Structural Phase Engineering in Epitaxial Perovskite Oxide Thin Films

Abstract

Control of structural phase is central to epitaxial oxide thin films as it governs polarization, lattice distortion, and electromechanical response. In these systems, the structural state is established by epitaxial constraint during growth and can be further tuned after growth by an external electric field. This dissertation investigates two complementary routes for controlling structural phases in oxide heterostructures: substrate-mediated strain control and direct flexoelectric polarization–electric field coupling. The first part focuses on （110）-oriented SrMnO3 thin films grown on LSMO-buffered PMN-PT piezoelectric substrate. In this system, biaxial tensile strain drives thickness-dependent lattice evolution and unit-cell volume expansion associated with oxygen-vacancy formation. X-ray photoelectron spectroscopy and electron energy-loss spectroscopy show that the vacancy concentration is higher near the strained interfacial region, linking defect formation directly to the lattice state. Piezoresponse force microscopy reveals switchable polar domains with both lateral and vertical components, indicating that the ferroelectric state is governed by the coupled effects of strain and defect-mediated lattice distortion. In situ X-ray diffraction further demonstrates that the piezoelectric strain of the PMN-PT substrate is transferred efficiently to the SrMnO3 thin film, providing a route for reversible electrical control of the lattice after growth. The second part addresses electric-field-driven phase transformation in epitaxial BiFeO3-BaTiO3 thin film. In this system, epitaxial relaxation produces a nanoscale interfacial layer between the coherently strained tetragonal phase and the relaxed rhombohedral phase. The strain gradient across this layer generates flexoelectric polarization, which couples directly to the applied electric field. Time-resolved X-ray microdiffraction shows that this coupling drives reversible phase transformation at the interface. When the applied electric field exceeds the built-in flexoelectric field, the net electrical polarization of the interfacial layer reverses, which in turn reverses the direction of the phase transformation. These results identify direct flexoelectric polarization–electric field coupling as the key mechanism for dynamic phase control in this system. Taken together, these two studies show that structural phases in epitaxial oxide thin films can be controlled after growth by electric field through two distinct routes: substrate-mediated strain transfer and direct flexoelectric polarization–electric field coupling. These results establish electromechanical coupling as a practical route for electrically tuning the lattice state and phase stability without changing chemical composition, and provide a framework for designing nanoscale oxide devices based on structural phase engineering.