Date of Award
Doctor of Philosophy (PhD)
Ceramic materials display a wide variety of valuable properties, such as ferroelectricity, superconductivity, and magnetic ordering, due to the partially covalent bonds which connect the cations and anions. While many breakthroughs have been made by mixing multiple cations on a sublattice, the equivalent mixed-anion ceramics have not received nearly as much attention, despite the key role the anion plays in the materials’ properties. There is great potential for functional ceramics design using anion engineering, which aims to tune the materials properties by adding and removing different types of anions in existing classes of ceramic materials. In this dissertation, I present our theoretical studies, — supplemented by experiments, where available, — of : 1) oxygen vacancies in superconducting oxides, 2) oxyfluorides to achieve multiferroic materials, 3) oxynitrides-based ferroelectric semiconductors, and 4) electride-based materials predicted to serve as promising fluorine-ion intercalation batteries. First, I will set the stage by describing the functions which we seek to gain or improve by anion engineering, and the theoretical methods which we use to predict improved materials. The first topic is the crystal site preference of oxygen vacancies in the high-temperature superconductor YBa2Cu3O7-x (YBCO). We find that it is favorable to form a small concentration of apical oxygen vacancies, in addition to the well-studied vacancies in the Cu-O chains. I will also show new experimental evidence that this apical vacancy occurs frequently under some conditions. Next, I describe our research on transition metal oxyfluoride perovskites. We predict that low-temperature fluorination of anion-deficient iron and manganese oxides can lead to multiferroic compounds with simultaneous ferroelectric and magnetic ordering. In the third topic, we employ a similar strategy of anion substitution to search for ABO2N semiconductors, where B is a p-block element. We propose a new class of stable tin oxynitrides-based ferroelectric semiconductors. These oxynitrides have tunable bandgaps across the entire visible spectrum, in addition to light electron effective masses which are expected to promote electrical conductivity. The new materials are also non-centrosymmetric, with a large polarization which may lead to switchable ferroelectricity. Finally, I will examine a new family of fluoronitrides and fluorocarbides that we predict to serve as fluorine-ion intercalation electrodes for use in high-capacity batteries. These materials utilize the unique chemistry of electrides to achieve excellent voltage and diffusion kinetics. Together, these examples demonstrate the power of anion engineering to create new functional ceramics with desirable properties.
Zohar Nussinov, Vijay Ramani, Bryce Sadtler, Elijah Thimsen,