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 degrees 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 x 10(-3)S cm(-1) at 700 degrees C). The crystal structure model can refine the profiles obtained by XRD with the P6(3) space group and the channel oxygen atoms were displaced from the ideal site at (0, 0, approximate to 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. (C)2012 The Ceramic Society of Japan. All rights reserved.