Oxide ionic conductivity of apatite-type lanthanum silicates

Dense ceramic disks of apatite-type lanthanum silicates have been prepared using sol-gel derived powders by sintering at first 1600 degrees C and then 1750 degrees C. AC impedance measurements showed that the oxide ionic conductivity of LaxSi6O1.5x+12 increases from similar to 1 ins cm(-1) to a maximum of similar to 20 mS cm(-1) with increasing the La content from x = 9.29 to 9.92. Rietveld refinements revealed that the observed powder X-ray diffraction patterns fit to a defect structure model which incorporates La ions into 4f cation vacancy of the apatite phase and contains extra oxide ions in an interstitial site near the conduction channel. The observed increase in the conductivity with the La content is explained by an interstitial conduction mechanism associated with an increase in the extra oxide ions. (c) 2005 Elsevier B.V. All rights reserved.