Local octahedral rotations and octahedral connectivity in epitaxially strained LaNiO3/LaGaO3 superlattices

For ABO3 perovskites, octahedral rotations and distortions couple strongly to the functional properties. However, in short period perovskite superlattices, the characterization of the octahedral behavior remains challenging due to the local structural variations of the BO6 octahedra. By aberration-corrected high-resolution transmission electron microscopy, we investigated the local octahedral rotations in a [(4 unit cell (u.c.)//4 u.c.) × 8] LaNiO3/LaGaO3 superlattice grown on a (001) SrTiO3 substrate. The octahedral behavior varies along the growth direction even though the superlattice is coherently strained. Near the substrate, octahedral rotations about [100] and [010] axes in the superlattice are suppressed due to the octahedral connectivity—rotational magnitudes and patterns—between the NiO6 and TiO6 octahedra. Away from the substrate, the magnitudes of [100] and [010] rotations are enhanced as a response to substrate-induced tensile strain. Near the surface of the superlattice, the [100] and [010] rotational magnitudes of NiO6 and GaO6 relax to the bulk values of LaNiO3 and LaGaO3, respectively. Our results indicate that the response of octahedral rotations to epitaxial strain in superlattices is significantly different from that in thin films.