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Bulk, polycrystalline (Co, Cu, Mg, Ni, Zn)O was synthesized using solid-state sintering. Micropillars were prepared and mechanically deformed along three crystallographic orientations: (001), (101), and (111). Pillars (001) and (111) cracked, while Pillar (101) remained intact. Pillars (001) and (101) exhibited activated slip systems, confirmed by a large stress drop, and the presence of slip bands, respectively. Schmid factor (SF) analysis was performed to examine the effect of grain orientations on dislocation activity and slip behavior. SF values range from 0 to 0.5, with non-zero values indicating potential for slip. Six slip systems exist in the (Co, Cu, Mg, Ni, Zn)O rock salt crystal structure: 1/2⟨110⟩11¯0. For the (001) orientation, four slip systems are potentially active (SF = 0.5). For the (101) orientation, there are four potentially active slip systems (SF = 0.25). For the (111) orientation, no potentially active slips systems exist (SF = 0). Dislocation structures, which were observed post-compression via transmission electron microscopy, demonstrated variations in size, number, and distribution across the pillar, depending on micropillar orientation. Entangled dislocations created misorientation in Pillar (001), which led to the possible formation of subgrains, while singular dislocations were observed in Pillar (101), and a lack of dislocations was observed in Pillar (111). Zener–Stroh type dislocation entanglement-mediated cracking is the proposed cause of the transgranular-type cracks in Pillar (001). The possible subgrain formation, or lack of formation, respectively, caused intergranular-type cracks to additionally form in Pillar (001), while Pillar (111) only exhibited transgranular-type brittle fracture. In combination, these findings highlight the importance of dislocation activity, without the need for elevated temperature, and grain orientation in controlling the mechanical deformation response in single-phase (Co, Cu, Mg, Ni, Zn)O. 

Abstract 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.