanthanum silicate oxyapatite (La 9.67 Si 6 O 26.5 , LSO), Fe-doped lanthanum silicate oxyapatite (La 10 Si 5 FeO 26.5 , LSFO) and co-doped lanthanum silicate oxyapatites (La 9.5 A 0.5 Si 5.5 Fe 0.5 O 26.5 , A ¼ Ba, Sr, and Ca) are characterized by X-ray diffraction (XRD) and scanning electron spectroscopy (SEM). The effect of co-doping of A (A ¼ Ba, Sr, Ca) and Fe on the microstructure, sinterability and oxide ion conductivity of lanthanum silicate oxyapatites is investigated in detail. The results show that, as compared to LSO and LSFO oxyapatites, co-doping of A (A ¼ Ba, Sr, and Ca) and Fe significantly benefits the sintering and densification process, and enhances the oxide ion conductivity. For co-doped oxyapatites, the oxide ion conductivities are related to the dopant size, the best properties are obtained for the oxyapatite co-doped with Ca and Fe. The co-doped La 9.5 Ca 0.5 Si 5.5 Fe 0.5 O 26.5 (LCSFO) oxyapatite is a good electrolyte for SOFCs, with an oxide ion conductivity of 1.39 Â 10 À2 S cm À1 at 800 C and a low activation energy of 90.71 kJ mol À1 . The bulk density and oxide ion conductivities of co-doped oxyapatite ceramics increase significantly with the increase of the sintering temperature. The grain bulk and grain boundary resistances of La 9.5 Ba 0.5 Si 5.5 Fe 0.5 O 26.5 (LBSFO), La 9.5 Sr 0.5 Si 5.5 Fe 0.5 O 26.5 (LSSFO) and LCSFO oxyapatite ceramics are significantly smaller than those of LSO and LSFO oxyapatite ceramics sintered under the identical conditions.

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