An investigation of the synthesis and conductivities of La-Ge-O based systems

Abstract

Recently apatite-type phases (e.g. La_10−x(Si/Ge)₆O_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 La₂GeO₅-type. In this paper we report a detailed investigation of the phase with composition La_9.33Ge₆O₂₆. We show that for synthesis temperatures in the range 1150–1300 °C, the hexagonal apatite-type structure is obtained (a=b=9.913(4), c=7.282(4) Å), but heating to higher temperatures (>1300 °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 La₂GeO₅. 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 °C) is significantly higher than for those prepared/sintered at lower temperatures (1150 °C). The changes observed appear to be due to loss of Ge, and if the sample is heated at high temperature (1500 °C) for several days, the formation of La₂GeO₅ 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_9.33Ge₆O₂₆, we also present conductivity data for La₂GeO₅ and La₄GeO₈ for comparison.