Entropy‐stabilized transition metal oxides ([MgNiCoCuZn]O) (ESO) in recent years have received considerable attention owing to their unique functional properties. Solution combustion and solid state syntheses resulted in crystallites varying from 5‐15 nm to 3‐5 μm respectively. Phase stability studies showed that all the systems containing Cu2+ ions in the ESO lattice segregated upon slow cooling in the furnace. It was only when ESO was quenched in air from 1000°C the lattice stabilized to a single phase. Experiments concomitant with molecular dynamics (MD) simulations demonstrated that the local stress fields around the cations played a critical role in stabilizing the single phase. The local stress fields are a result of Jahn‐Teller distortion induced by the Cu2+ ions in the lattice. It is clearly established that in the absence of the minimization of the local stress fields around the Cu2+ ions, segregation leading to the formation of a multi‐phase material is imminent for this particular composition.

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The effect of various cooling regimes on the evolution of the crystal structure of entropy- stabilized oxides (MgNiCoCuZn)O has been studied using a high-resolution time-of-flight neutron Fourier diffractometer. It has been established that cooling in a furnace at a rate of 1, 2, and 5 K/min leads not only to the segragation of a part of copper oxide into a separate phase, but also to nanostructuring of both the main matrix and CuO. The characteristic sizes of the formed nanoparticles are determined. It is shown that the expected rhombohedral or tetragonal distortions due to the possible Jahn-Teller effect are not observed in these samples.