Improved Deformability of Co‐Free Medium Entropy Oxides by Controlled Entropy Destabilization

High-entropy oxides (HEOs), such as (CoCuMgNiZn)O, are attractive structural ceramics due to their compositional complexity and single-phase stability. However, their intrinsic brittleness remains a major limitation. Here, we introduce a strategy termed as "controlled entropy destabilization" to activate defect-mediated plasticity in a Co-free medium-entropy oxide (MEO) (CuMgNiZn)O. Spark plasma sintering produces a Rocksalt structure containing pre-existing Cu-rich secondary phases that act as defect sources during deformation. In situ SEM micropillar compression studies from room temperature to 600°C, combined with transmission electron microscopy and crystallographic orientation mapping, reveal a clear temperature-dependent transition in deformation mechanisms. At room temperature, deformation is dominated by crack initiation with limited dislocation activity. At 400°C, thermally activated dislocation motion leads to delayed cracking and increased plastic strain accommodation. At 600°C, extensive dislocation glide and stacking-fault formation produce pronounced plasticity and strain hardening. The fracture strain increases from 2.0% ± 0.3% at room temperature to 15.9% ± 0.1% at 600°C. These findings demonstrate that the controlled entropy destabilization approach can activate dislocation nucleation, migration, and storage, enabling intrinsic plasticity in multielement oxide ceramics at moderate temperatures.