Synthesis of lanthanum silicate oxyapatite materials as a solid oxide fuel cell electrolyte

Lanthanum silicate oxyapatites of structural formula La9.33+x(SiO4)6O2+1.5x (0 < x < 0.27) are currently investigated for their high ionic conductivity. We have studied the synthesis of pure apatite powders, especially La9.33(SiO4)6O2 and La9.56(SiO4)6O2.34, by solid state reactions. We have finalized a synthesis process which limits the formation of the often encountered secondary phases La2SiO5 and La2Si2O7 by using an appropriate thermal treatment of the starting mixture. Lanthanum oxyapatite powders were synthesized at a temperature much lower (1200 °C) than that used in the conventional powder solid-state synthesis routes (T > 1600 °C). X-ray diffraction, scanning electron microscopy and specific surface area measurements were used to analyze the structural and microstructural changes of the as prepared powders heated at different temperatures. Electrical characterization of pure sintered materials was conducted and showed that the incorporation of extra oxide ions, corresponding to the La9.56(SiO4)6O2.34 composition, enhanced the ionic conductivity by one order of magnitude when comparing to that of the La9.33(SiO4)6O2 sample.