Defects and disorder in apatite-type silicate oxide ion conductors: implications for conductivity

Abstract

The structure and composition of La-silicate apatite oxide ion conductor, La₁₀Si₆O₂₇, in which the types of defects it contains are controversial, has been thoroughly investigated. Large crystals were grown using the floating zone method, and their structure was characterised by X-ray and neutron single crystal diffraction and second harmonic generation measurements. Structure refinements reveal extensive positional disorder on the oxygen substructure, the presence of La vacancies and no evidence of sites occupied by excess oxide ions. The actual composition of the crystal was determined through SEM-EDX and density measurements and was found to be La_9.64Si_5.77O₂₆, differing significantly from the nominal composition but in agreement with the structure refinements. Structural models considering the actual composition, the disorder on the O sublattice, and local relaxations around these defects were able to reproduce the key features of the diffuse scattering observed in single crystal neutron diffraction patterns.