Three-dimensional atomic scale electron density reconstruction of octahedral tilt epitaxy in functional perovskites

ARTICLE DOI: 10.1038/s41467-018-07665-1 OPEN Three-dimensional atomic scale electron density reconstruction of octahedral tilt epitaxy in functional perovskites 1234567890():,; Yakun Yuan1,2, Yanfu Lu1,2, Greg Stone1,2, Ke Wang2, Charles M. Brooks3, Darrell G. Schlom3,4, Susan B. Sinnott1,2, Hua Zhou5 & Venkatraman Gopalan1,2,6 Octahedral tilts are the most ubiquitous distortions in perovskite-related structures that can dramatically influence ferroelectric, magnetic, and electronic properties; yet the paradigm of tilt epitaxy in thin films is barely explored. Non-destructively characterizing such epitaxy in three-dimensions for low symmetry complex tilt systems composed of light anions is a formidable challenge. Here we demonstrate that the interfacial tilt epitaxy can transform ultrathin calcium titanate, a non-polar earth-abundant mineral, into high-temperature polar oxides that last above 900 K. The comprehensive picture of octahedral tilts and polar distortions is revealed by reconstructing the three-dimensional electron density maps across film-substrate interfaces with atomic resolution using coherent Bragg rod analysis. The results are complemented with aberration-corrected transmission electron microscopy, film superstructure reflections, and are in excellent agreement with density functional theory. The study could serve as a broader template for non-destructive, three-dimensional atomic resolution probing of complex low symmetry functional interfaces. 1 Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802, USA. 2 Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA. 3 Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, USA. 4 Kavli Institute at Cornell for Nanoscale Science, Ithaca, NY 14853, USA. 5 Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439, USA. 6 Department of Physics, Pennsylvania State University, University Park, PA 16802, USA. Correspondence and requests for materials should be addressed to H.Z. (email: ) or to V.G. (email: ) NATURE COMMUNICATIONS | (2018)9:5220 | DOI: 10.1038/s41467-018-07665-1 | www.nature.com/naturecommunications 1 ARTICLE C NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-07665-1 omplex oxides interfaces have become a vibrant research focus in condensed matter physics and material science1–5, since they are a fertile playground for emergent phenomena such as, magnetism6, ferroelectricity7, interface charge transfer8, two-dimensional free electron gases9, superconductivity10, and topological states11 through strategies in modern materials design, including strain tuning12–14, artificial layering15,16, spatial confinement17, and interfacial coupling18–25. Control of octahedra tilts in complex oxides via film-substrate interface design, or tilt epitaxy, has been predicted to be a powerful knob for tuning various functional properties, including inversion symmetry breaking26–28, magnetism18,22,29, and electronic orders30. Although the tilt epitaxy promises a potentially wonderful route for designing these functionalities in various materials, the experimental reports on realizing tilt epitaxy are still very limited. So far, the tilt epitaxy has been used to stabilize polar distortions in metallic ultrathin nickelates films20 and to manipulate magnetic anisotropy in SrRuO318,29 and La2/3Sr1/ 22 3MnO3 . However, in these works, only in-phase octahedra tilt along one of the three crystallographic axes are experimentally resolvable. Moreover, strain and substrate termination effects can convolute with tilt epitaxy, which remain unexplored. In general, there are three outstanding challenges in implementing tilt epitaxy. The first is that substrate tilts can transfer into the film only to within ~10 unit cells, thus necessitating ultrathin films to observe these dramatic effects. Secondly, experimentally determining the complete three-dimensional (3D) structure of such tilt epitaxy interfaces with atomic resolution is quite a formidable task. Direct aberration-corrected scanning transmission electron microscopy (STEM) is now routinely used for probing atomic structures with picometers metrology; however, they probe the potential of two-dimensional projections of atomic columns, and deconvolving the information along depth direction is a challenge22, as we illustrate in this work. Coherent Bragg rods analysis (COBRA)31–41, which reconstructs 3D electron density with atomic resolution based on a phase retrieval algorithm taking advantage of the interference between the diffracted X-ray beams from the thin film and the substrate, is a promising technique for such purpose. COBRA requires no special sample preparation (such as in STEM) and is readily applicable to any epitaxial system with film thickness <20 nm. However, previous COBRA studies have mostly focused on systems with high symmetry, e.g., 4mm point group, and heavy cations. The complete 3D analysis of oxygen octahedra for a low symmetry system is still an outstanding challenge. Other emerging 3D imaging techniques (see Supplementary Table 1) include coherent diffraction imaging42, tomography43, topography44, ankylogaphy45 using X-ray or electrons, each with its own merits and drawbacks. The third challenge is to be able to deconvolve the influence of the tilt epitaxy from that of strain and surface termination effects that may coexist. In this work, we tackle all three of these outstanding challenges. We study ultrathin films of a prototypical perovskite with a complex tilt pattern, namely calcium titanate on various substrates that provide a range of tilt and strain states. We report the atomic scale 3D reconstruction of the electron density across these low symmetry epitaxial complex oxides interfaces by COBRA, the first such feat where both substrate and film possess three octahedral tilts in addition to polar distortions. The reconstruction requires high quality mapping of diffractions in a large reciprocal space volume and generalized computer routines for handling the large experimental data set. Specifically, we present COBRA reconstructed electron densities (EDs) of ultrathin epitaxial CaTiO3 films on NdGaO3(110), DyScO3(110) and La0.29Sr0.71Al0.65Ta0.35O3(001) (LSAT) substrates, each offering a unique combination of strain and octahedral tilt patterns across 2 the interface. Combining COBRA studies with complementary scanning transmission electron microscopy (STEM) and density functional theory (DFT) reveals the distinct roles of tilt epitaxy, strain and surface termination. We find that, in addition to epitaxial strain effect inducing polar distortion in the film, the tilt epitaxy monoclinically distorts the film and clamps the in-plane oxygen octahedral tilts of CaTiO3 on LSAT substrate, giving rise to significantly higher polar transition temperatures (>900 K) in ultrathin CaTiO3 films (8 u.c. or ~3.0 nm thick) than previously report (...truncated)