Lanthanum silicate La9.5Si6O26.25 (LSO) with an apatite-type structure has been synthesized via a sol-gel process. The microstructure and ionic conductivity of such samples have been evaluated as a function of sintering conditions by X-ray diffraction, scanning electron microscopy, Raman spectra and AC impedance spectroscopy. The result shows that dense pellet LSO of relative density higher than 95% with pure apatite phase can be obtained at low sintering temperature of 1500 degrees C. The samples exhibit an increased conductivity with sintering temperature, owing to grain size effect as well as local modulation of the [SiO4](4-) tetrahedra. LSO sintered at 1650 C exhibits the highest ionic conductivity of 2.70 x 10(-2) S cm(-1) at 800 degrees C. Probed by Raman spectroscopy, the local relaxation of the [SiO4]4- tetrahedra in LSO, tuned by sintering temperature, contributes to a fast conduction pathway for interstitial oxide ions accompanying with low activation energy, which results in a high ionic conductivity. (C) 2013 Elsevier B.V. All rights reserved.