Effect of La vacancies on the oxide-ion conduction in lanthanum silicate apatites

Abstract

La-excess compositions of lanthanum silicate apatites (La9.33+0.67xSi6O26+x or La9.33+0.67xSi6–0.5xO26; x > 0) exhibit higher oxide-ion conductivity than basic composition (La9.33Si6O26) due to smaller activation energies. On the contrary, the activation energies for oxide-ion conduction tend to increase with more La vacancies, less La content, at the La 4f site. However, the effect of La vacancies on the activation energy of oxide-ion conduction is not fully understood. In the present work, first-principles calculations are performed for La9.33+0.67xSi6O26+x (x = 0.0, 0.5, 1.0) to clarify the relationship between the amount of La vacancy at the La 4f site and the activation energy of oxide-ion migration in the O4 channel. It is found that the potential barrier of oxide-ion migration is extremely low in La10Si6O26 (x = 1.0) whereas that becomes higher with increasing the amount of La 4f vacancies. This is closely related to sizes of La 6h triangles that surround the O4 channel. In the presence of more La 4f vacancies, La 6h triangles tend to be more expanded than those in La10Si6O26. Since migrating oxide ions have attractive interactions with their surrounding La 6h ions, more expanded La 6h triangles lead to more energy costs for atomic displacements of the La 6h ions during the oxide-ion migration, resulting in their higher potential energy barriers. The effect of La 4f vacancies on the oxide-ion migration can be explained by such local atomic arrangements of La 6h and their interactions with migrating oxide ions.