Synthesis and Ionic Conduction of Cation-deficient Apatite La9.332x/3MxSi6O26 Doped with Mg, Ca, Sr*

Apatite-type lanthanum silicate with special conduction mechanism via interstitial oxygen has attracted considerable interest in recent years. In this work, pure powder of La9.33–2x/3MxSi6O26 (M=Mg, Ca, Sr) is prepared by the sol-gel method with sintering at 1000°C. The powder is characterized by X-ray diffraction (XRD) and scanning electron micrograph (SEM). The apatite can be obtained at relatively low temperature as compared to the conventional solid-state reaction method. The measurements of conductivity of a series of doped samples La9.33–2x/3MxSi6O26 (M=Ca, Mg, Sr) indicate that the type of dopant and the amount have a significant effect on the conductivity. The greatest decrease in conductivity is observed for Mg doping, following the Ca and the Sr doped apatites. The effect is ultimately attributed to the amount of oxygen interstitials, which is affected by the crystal lattice distortion arising from cation vacancies.