Enhancement of Ionic Transport in Complex Oxides through Soft Lattice Modes and Epitaxial Strain

Lattice dynamics is increasingly acknowledged as playing an important role in the ionic transport mechanisms of many oxide ion conductors. In particular, specific structural distortions–so-called octahedral rotations–have been suggested as the origin of the enhanced mobility observed in Ln 2 NiO 4+δ Ruddlesden–Popper phases (Ln = La, Pr, Nd), where oxide interstitial diffusion occurs through an interstitialcy mechanism. In this work, we use theory and first-principles calculations to unravel and quantify the microscopic link between soft lattice modes and migration barriers in the Ln 2 NiO 4+δ family of materials. We show that the magnitude of the migration barriers can be correlated with the tendency of each material to undergo an octahedral rotation distortion: as the tendency of a material to undergo such a distortion increases, the migration barrier decreases. We then use this insight to formulate simple design guidelines for further decreasing migration barriers through epitaxial strain and that connect trends in the ionic transport properties of the Ruddlesden–Popper phases with the structures of the parent ANiO 3 perovskites.