Date of Award
6-27-2024
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Ferroic materials are important components of modern day electronics. They can be described by an order parameter that gains a non-zero value upon undergoing a symmetry-lowering phase transition. Ferroic materials include ferroelectrics, ferromagnets, ferrotoroidal materials and ferroaxial materials. Their physical properties can be controlled using external stimuli such as temperature, strain or electric field. Their physical properties are highly sensitive to small changes in structure and composition. First-principles calculations based on density-functional theory (DFT) have proven instrumental in describing material properties right from the atomic scale. Concurrently, advanced scanning transmission electron microscopy (STEM) has enabled the simultaneous imaging and spectroscopy of materials with unprecedented spatial and energy resolution. Therefore, the integration of theoretical modeling and microscopy provides an unparalleled approach to unravel the intricate structure-property correlations in ferroic materials. In this dissertation, I present our theoretical studies combined with quantitative STEM characterization of various ferroic systems and their physical properties. Specifically, I investigate a variety of structural distortions that undergo in quasi-one-dimensional AxTiS3 (A = Ba, Sr; x ≥ 1) chalcogenides, multiferroic hexagonal YbFeO3 and fluorite-structure ferroelectrics BO2 (B = Hf, Zr), along with the effect these distortions have on the optical, chiral and polar properties. The objective of this research is to delve into the underlying mechanisms driving the emergence of ferroic distortions in different materials, and providing guidance for manipulating their properties. Following an introduction of ferroic materials in chapter 1, and a discussion of the methods in chapter 2, in chapter 3, I discuss my studies on how structural disorder enhances the optical anisotropy in quasi-one-dimensional (quasi-1D) BaTiS3. Aberration-corrected STEM observations reveal long-range disordered, but locally ordered, Ti displacements within the ab-plane. DFT calculations show that the refractive index tensor of BaTiS3 is highly sensitive to the magnitude of the Ti displacements. In the transparent region (> 5 μm), the ordinary (n_o) refractive index decreases while the extraordinary refractive index (n_e) remains nearly constant with increasing Ti ab-plane displacements. Together, these ab-plane Ti displacements lead to a large birefringence ("Δn" ≈ 0.8) — that had been previously observed in experiments, but had remained poorly understood. In chapter 4, I report on the presence of a relatively new multiferroic order — ferrochiral order — in strained quasi-1D BaTiS3 with P63 space group, which shows simultaneous ferroelectric and ferroaxial order, both of which are strongly coupled. Under biaxial tensile strain on ab-plane, a ferrochiral transition is energetically favored transforming achiral P63cm-BaTiS3 to chiral P63-BaTiS3, which involves Ti polar displacements along and TiS6 octahedral rotations about the c-axis. We also demonstrate atomic-scale mapping of ferrochiral domains in P63-BaTiS3 using HAADF-STEM and 4D-STEM techniques. DFT calculations coupled with Landau-based modeling shows the presence of a strong coupling between the two order parameters. We predict that the chirality in this material can be controlled using an external electric field. In chapter 5, I have investigated how structural modulations in quasi-1D SrxTiS3 (1 < x < 1.5) selectively boost the extraordinary index n_e and result in a world-record birefringence ("Δn > 2.1" within the IR region). Structural characterization from X-ray diffraction and STEM imaging indicate that the structural modulations in SrxTiS3 arise from an overall trigonal twist distortion of TiS6 polyhedral units compared to the unmodulated SrTiS3 structure. The excess Sr, compared to stoichiometric SrTiS3¬, result in the formation of TiS6 trigonal-prismatic units that break the infinite chains of face-shared TiS6 octahedra in SrTiS3 into periodic blocks of five TiS6 octahedral units. The additional electrons introduced by the excess Sr subsequently occupy the TiS6 octahedral blocks to form highly oriented and polarizable electron clouds, which selectively boost the extraordinary index n_e and result in record birefringence. In the chapter 6, I have investigated the interaction of atomically sharp antiphase boundaries (APB) with ferroelectric domains within the multiferroic material YbFeO3. Our atomic-resolution STEM results reveal ferroelectric domain-walls pinned at the APB in YbFeO3 films. Elemental mapping using EELS and simultaneous HAADF-STEM observations suggest the presence of interstitial Fe at the APB. DFT total-energy calculations suggest that the pinning of ferroelectric DW at the APB is energetically favorable due to the geometric constraints imposed by the FeO6 polyhedra at the APB. In chapter 7, atomic-scale characterization of polar domains in fluorite-structure ferroelectrics, HfO2 and ZrO2, is presented. To resolve the polar displacement of oxygen, we have conducted annular bright-field (ABF) and differential phase contrast (DPC) imaging using STEM. In polycrystalline ZrO2, we find the presence of alternating head-to-head and tail-to-tail polar domains along the c-axis within a single grain. In a single crystalline HfO2 thin-film doped with 10%-La (LHO), sub-nanometer-sized polar domains separated by non-polar spacer layers were observed. Theoretical calculations using DFT have been performed to get insights into the stabilization mechanism of different polar domains. In the final chapter 8, we propose a workflow combining data mining from pre-existing databases with high-throughput first-principles calculations to search for undiscovered, polar binary oxides. 25 new polar binary oxides are predicted to be formable. Our theoretical assessments identified 9 promising polar candidates — P1-As12O24, P1-Ca4O12, P1-Sb6O15, C2-P4O8, Cm-Ge12O24, Cm-Nb6O15, Cm-Sb6O15, Pmn21-Pb2O2, Pna21-Ge4O8 — as switchable ferroelectrics due to their low energy barriers (< 60 meV/atom) for polarization reversal. Pmn21-Pb2O2 was predicted as a candidate for electro-optical applications due to its strong Pockels response. P1-Ca4O12 was computed to have large optical anisotropy (Δn ≈ 0.33) over the broadband wavelength range (> 1 μm).
Language
English (en)
Chair
Rohan Mishra