Effects of biaxial stress and layer thickness on octahedral tilts in LaNiO₃

Using first-principles calculations based on the density functional theory, we investigated the structural properties of superlattices of the perovskites LaNiO₃ and SrTiO₃. Inspection of the tilt angles of the NiO₆ octahedra in the biaxially stressed LaNiO₃ layers reveals the important role of octahedral connectivity at the interface. For tensile biaxial stress, we find that the LaNiO₃ layers exhibit a negative Poisson's ratio, in contrast to the positive Poisson's ratio observed in bulk material. These results can be explained in terms of simple models, and are discussed in light of recent experiments.