Insight into doping effects in apatite silicate ionic conductors

Novel apatite-type silicates are attracting considerable interest as a new family of oxide-ion conductors with potential use in fuel cells and ceramic membranes. Combined computer modeling and X-ray absorption (EXAFS) techniques have been used to gain fresh insight, at the atomic level, into the site selectivity and local structures of a wide range of dopants in these apatite materials. The results indicate that an unusually broad range of dopant ions (in terms of size and charge state) can substitute for La in the La9.33Si6O26 apatite, in accord with current experimental data. The range is much wider than that observed for doping on a single cation site in most other oxide-ion conductors, such as the perovskite LaGaO3. In addition, our local structural investigation demonstrates that this dopant behavior is related to the flexibility of the silicate substructure, which allows relatively large local distortion and alteration of the site volumes. This could be a key factor in the high oxide-ion conductivity exhibited by these apatite silicates. Indeed, the breadth of possible doping regimes in these novel materials provides new opportunities to design and optimize the conduction properties for fuel cell electrolytes.