Reversible Enhancement of Electronic Conduction Caused by Phase Transformation and Interfacial Segregation in an Entropy-Stabilized Oxide

Entropy-stabilized oxide (ESO) research has primarily focused on discovering unprecedented structures, chemistries, and properties in the single-phase state. However, few studies discuss the impacts of entropy stabilization and secondary phases on functionality and in particular, electrical conductivity. To address this gap, electrical transport mechanisms in the canonical ESO rocksalt (Co,Cu,Mg,Ni,Zn)O are assessed as a function of secondary phase content. When single-phase, the oxide conducts electrons via Cu+/Cu2+ small polarons. After 2 h of heat treatment, Cu-rich tenorite secondary phases form at some grain boundaries (GBs), enhancing grain interior electronic conductivity by tuning defect chemistry toward higher Cu+ carrier concentrations. 24 h of heat treatment yields Cu-rich tenorite at all GBs, followed by the formation of anisotropic Cu-rich tenorite and equiaxed Co-rich spinel secondary phases in grains, further enhancing grain interior electronic conductivity but slowing electronic transport across the tenorite-rich GBs. Across all samples, the total electrical conductivity increases (and decreases reversibly) by four orders of magnitude with heat-treatment-induced phase transformation by tuning the grains' defect chemistry toward higher carrier concentration and lower migration activation energy. This work demonstrates the potential to selectively grow secondary phases in ESO grains and at GBs, thereby tuning the electrical properties using microstructure design, nanoscale engineering, and heat treatment, paving the way to develop many novel materials. Heat treating a (Co,Cu,Mg,Ni,Zn)O entropy-stabilized rocksalt oxide yields Cu-rich tenorite particle formation at grain boundaries and in grains, followed by Co-rich spinel particle formation in grains. Charge compensating Co and Cu vacancies enhance Cu+/Cu2+ small polaron conductivity by up to 10 000 times by minimizing the Cu+ formation energy and activation energy of polaron hopping. image