Effect of Al-doping on crystal structure and ionic conductivity of apatite-type La9.33+x/3Si6-xAlxO26 solid electrolytes

La9.33+x/3Si6−xAlxO26 (0 ≤ x ≤ 2.0) sinters with an apatite-type crystal structure were synthesized by solid-state reaction at 1600°C for 5 h, and the sintered discs were examined by AC impedance analysis and X-ray Diffraction (XRD). The total conductivity was significantly increased with an increase in the Al-content and showed a maximum value for x = 1.5 (8.8 × 10−3 S cm−1 at 700°C). The crystal structure model can refine the profiles obtained by XRD with the P63 space group and the channel oxygen atoms were displaced from the ideal site at (0, 0, ≈0.25) with an increase in the Al-content and showed a maximum displacement for x = 1.5. These results show that the position of the channel oxygen atoms is the key parameter that governed the conductivity.