Characterizing Disorder in High Entropy Oxides at Every Length Scale

The field of high entropy oxides (HEOs) flips traditional materials science paradigms on their head by seeking to understand what properties arise in the presence of profound configurational disorder. This disorder, which emerges as the result of multiple elements sharing a single crystalline lattice appears to imbue some HEOs with functional properties that far surpass their conventional analogs. However, there are significant questions surrounding the actual degree of configurational disorder, its role in stabilizing the HEO phase, and its effect on other physical properties. Grasping the true extent of the elemental disorder in HEOs requires advanced characterization across orders of magnitude in length scales - from the atomic scale to the average structure, preferably with elemental sensitivity. <br/> <br/>In my talk, I will discuss my group's efforts towards addressing these questions using x-ray and neutron methods. Our measurements extend from the nanoscale (x-ray absorption and extended x-ray absorption fine structure, both of which are sensitive to the immediate environment at each metal site) to the microscopic (scanning electron microscopy and x-ray fluorescence microscopy) to the average (bulk diffraction). We find that the true configurational disorder is greatly influenced by synthesis method and that significant kinetic and thermodynamic control is needed to ensure the most random elemental distributions. The most profound differences between samples are, surprisingly, observed at intermediate length scales, in the mesoscopic regime. However, importantly, we find that these sample-to-sample variations do not strongly influence the functional magnetic properties. The most technologically important properties, including ordering temperature and saturated moment, are highly robust to preparation method, and therefore are highly suitable for real world applications.