Synthesis and characterization of Al3+ and M (M = W6+, In3+, Nb5+, Mg2+) co-doped lanthanum silicate oxy-apatite electrolytes

Abstract

Apatite-type La10Si5Al0.9M0.1O27−δ (M = W6+, In3+, Nb5+ or Mg2+) are successfully synthesized by the high-temperature solid state reaction method. The composition, microstructure and electrical conduction performance of the samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS), respectively. The dense co-doped lanthanum silicates electrolytes with pure hexagonal apatite-type structure are obtained after sintered at 1873 K for 6 h. La10Si5Al0.9W0.1O26.65 has the highest total conductivity among the co-doped samples in the temperature range of 673–1073 K. The influences of the co-doped W6+ content on the properties of the La10Si5Al1−xWxO27±δ (0 ≤ x ≤ 1) are investigated. It is found that with increasing the W6+ content, the equiaxed apatite-type grains are replaced gradually by the rod-like grains, accompanied by the formation of impurity phases (La6W2O15, La2SiO5 and La2Si2O7) in the samples. The La10Si5Al0.9W0.1O26.65 exhibits the highest conductivity of 3.03 × 10−2 S cm−1 at 1073 K. The oxygen pressure independency of the total conductivity suggests that La10Si5Al0.9W0.1O26.65 remains an almost pure oxygen ionic conductor. La10Si5Al0.9W0.1O26.65 also shows good stability in 20% H2/Ar, wet Ar and pure CO2 atmospheres, respectively.