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. La-10(SiO4)(6)O-3 and La-9.33(SiO4)(6)O-2 via a sol-gel process. The properties of the resulting apatite phases have been characterised by thermal analysis (TCA-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 degreesC. On further reaction at 1400 degreesC. crystallinity improved. with the apatite structure retained. The room-temperature structure is hexagonal, space group P6(3) or P6(3)/m, with a = 9.722(1). c = 7.182(1)Angstrom for La-10(SiO4)(6)O-3 and a = 9.717(2), c = 7.177(1)Angstrom for La-9.33(SiO4)(6)O-2 i.e., the cell volume of La-10(SiO4)(6)O-3 is a little greater than that of La-9.33(SiO4)(6)O-2. 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(-3)Scm(-1) In general. the conductivity of La-10(SiO4)(6)O-3 is higher than La-9.33(SiO4)(6)O-2 indicating that oxygen interstitials may be introduced into the apatite lattice for La-10(SiO4)(6)O-3 which may benefit the oxygen ion transportation. The a.c. and d.c. conductivitis are comparable for La-10(SiO4)(6)O-3 but not for La-9.33(SiO4)(6)O-2 (C) 2001 Elsevier Science Ltd. All rights reserved.