Addressing Nanoscale Heterogeneity in Compositionally Complex Ceramic Oxides

The recent emergence of multicomponent, high-entropy oxides (HEOs) has sparked research into novel ceramics with significant compositional and structural diversity. The wide range of achievable materials presents promising research opportunities, as they may be custom-designed to enhance performance in any of a number of important applications, such as thermal barrier coatings and solid oxide fuel cells. In these arenas and in many others, the extent to which high entropic lattice disorder versus specific chemical short-range order, nanoscale phase segregation, and intrinsic defect/disorder states are achievable remains an open research question. We present detailed investigation of the defects, local atomic order, microstructure, and thermomechanical properties achieved in a family of rare-earth and transition metal-based pyrochlore and fluorite HEOs prepared through compositional tuning and variation in synthesis/processing conditions. A combination of local to long-range electron, X-ray and neutron scattering probes are employed to investigate the complex configurational diversity and associated structure-property trends in the series. Experimentally derived models are supported by Density Functional Theory calculations and Metropolis Monte Carlo simulations. We demonstrate that nanostructured oxides produced through polymeric steric entrapment and other low temperature reaction routes are promising precursors for the synthesis of compositionally complex pyrochlore/fluorite ceramics with specific nanoscale ordering motifs. We also discuss the exquisite level of experimental characterization and theoretical modeling that may be required to understand the phase stability, atomic structure, and respective properties of emerging HEO materials classes, highlighting some remaining challenges.