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

Yakun Yuan,Yanfu Lu,Darrell G Schlom,Charles M Brooks,Venkatraman Gopalan,Hua Zhou,Ke Wang,Susan B Sinnott,Greg Stone

doi:10.1038/s41467-018-07665-1

Abstract

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.

Highlights

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
Direct aberration-corrected scanning transmission electron microscopy (STEM) is routinely used for probing atomic structures with picometers metrology; they probe the potential of two-dimensional projections of atomic columns, and deconvolving the information along depth direction is a challenge[22], as we illustrate in this work
We find that the story changes dramatically for ultrathin (8 u.c. or ~3.0 nm) films used in this study, where out-of-plane polarization arises, Curie temperatures are significantly higher, the effect of chemical termination at the interface and geometric oxygen octahedral tilt mismatch between substrate and CaTiO3 becomes important

Summary

Introduction

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. 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. 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. An unexpected out-of-plane polarization is observed in tensile strained CaTiO3 thin films with directions dictated by the interfacial valence mismatch These tilt epitaxy as well as valence mismatch effects should be present in all epitaxial complex oxides systems and strongly mediate the properties of ultrathin epitaxial films, which provide new routes to artificially control materials functionalities

Methods

Results

Conclusion