Complex Oxides An Introduction PDF Download

The discovery of novel materials brings diverse scientific opportunities by introducing novel physical phenomena, opening ways to verify and formulate theories, and enabling revolutionary industrial applications. Stabilization of novel structures in complex oxide materials is in particular of strong interest, as these materials display a broad spectrum of physical and chemical properties, along with dramatic variations in the material properties upon subtle changes in their structures. The stabilization of a material is largely governed by thermodynamic variables such as temperature, pressure, and chemical potential. Hence, stabilizing a novel, unconventional material usually requires that one or more of these thermodynamic variables are maintained at values far from the standard conditions. Epitaxial thin film growth, in which the target material is artificially exposed to large effective pressure due to the forced geometrical match with an underlying substrate with different unit cell dimensions, is a well-established example that satisfies this criterion. Another relevant example is ion intercalation, where relatively small ions such as lithium are inserted into a material (usually a two-dimensional van der Waals material) by electrochemically tuning the chemical potential of the ions, thereby changing the material's physical and chemical properties. In this work, I will show that controlled ionic modification via deintercalation can be an effective approach to stabilize novel complex oxides and demonstrate this in two material systems. First, I will discuss the meta-stable formation of IrO$_{x}$ via deintercalation of strontium in SrIrO$_{3}$, which demonstrates high oxygen evolution reaction (OER) activity and good chemical stability in acid. This motivates design strategies for high-activity OER catalysts such as the stabilization of a novel columbite polymorph of IrO$_{2}$, which I demonstrate can be achieved via epitaxial thin film growth. Second, I will show that oxygen deintercalation of perovskite nickelate stabilizes infinite-layer \textit{Ln}NiO$_{2}$ (\textit{Ln} = lanthanide), which upon hole-doping hosts unconventional superconductivity and shows striking similarities in the superconducting phase diagram to that of the cuprates despite marked differences in the two systems. Overall, this work highlights deintercalation as a powerful technique to stabilize novel complex oxides that is widely applicable to various material systems. As this approach has been rather underutilized relative to other conventional methods, there is great potential to further discover novel materials using this technique.