Entropy‐Enabled Site Inversion and Defect Activation in Medium‐Entropy Spinels for Near‐Black Infrared Emission

ABSTRACT Spinel infrared (IR) coatings are attractive for high‐temperature thermal management but often deliver sub‐black emissivity and suffer thermal drift. Although entropy‐stabilized oxides have been explored, most prior demonstrations remain case‐by‐case without a unifying principle for tailoring infrared radiation. Here, we report a medium‐entropy inverse spinel, where configurational entropy drives partial site inversion and defect formation, coherently tuning both band structure and lattice dynamics. The ceramic achieves near‐blackbody emissivity (∼0.90, 0.78–16 µm) and more than doubles the parent oxide in the 2–8 µm band critical for radiative heat transfer above 1000°C. It remains a single‐phase and higher emissivity after 200 h at 1300°C. The material exhibits low thermal conductivity (0.53 W·m ⁻¹ ·K ⁻¹ , 800°C), and as a sprayable coating reaches a hemispherical emissivity of ∼0.96 on steel and refractories, increasing furnace temperature by ∼44.3°C. Spectroscopy and density functional theory (DFT) reveal entropy‐driven mixed‐valence states, abundant oxygen vacancies, band gap narrowing (∼0.7 eV), and activation of IR phonons. This work establishes a generalizable strategy, coupling configurational entropy and site inversion, to convert robust spinels into near‐black, durable radiators, offering a scalable platform for energy‐saving coatings in extreme environments.