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.) 9 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. Introduction ABO3 perovskites exhibit fascinating functionalities such as colossal magnetoresistance, metal–insulator transitions, multiferroicity, and superconductivity due to the strong correlation between charge, spin, and orbital degrees of freedom [1, 2]. Heterostructures of different ABO3 perovskites provide the possibility to not only manipulate the existing functionalities but also create new ones which are enabled by structural and electronic reconstructions at the heterointerfaces [3–5]. It has been demonstrated that the electronic and magnetic properties of ABO3 perovskites are strongly coupled to the B–O bond lengths and O–B–O bond angles; therefore, precise control of the BO6 octahedral rotations and distortions by epitaxial strain and interfacial octahedral connectivity has become the key to engineering desired functionalities in ABO3 perovskite heterostructures [6–22]. One example is the design of new high-Tc superconductors. It has been predicted that epitaxial strain and quantum confinement can be used to manipulate the electronic structure of LaNiO3/RXO3 superlattices (R = rare earth cation, X = trivalent cation such as Al, Ga...) to match that of cuprate high-Tc superconductors [23]. In bulk LaNiO3, the electron configuration of the Ni ions is t2g 6 eg 1 with the eg electron occupying either dx2 y2 or d3z2 r2 orbital. Substrate-induced tensile strain increases the in-plane Ni–O bond lengths thus favors the occupation of the in-plane dx2 y2 orbital. The in-plane orbital polarization of the eg electron is further supported by the insulating RXO3 layers which block the c-axis charge transfer. Subsequent experiments have revealed orbital polarization, metal–insulator transition, and antiferromagnetism in epitaxial-strained LaNiO3/RXO3 superlattices [24–27]. However, superconductivity has not Electronic supplementary material The online version of this article (doi:10.1007/s10853-015-9077-y) contains supplementary material, which is available to authorized users. & H. Y. Qi [email protected] 1 Central Facility of Electron Microscopy, Electron Microscopy Group of Materials Science, University of Ulm, Albert Einstein Allee 11, 89081 Ulm, Germany 2 Max Planck Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany 123 J Mater Sci (2015) 50:5300–5306 DOI 10.1007/s10853-015-9077-y