Synthesis and ionic conductivities of M (Mg, Ba, Zr) and Al co-doped apatite-type lanthanum germanate electrolytes for IT-SOFC

In this paper, the structure and ionic conductivities have been investigated for M (Mg, Ba, Zr) and Al co-doped apatite-type lanthanum germanates. La_9.0.5Ge_5.5Al_0.5O_26±δ (M = Mg, Ba and Zr) were prepared by solid state reaction. The results of XRD analysis showed that the space group is P6₃/m. M (Mg, Ba, Zr) substituting La site and Al substituting Ge site increased the cell volume and improved the lattice distortion of the apatite structure. In addition, M (Mg, Ba, Zr) and Al doping facilitated sintering and improved the relative density. Analysis of AC impedance spectroscopy showed that M (Mg, Ba, Zr) and Al co-doping in lanthanum germanates introduced local distortion and improved the ionic conductivities. Moreover, the thermal expansion coefficient (TEC) of La_9.5M_0.5Ge_5.5Al_0.5O_26±δ (M = Mg, Ba and Zr) was improved after doping with M (Mg, Ba, Zr) and Al ions. La_9.5Mg_0.5Ge_5.5Al_0.5O_26.5 sintered at 1400 °C exhibited the highest conductivity (1.26 × 10⁻² S/cm, 800 °C) and improving TEC, which suggests that the (M (Mg, Ba, Zr), Al) doped apatite-type lanthanum germanate can be a potential material for the IT-SOFC electrolyte.