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 La 1+x Sr 1−x Ga 3 O 7+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.

High-performing solid-state electrolytes (SSE) are a crucial contribution to future energy conversion devices. Recent research shows that the melilite system can yield highly conductive candidates for certain compositions, worth investigating. Based on a previously introduced kinetic Monte Carlo (KMC) model for oxygen ion conductivity in lanthanum-rich melilite structures La 1+x Sr 1−x Ga 3 O 7+0.5x by Schuett et al., simulations were extended to Ga site doping. Various dopants were investigated concerning their effect on oxygen ion conductivity in La 1.5 Sr 0.5 Ga 3−y X y O 7.25 (X = B, Al, In, Si, Sc, Zn). The density functional theory (DFT) analysis of site and migration energies identified different trapping and blocking behaviors for the respective dopants, which were integrated in the existing energy model. Subsequent KMC simulations revealed Al to be the only dopant with beneficial impact on oxygen ion conductivity and activation energy. For this dopant, the energy model was refined by including energy contributions for higher dopant concentrations. Two different approaches were chosen: one based on pair interactions, and the other relying on cluster interactions. The subsequent KMC simulations revealed promising improvements in oxygen ion conductivity and the respective activation energies that need to be confirmed by experimental investigation. Furthermore, a basis has been provided for simulations on complex melilite structures.