A powder neutron diffraction study of the oxide-ion-conducting apatite-type phases, La9.33Si6O26 and La8Sr2Si6O26

In this paper we report an investigation into the relationship between structure and conductivity for the two apatite-type phases, La9.33Si6O26 and La8Sr2Si6O26. High oxide ion conduction has been observed in the former (σ=1.2×10−4 S cm−1 at 700°C, Ea=0.73 eV), whereas the oxide ion conductivity of La8Sr2Si6O26 is low (σ=2.9×10−7 S cm−1 at 700°C, Ea=1.34 eV). Structural refinement of powder neutron diffraction data showed that both compounds have a hexagonal cell (space group P-3; a=b≈9.7, c≈7.2 Å). In addition to the presence of vacancies on the cation sites, the sample La9.33Si6O26 also showed a key difference in the channel oxygen sites compared to La8Sr2Si6O26. For the latter the channel oxygen atoms all appear to be located close to the ideal (0,0,0.25) site, whereas for La9.33Si6O26, ∼14% of these oxygens are displaced to a new position around (0,0,≈0.38). From these studies we can propose that the higher conductivity and lower activation energy for oxide ion conduction in La9.33Si6O26 compared to La8Sr2Si6O26 is due to this observed disorder on the channel sites.