Structure-property relationships of nanoscale engineered perovskite oxides

Recent advances in the synthesis of nanoscale customized structure have demonstrated that reactive molecular beam epitaxy (MBE) can be used to construct nanostructure of oxides with atomic control. The ability to engineer the structure and chemistry of oxides at the nanometer scale makes possible for the creation of new functional materials that can be designed to have exceptional properties.; This thesis focused on understanding structure-property relationships of such nanoscale customized oxides utilizing state-of-the-art transmission electron microscopy (TEM).; Epitaxial thin films of n = 1–5 members of Ruddlesden-Popper homologous series Sr_n+1Ti _nO_3n+1 were synthesized by reactive MBE. We investigated the structure and microstructure of these thin films by x-ray diffraction along with high-resolution transmission electron microscopy (HRTEM) in combination with computer image simulations. We found that the thin films of n = 1–3 members are nearly free of intergrowths, e.g. phase-pure, while n = 4 and 5 thin films contain noticeably more intergrowth defects and anti-phase boundaries in their perovskite sheets. We show that these results are consistent with what is known about the thermodynamics of Sr _n+1Ti_nO_{3 n+1} phases.; We also investigated the atomic structure and interfacial structure of artificial PbTiO₃/SrTiO₃ and BaTiO₃/SrTiO ₃ superlattices grown by MBE both with and without digital compositional grading. Both of these systems form a solid solution over their entire composition range. Thus, these layered heterostructures are metastable. We demonstrated, however, that the thermodynamically metastable superlattices can be kinetically stabilized via layer-by-layer growth. In addition, we found that the interfaces between two constituents in the heterostructures are atomically-abrupt. The superlattice thin films were made fully coherent with the substrates, resulting in a homogeneous large strain in the BaTiO₃ layers due to the lattice mismatch between BaTiO₃ and SrTiO₃ (∼2.3%). Atomic positions of cations and anions in the strained BaTiO₃ unit cell were determined by quantitative HRTEM. A strain-induced enhancement of spontaneous polarization of BaTiO₃ was found. The experimental results agree quantitatively with the theoretical calculations. Finally, the structural stability of the layered heterostructures at high temperatures was investigated.