First principles study of LaGaO$_3$/MgAl$_2$O$_4$ (001) polar interfaces

Materials with high oxygen ion conductivity have been the center of much attention due to both fundamental interest and technological applications. One of the most remarkable ionic conductors and an excellent candidate for future solid oxide fuel cells is LaGaO$_3$ (LGO), as it exhibits very high ionic conductivity when doped with Sr or Mg. To achieve enhanced ionic transport in this system, where oxygen vacancies (V$_{\rm O}$) are the dominant carriers, we propose a negatively charged interface as a way of inducing a V$_{\rm O}$ enrichment layer. In this study, the interface is comprised of LGO and the spinel MgAl$_2$O$_4$, both of which exhibit nominally charged (001) planes. We consider an interface where the (GaO$_2$)$^{1-}$ layer of LGO is in contact with the (AlO$_2$)$^{1-}$ layer of the spinel. Such negatively charged interfaces require compensating defects, providing a strong driving force for enhancing the V$_{\rm O}$ concentration, and hence, the in-plane ionic conductivity in the space charge region adjacent to the boundary. We report results from first-principles calculations which provide information on the structure and relative stability of these polar interfaces, compensation mechanisms, and defect formation energies as a function of distance from the interface.