Energy Level Engineering for Multi Rare Earth Doped Hafnate With Broadband High Infrared Emissivity

Abstract Conventional entropy‐driven strategies introduce lattice distortion to enhance phonon scattering, yet do not always lead to improved photon absorption for high emissivity thermal protection. Herein, an effective strategy of energy‐level engineering driven multi rare earth ion doping in rare earth hafnate systems for broadband high emissivity, aiming to solve the transparency to thermal radiation of rare earth hafnate materials is proposed. The multiple dopants, including La, Sm, Eu, and Gd, are utilized to absorb the thermal radiation energy alternatively by their characteristic 4 f ‐4 f transition of electronic energy levels. According to theoretical optimization and experimental verification, (La _0.25 Sm _0.25 Eu _0.25 Gd _0.25 ) ₂ Hf ₂ O ₇ (4RH‐Eu) exhibits the highest average emissivity of 0.86 at 400 °C across 2.5–14 µm and persist such the high level up to 1200 °C. Such high emissivity of 4RH‐Eu is beneficial to its thermal radiation shielding performance improvement, leading to its transmittance below 0.12. The synergistic effect of those dopants contourites to a broadband high photon absorption, and accordingly high emissivity across the whole thermal radiation range. These findings lay the theoretical foundation for the next‐generation radiation thermal protection materials.