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

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.

Atomic‐Scale Control of Electronic Structure and Ferromagnetic Insulating State in Perovskite Oxide Superlattices by Long‐Range Tuning of BO6 Octahedra

“…[15][16][17] However, as shown in the left sketch in Figure 1a, the major drawback of OOC is that its impact length scale is only 2-6 unit cells (uc) confined at the interface region, and the modulation of BO 6 octahedral rotations decays very quickly away from interface. [15][16][17][21][22][23][24] Therefore, an enhancement of the impact length is highly desirable in order to extend the tuning of emergent effects beyond the interfacial region and to achieve a large volume of material, comprising novel functionalities for both fundamental studies and device applications.…”

“…In this study, by carefully growing a series of (LBMO)N-(STO)N (noted as LNSN; N = 4, 5, 6, 8, 9, 10, 12, and 14) superlattices, we demonstrate that the modulation of MnO 6 octahedral rotations in LBMO could be extended up to 12 uc which is significantly increased in comparison to previously reported OOC length scale of 2-6 uc. [15][16][17][21][22][23][24] O K-edge X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) and linear dichroism (XLD) and density functional theory (DFT) calculations reveal that when interfacial MnO 6 octahedral rotations are strongly suppressed, the Mn e g bandwidth is reduced and orbital reconstructions occur in the Mn 3d orbitals. This in turn modifies the magnetic and electronic ground states of the manganite and creates an emergent ferromagnetic insulating state in LBMO with an increased T C of 235 K.…”

Interfacial Octahedral Manipulation Imparts Hysteresis‐Free Metal to Insulator Transition in Ultrathin Nickelate Heterostructure

“…Others, using HR‐TEM, have measured bond angles and OORs, but the samples were thinned for creating the cross‐sections, making quantification of the γ angle a major challenge. In contrast, synchrotron X‐ray diffraction is a nondestructive technique for quantifying the OORs, as demonstrated by May et al for LaNiO 3 films and Yuan et al for CaTiO 3 . Here, the relative intensities of the half‐order reflections due to OORs can be used to quantify ϴ in the different directions.…”

High‐Resolution Crystal Truncation Rod Scattering: Application to Ultrathin Layers and Buried Interfaces

“…[ 29 ] It was also shown that a rough estimate of the accuracy of the retrieved electron density of a surface or thin film may be obtained by including several bulk/substrate unit cells in the analysis and comparing the reconstructed structure deep into the bulk with that of the known structure. [ 93 ]…”

“…A survey of recent high‐resolution CTR results using model refinement, direct methods, and hybrid approaches shows that the estimated uncertainty in the relative atomic positions (along the surface normal) is generally less than 0.1 Å (10 pm), the uncertainty in the occupancies is on the order or ≈5–10%, and bond/tilt angles can be determined with a precision a few degrees. [ 1,31,93–100 ]…”