Cation‐Framework‐Preserving Oxygen‐Vacancy Engineering for Broadband Infrared Emission in High‐Entropy Spinel Oxides

High‐temperature radiative ceramics require broadband infrared emission and stable structures, but the defects that enhance emission can also weaken oxide lattices. Here, we regulate oxygen vacancies in (MnCrFeNiCu) 3 O 4 , a high‐entropy spinel oxide (HEO), by vacuum annealing without changing the nominal cation composition. This approach regulates oxygen nonstoichiometry without introducing an additional cation‐substitution variable. The binary Mn–Cr reference has band‐averaged emissivities of 0.668 and 0.726 over 0.78–2.5 and 2.5–16 µm, respectively. Pristine HEO increases these values to 0.919 and 0.863, while the optimized HEO–O V –5 h reaches 0.932 and 0.902. Multication incorporation broadens electronic states near the band edges. Oxygen removal creates vacancy‐derived states, redistributes charge, and changes local M─O coordination. The vacancy‐derived states are consistent with localized d – d excitation, intervalence charge transfer, and polaron‐related absorption, while changes in M─O coordination broaden vibrational absorption. Together, these electronic and local structural changes are consistent with the enhanced infrared emissivity. The spinel framework and high emissivity remain largely preserved after annealing at 1000°C for 5 h. These results show that oxygen vacancies can be regulated within a stable multication spinel to combine broadband infrared emission with high‐temperature stability.