Defect chemistry and oxygen ion migration in the apatite-type materials La9.33Si6O26 and La8Sr2Si6O26

Abstract

Computer modelling techniques have been used to examine the mechanistic features of oxygen ion transport in the La₈Sr₂Si₆O₂₆ and La_9.33Si₆O₂₆ apatite-oxides at the atomic level. The potential model reproduces the observed complex structures of both phases, which are comprised of [SiO₄] tetrahedral units and La/O channels. Defect simulations have examined the lowest energy interstitial and vacancy sites. The results suggest that oxygen ion migration in La₈Sr₂Si₆O₂₆ is via a vacancy mechanism with a direct linear path between O5 sites. Interstitial oxygen migration is predicted for La_9.33Si₆O₂₆via a non-linear (sinusoidal-like) pathway through the La3/O5 channel. The simulations demonstrate the importance of local relaxation of [SiO₄] tetrahedra to assist in the facile conduction of oxygen interstitial ions. In general, the modelling study confirms that the high ionic conductivity in silicate-based apatites (with oxygen excess or cation vacancies) is mediated by oxygen interstitial migration.