Oxygen Ion Migration and Conductivity in LaSrGa3O7 Melilites from First Principles

High oxygen ion conductivity of solid electrolytes is a crucial prerequisite for the efficiency of electrochemical devices such as solid oxide fuel cells and rechargeable oxide batteries. Lanthanum melilites of the composition La1+xSr1-xGa3O7+x/2, with x > 0, show promising oxygen ion conductivity because of the high mobility of oxygen interstitials formed for charge compensation. In this study, we investigate the site and migration energies of oxygen interstitials (x > 0) and vacancies (x < 0) using density functional theory, taking into account various cation environments. It is shown that energy of oxygen defects strongly depends on the local environment. In addition, the results prove that the transport of interstitial ions follows an interstitialcy mechanism rather than a direct interstitial migration, whereas the transport of oxygen vacancies is associated with considerably higher migration barriers. Based on our calculations, we present an energy model to predict the site and migration energies for arbitrary ionic configurations in Kinetic Monte Carlo simulations, thus linking the microscopic ionic motion with the macroscopic transport properties. Our simulations show that the macroscopic activation energy is essentially determined by the interaction of the interstitials with the local cation and anion environment, which explains the decrease of the activation energy with the increasing lanthanum content. Consequently, this factor is substantial for the accurate prediction of ionic conductivity in strongly disordered materials.