Preparation and characterisation of apatite-type lanthanum silicates by a sol-gel process

Recent reports have indicated good fast oxide ion conductivity in apatite silicates. In this article we report on the successful low temperature synthesis of the apatite-type lanthanum silicates, La10(SiO4)6O3 and La9.33(SiO4)6O2, via a sol-gel process. The properties of the resulting apatite phases have been characterised by thermal analysis (TGA-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and conductivity measured by both a.c. impedance spectroscopy (IS) and d.c. methods. The apatite phases may be obtained at 800°C. On further reaction at 1400°C, crystallinity improved, with the apatite structure retained. The room-temperature structure is hexagonal, space group P63 or P63/m, with a = 9.722(1), c = 7.182(1)إ for La10(SiO4)6O3 and a = 9.717(2), c = 7.177(1)إ for La9.33(SiO4)6O2, i.e., the cell volume of La10(SiO4)6O3 is a little greater than that of La9.33(SiO4)6O2. Both compositions exhibit high ionic conductivity, although the grain boundary resistance is the dominant feature in the impedance spectrum of both. At 1000K the total conductivity is 10-3Scm-1. In general, the conductivity of La10(SiO4)6O3 is higher than La9.33(SiO4)6O2 indicating that oxygen interstitials may be introduced into the apatite lattice for La10(SiO4)6O3, which may benefit the oxygen ion transportation. The a.c. and d.c. conductivitis are comparable for La10(SiO4)6O3 but not for La9.33(SiO4)6O2.