An investigation was made of the relationship between structure and conductivity in these 2 apatite-type phases. A high oxide-ion conductivity (1.2 x 10-4S/cm at 700C, E = 0.73eV) was observed in La9.33Si6O26. The oxide-ion conductivity of La8Sr2Si6O26 was low (2.9 x 10-7S/cm at 700C, E = 1.34eV). Structural refinement of powder neutron diffraction data showed that both compounds had an hexagonal structure (P¯3; a = b = 0.97, c = 0.72nm). In addition to vacancies on cation sites, the La9.33Si6O26 also exhibited a key difference in the channel O sites, as compared with La8Sr2Si6O26. In the latter, the channel O atoms all appeared to be located close to the ideal (0, 0, 0.25) site whereas, in La9.33Si6O26, some 14% of these O atoms were displaced to a new position near to (0, 0, 0.38). It was concluded that the higher conductivity, and lower activation energy for oxide-ion conduction, in La9.33Si6O26 as compared with La8Sr2Si6O26, was due to the observed disorder on the channel sites.

A Powder Neutron Diffraction Study of the Oxide-Ion-Conducting Apatite-Type Phases, La9.33Si6O26 and La8Sr2Si6O26. J.E.H.Sansom, D.Richings, P.R.Slater: Solid State Ionics, 2001, 139[3-4], 205-10