UC Berkeley

This dissertation explores the lattice degree of freedom in several heavily-substituted, highly disordered, and technologically relevant perovskite systems utilizing epitaxial thin-film synthesis and characterization. By creating controlled changes to the boundary conditions imposed on thin films, effecting different surface orientations, strain states, film thicknesses, and elastic stiffness, connections can be drawn between the resulting structural, chemical, and electronic changes that are driven by the film’s boundary conditions, and the resulting properties that can be achieved in these films. First, I focus on the impact such changes have on the surface properties of La1-xSrxCo1-yFeyO3-δ transition metal perovskites that are being considered for cathodes for oxygen electrocatalysis. There, growth-mediated control of the surface orientation gives rise to distinct evolution of local non-stoichiometry and increased oxygen reactivity in (111)-oriented films. Application of epitaxial strain on the films is then found to impact the electronic structure of these films, modulating the occupancy of transition-metal orbitals at the surface, which ultimately impacts the ability of these materials to perform oxygen exchange. The focus is then changed to relaxor ferroelectrics, and understanding how changes to elastic and electrostatic boundary conditions impact the response of the disordered nanoscale polar structure. In PbSc0.5Ta0.5O3, reducing the thickness of films results in suppression of the polarization response that undergoes a rapid collapse when film thickness is reduced below the polar correlation length. It is then demonstrated that the polarization and piezoelectric response of 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 is suppressed by the mechanical clamping of the substrate, and methods for removing films from the substrate allow for enhanced responses. Together, these studies point to the connection between lattice constraints and the achievable responses in perovskites, as well as point to new engineering approaches for enhancing desirable properties across a range of applications.