The synthesis and conductivities of the apatite-type phases La9.33Si6-xGexO26

Recently apatite-type phases, e.g. (La/Sr)(10-x)Si(6)O(26+y), have been attracting significant interest due to their high oxide ion conduction, with the Ge based analogues, (La/Sr)(10-x)Ge(6)O(26+y), showing the highest conductivities. These latter systems are, however complicated by Ge loss, and so it is of interest to examine mixed Si/Ge systems, to potentially obtain the higher conductivities associated with Ge based systems, whilst minimising-the problems associated with Ge loss. In this study we have therefore prepared and characterised. a range of novel mixed silicon/germanium apatite samples, La(9.33)Si(6-x)Ge(x)O(26), (0 less than or equal to x less than or equal to 6). We-report data on the conductivities for these systems and show that there appears to be an optimum nominal Si:Ge ratio of 0.5 where conductivity at high temperatures reaches a maximum value (0.06 S cm(-1) at 800 degreesC). We also show that the activation energy for oxide ion conduction shows a general increase with increasing-Ge content. This increase is, however, not uniform, with a plateau,value being reached of approximate to1.1 eV, when the Si:Ge ratio is in the range of 2 - 0.5, before a subsequent increase to a maximum value of 1.3 eV is observed.