The Synthesis and Characterisation of Ge Containing Apatite-Type Oxide Ion Conductors
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The Synthesis and Characterisation of Ge Containing Apatite-Type Oxide Ion Conductors P.R. Slater, J.E.H. Sansom, J.R. Tolchard, M.S. Islam Department of Chemistry, University of Surrey, Guildford, Surrey. GU2 7XH. UK ABSTRACT Apatite-type oxides, La10-x(Si/Ge)6O26+z, have been attracting significant interest recently due to their high oxide ion conductivities. Most of the work so far has focused on the Si based systems, since the Ge based systems suffer from problems attributed to Ge loss. In this paper we show that doping divalent cations on the La site or B on the Ge site helps to stabilise the hexagonal apatite lattice for these Ge based systems. These doped phases show high oxide ion conductivities, although results from extended sintering studies suggest that Ge loss is still a problem. In order to limit Ge loss, we have also examined Bi doping to lower the sintering temperature and preliminary results for the novel Bi containing apatite-type phases, La6Bi2M2Ge6O26 (M=Mg, Sr, Ba) and La8-xBi2Ge5GaO26+y, are also reported. INTRODUCTION Following the pioneering early work by Nakayama et al. on high oxide ion conduction in apatite-type La10-xSi6O26+z there has been significant interest in the oxide ion conducting properties of these apatite materials [1-14], due to potential technological applications in high temperature devices such as solid oxide fuel cells. Most of the initial work has focused on the Si based materials, since high quality samples can be readily made in this system. Early studies showed the importance of non-stoichiometry, either in terms of cation vacancies or oxygen excess, to maximise the oxide ion conductivity of these phases. Neutron diffraction studies of samples with cation vacancies, e.g. La9.33Si6O26, indicated the presence of disorder within the oxide channels, in the form of Frenkel-type defects [5], while in the oxygen excess samples, disorder in the sense of excess interstitial oxide ions is present. A range of doping studies has also been reported for the Si based samples [6,8,11,13]. In contrast, studies of comparable samples containing Ge in place of Si are limited. Initial work in this area was reported by Arikawa et al., who showed that corresponding Ge containing compositions, (La/Sr)10-xGe6O26+z, exhibited higher conductivities than the Si based systems at high temperatures [7]. However, the X-ray patterns showed severe peak broadening/extra peaks, and subsequently the same group claimed that the conducting phase was cation deficient La2GeO5 [15]. In contrast, studies by Nakayama et al. and our previous studies supported the conclusion that the highly conducting phase was indeed apatite-type [9, 12]. In our previous work, we showed that La9.33Ge6O26 with the hexagonal apatite lattice could be prepared at intermediate temperature (1150°C). Heating to higher temperature (required to obtain dense pellets for conductivity measurements) resulted in, firstly peak broadening, and then the occurrence of distinct extra peaks either side of the main apatite peaks (figure 1).
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