Abstract
Ionic conduction mechanisms in apatite-type lanthanum silicate and germanate studied by first-principles calculations were reviewed, along with the ones previously proposed by experiments and atomistic simulations. It was found that the most stable interstitial oxygen sites in La10(SiO4)6O3 are located at the vicinity of the O4 column and the metastable sites are present between SiO4 tetrahedra. In contrast, La10(GeO4)6O3 showed the most stable sites between GeO4 tetrahedra, forming Ge2O9 units. These can reasonably explain interstitial sites reported experimentally. The fundamental mechanism for ionic conduction in both systems is the interstitialcy mechanism, where interstitial oxygen ions and oxygen ions at regular lattice sites undergo cooperative motion through the lattices. In particular, the interstitialcy mechanism via Si/GeO4 tetrahedra takes place by sequential bond-forming and bond-breaking events between Si/Ge and oxygen ions, and play an important role to realize rapid ionic conduction.
