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

TLDR

Computer modelling techniques have been used to examine the mechanistic features of oxygen ion transport in the La8Sr2Si6O26 and La9.33Si6O26 apatite‑oxides at the atomic level. Defect simulations examined the lowest‑energy interstitial and vacancy sites to study oxygen ion transport. The simulations confirm that La8Sr2Si6O26 conducts oxygen via a vacancy mechanism along a linear O5–O5 path, while La9.33Si6O26 conducts via a non‑linear interstitial pathway through the La3/O5 channel, with local [SiO4] relaxation facilitating conduction and overall supporting that high ionic conductivity in silicate apatites is mediated by oxygen interstitial migration.

Abstract

Computer modelling techniques have been used to examine the mechanistic features of oxygen ion transport in the La8Sr2Si6O26 and La9.33Si6O26 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 La9.33Si6O26via 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.