| This thesis discusses first-principles modeling of functional perovskite oxides and perovskite superlattices. In the past few decades, first-principles density functional theory has driven tremendous advances in the theoretical study of materials. However, it does not give us a conceptual understanding of the physics of these materials, which makes the first-principles modeling necessary.;In the first project, we use the first-principles method to study the epitaxial strain-induced ferroelectricity in the orthorhombic CaTiO3 structure and construct the energy expansion from first principles to illustrate the mechanism of the induced ferroelectricity. We also discover an unexpected polar phase of CaTiO3 with in-plane polarization under compressive strain. Symmetry analysis shows that this phase is a realization of a new mechanism of geometric ferroelectricity. In the second project, we collaborate with an experimental group at SUNY Stony Brook to study the perovskite superlattices PbTiO3/BaTiO3. A variety of properties, including electric polarization, tetragonality, piezoelectricity and dielectric constant, have been studied from first principles. We also construct a slab model, in which different constituents are treated as bulk-like materials with appropriate electrostatic constraints, to investigate the origin of the enhanced piezoelectricity in PTO/BTO superlattices. The third project is our first-principles study of the BaTiO3/CaTiO3 superlattices, in which the oxygen octahedron rotations play a substantial role. We observe the phase transitions among three competing phases and enhanced piezoelectricity in all of the three phases at intermediate BaTiO3 concentration. The slab models of BTO/CTO superlattices consistently underestimate the polarization, which indicates the interfacial enhancement of polarization. |
