Synthesis and electrical characterisation of the apatite-type oxide ion conductors Nd9.33+xSi6-yGayO26+z

Apatite-type oxides have been attracting interest as a new class of oxide ion conductors. In this paper we examine the effect of Ga doping on the conductivity of the apatite silicate system, Nd_9.33+xSi₆O_26+3x/2 and compare the results to those reported for similar doping studies in La_9.33+xSi₆O_26+3x/2. The highest conductivities are observed for samples containing oxygen excess, which is in agreement with previous reports that interstitial oxide ions are important for high oxide ion conduction in these materials. For oxygen stoichiometric materials, i.e. Nd_9.33+x/3Si_6-xGa_xO₂₆, the Ga doping results in a significant increase in activation energy and a consequent lowering of the low temperature conductivity. This is contrary to results previously reported for the La containing analogues, which showed an enhancement of conductivity on Ga doping up to x=1.5. Possible explanations for the differences between the two systems are discussed. Paper presented at the Patras Conference on Solid State Ionics — Transport Properites, Patras, Greece, Sept. 14 – 18, 2004.