The effect of local structure on ionic conductivity of apatite-type La9.5Si6O26.25

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 °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 × 10−2 S cm−1 at 800 °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.