| The lack of an inversion center in a material's crystal structure can result in many useful material properties, such as ferroelectricity, piezoelectricity and non-linear optical behavior. Recently, the desire for low power, high efficiency electronic devices has spurred increased interest in these phenomena, especially ferroelectricity, as well as their coupling to other material properties. By studying and understanding the fundamental structure-property relationships present in non-centrosymmetric materials, it is possible to purposefully engineer new compounds with the desired "acentric" qualities through crystal engineering. The families of ABO3 perovskite and ABO2.5 perovskite-derived brownmillerite oxides are ideal for such studies due to their wide range of possible chemistries, as well as ground states that are highly tunable owing to strong electron-lattice coupling. Furthermore, control over the B-O-B bond angles through epitaxial strain or chemical substitution allows for the rapid development of new emergent properties. In this dissertation, I formulate the crystal-chemistry criteria necessary to design functional non-centrosymmetric oxides using first-principles density functional theory calculations. Recently, chemically ordered (AA')B2O 6 oxides have been shown to display a new form of rotation-induced ferroelectric polarizations. I now extend this property-design methodology to alternative compositions and crystal classes and show it is possible to induce a host of new phenomena. This dissertation will address: 1) the formulation of predictive models allowing for a priori design of polar oxides, 2) the optimization of properties exhibited by these materials through chemical substitution and cation ordering, and 3) the use of strain to control the stability of new phases. Completion of this work has led to a deeper understanding of how atomic structural features determine the physical properties of oxides, as well as the successful elucidation of new design principles to help guide experimental synthesis of novel materials. |
