Oxide surface and bulk atomic structures studied by real space and reciprocal space probes [electronic resource]

The versatile and intriguing properties of complex oxide materials make them important components of electronic, optical, electrochemical and catalytic devices and applications. However, the understanding of such materials is often constrained by our ability to characterize the oxide crystals in full details. One difficulty is to characterize the location and the fractions of transition metals with similar atomic numbers and radii in the bulk of multi-cation oxide systems whose properties are can be affected by the cation disorders. Another example is pertinent to surfaces on which important catalytic reactions take place between the oxide surfaces and the gas molecules in the surrounding environment. Many surface characterization techniques are electron based and limit their usage to unrealistic conditions (e.g. ultrahigh vacuum). Synchrotron X-ray radiation provides unique advantages for characterizing oxide materials, particularly its tunable energy and extremely large intensity. In this dissertation, I will demonstrate how we enable two diffraction techniques at the Stanford Synchrotron Radiation Lightsource to obtain important information about the bulk and surface atomic structure of materials. Specifically, in the first part, we employ the resonant diffraction to investigate the site-specific cation occupancies in the bulk A2BO4 spinel oxides. The quantification of the anti-site defect concentrations in materials such as Ga2ZnO4, Cr2MnO4, Co2NiO4, and Co2ZnO4 explains the electrical behaviors of several distinctive classes of spinels. The excellent agreement between experiment results and theoretical predictions validate doping rules in spinels. In the second part, I detail the refinement approach to model the atomic structure of oxide single crystals and thin films based on crystal truncation rods, diffuse scattering features that are sensitive to surface structures. Cerium dioxide (CeO2), an electrode material for fuel cells and electrolyzers, is grown commensurately on yttria-stabilized zirconia (YSZ) and the oxide heteroepitaxy is used as a model system for surface structural determination. The interface and surface roughness is investigated by surface X-ray diffraction at ambient pressure and at elevated temperatures. We show that the surface roughness has two components of different physical origins over different length scales. Combining other characterization performed on films of different thicknesses, we determine the growth mode transition of ceria ultrathin films induced by the large biaxial strain. We propose the dislocation formation mechanisms at the heterointerface of ceria and YSZ to explain why it is possible to grow commensurate ceria films beyond the critical thickness predicted by equilibrium theory. The model system of coherently strained ceria ultrathin films will enable us to study the interplay of strain, point defects, and the interface and their effect on transport and catalytic properties in heteroepitaxial oxide systems.

Keywords: Materials Science and Engineering.
Advisor: Chueh William
Toney Michael Folsom
Salleo Alberto