Text

theses

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 CaTiO$_3$ 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 CaTiO$_3$ 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 PbTiO$_3$/BaTiO$_3$. 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 BaTiO$_3$/CaTiO$_3$ 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 BaTiO$_3$ concentration. The slab models of BTO/CTO superlattices consistently underestimate the polarization, which indicates the interfacial enhancement of polarization.

ETD_5943

Ph.D.

Includes bibliographical references

by Qibin Zhou

doi:10.7282/T3DN46PB

ETD doctoral

The author owns the copyright to this work.