Compositionally complex oxides (CCOs) are an emerging material class that includes high-entropy oxide (HEOs) and entropy-stabilized oxides (ESOs), whose unprecedented properties stem from disorder-induced distributions in electronic structure and chemistry caused by stabilizing many-cation (typically > 5) solid solutions [1,2]. This contribution presents our lab’s recent works elucidating the formation mechanisms and property implications of CCO-derived composite and thin film electroceramics, with a theme of atomic- and nanoscale characterization by scanning transmission electron microscopy (STEM).

Research on HEOs/ESOs has primarily focused on exploring new structures, chemistries, dislocations [3], or unique properties. However, few studies discuss the impact of secondary phases on functionality. Here, electrical transport mechanisms in the canonical ESO (Co,Cu,Mg,Ni,Zn)O were assessed as a function of secondary phase content [4]. When single-phase, the oxide is a small polaron electronic conductor. After heat treatments, Cu-rich tenorite particles form at grain boundaries, which enhances the grain interior rocksalt oxide electronic conductivity due to increased Cu cation vacancies and compensating small hole polarons. While Cu depletion tailors grain interior the conduction mechanism, Cu-rich tenorite grain boundary phases create a pathway for Cu²⁺/Cu³⁺ small hole polarons after longer heat treatment times. The ability to selectively grow secondary phases nucleated at grain boundaries enables tuning of electrical properties in CCOs, HEOs, and ESOs using microstructure design, nanoscale engineering, and heat treatment, paving the way to develop many novel materials.

Integrating novel CCOs/HEOs/ESOs into composites with nanoscale tunability, using methods such as self-assembly [5] and exsolution [6], will enable tailored (multi)functionality beyond what is possible in a single phase. Here, we demonstrate a novel, highly extensible approach, exsolution self-assembly (ESA), to realize CCO-based nanocomposite thin films with intricate multi-element nanostructures [7,8]. Using pulsed-laser deposition (PLD) [9], we selectively reduce cations in a model perovskite CCO LaFe_0.7Ni_0.1Co_0.1Pd_0.05Ru_0.05O_3-δ, inducing defect-interaction-driven exsolution and simultaneous self-assembly of metal nanorods and metal-oxide core-shell nanoparticles, depending on oxygen partial pressure (P_O2). A correlated analysis using STEM imaging and spectroscopy, strain mapping, atom probe tomography with 3D mass spectrometry, and X-ray photoemission spectroscopy was performed to characterize the ESA nanostructures and elucidate the nanostructure formation mechanisms underlying the highly tailorable synthesis approach. With decreasing P_O2 from 3 mtorr, 0.15 mtorr, to 0.015 mtorr, concentration of oxygen vacancy increases, which tunes the extent of exsolution for different ESA nanostructures. At P_O2 of 3 mtorr, the LaFeO₃-based CCO thin film matrix shows uniform cation distribution. When P_O2 drops to 0.15 mtorr, ESA Pd nanorods grow from bottom of the thin film to top surface, with growth restricted by compressive stress exerted by the matrix in the in-plane direction and Pd availability in surroundings. When P_O2 further reduce by 10 times to 0.015 mtorr, Pd-Ni_xCo_1-xO metal-oxide core-shell nanoparticles embedded in the matrix form via seed growth effect triggered by growth of Pd followed by subsequent exsolution of Ni³⁺ and Co³⁺ in the CCO matrix. ESA is expected to synthesize complex and multi-dimensional nanostructures for electrochemical devices via integration of novel compositions and crystal structures of CCOs as well as PLD conditions.