First-principles analysis of oxide-ion conduction mechanism in lanthanum silicate

First-principles calculations were performed for an interstitial oxide ion in lanthanum silicate with the apatite-type crystal structure. The minimum energy diffusion pathway and the potential barrier were investigated by the nudged elastic band method. It was found that the energetically most stable interstitial O5 site is located close to the O4 column. The O5 diffusion along and normal to the c axis gave rise to the calculated activation energies for diffusion of around 0.3 eV to 0.4 eV, which are much smaller than those expected for diffusion of a large-sized oxygen ion in the complicated crystal structure. During the diffusion process, the interstitialcy mechanism with O4 is likely to occur for O5 diffusion along the c axis. In contrast, for O5 diffusion normal to the c axis, it was found that SiO4 groups play a role in acting as relay points for the interstitial O5 ion to diffuse.