An Investigation of the Synthesis and Conductivities of La-Ge-O Based Systems

Recently apatite-type phases (e.g. La(10-x)(Si/Ge)(6)O(26 +/- z)) have been attracting significant interest due to their high oxide ion conduction. In the case of the Ge based systems there is some uncertainty regarding the nature of the conducting phase, whether it is indeed apatite based or cation deficient La(2)GeO(5)-type. In this paper we report a detailed investigation of the phase with composition La(9.33)Ge(6)O(26). We show that for synthesis temperatures in the range 1150-1300 degrees C, the hexagonal apatite-type structure is obtained (a = b = 9.913(4), c = 7.282(4) angstrom), but heating to higher temperatures (> 1300 degrees C) leads to the occurrence of extra peaks in the XRD pattern around the apatite peaks. Attempts to refine the extra peaks on a monoclinic apatite-type cell have so far proved unsuccessful, and the exact nature of this system is not clear, although the XRD pattern appears to resemble that of an apatite-type phase more closely than that of La(2)GeO(5). In addition to the change in the XRD pattern, there is also a significant change in the oxide ion conductivity. Specifically the activation energy for samples prepared/sintered at high temperatures (1500 degrees C) is significantly higher than for those prepared/sintered at lower temperatures (1150 degrees C). The changes observed appear to be due to loss of Ge, and if the sample is heated at high temperature (1500 degrees C) for several days, the formation of La(2)GeO(5) becomes apparent. This loss of Ge is a significant problem for the possible use of these materials in SOFCs. In addition to the data on La(9.33)Ge(6)O(26), we also present conductivity data for La(2)GeO(5) and La(4)GeO(8) for comparison.