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

Abstract 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 (SiO 4 ) 6 O 3 and La 9.33 (SiO 4 ) 6 O 2, 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 P6 3 or P6 3 /m, with a = 9.722(1), c = 7.182(1)A for La 10 (SiO 4 ) 6 O 3 and a = 9.717(2), c = 7.177(1)A for La 9.33 (SiO 4 ) 6 O 2, i.e., the cell volume of La 10 (SiO 4 ) 6 O 3 is a little greater than that of La 9.33 (SiO 4 ) 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 (SiO 4 ) 6 O 3 is higher than La 9.33 (SiO 4 ) 6 O 2 indicating that oxygen interstitials may be introduced into the apatite lattice for La 10 (SiO 4 ) 6 O 3, which may benefit the oxygen ion transportation. The a.c. and d.c. conductivitis are comparable for La 10 (SiO 4 ) 6 O 3 but not for La 9.33 (SiO 4 ) 6 O 2.