Fractal Structure Markedly Suppresses Near-Infrared Thermal Quenching in Yb ³⁺ -Doped Double Perovskite Cs ₂ NaHoBr ₆ and Enables Ultrahigh-Temperature Sensing

Doping perovskite materials with rare-earth ions is an effective strategy for achieving near-infrared (NIR) emission, yet their practical applications are severely limited by the thermal quenching of luminescence at elevated temperatures. Here, a disordered double perovskite (DDP) powder, Cs 2 NaHoBr 6, was synthesized by solvent–evaporation crystallization. Under 365 nm excitation, the material exhibited red and NIR emission. We therefore introduced Yb 3+ doping to exploit the energy transfer and cross-relaxation between Yb 3+ and Ho 3+, achieving a quantum efficiency of 20.94% in the NIR band. While the DDP Cs 2 NaHoBr 6:Yb 3+ demonstrated antithermal quenching behavior, fluorescence dropped once the temperature exceeded 373 K. To simultaneously enhance high-temperature luminescence and thermometric performance, we constructed an ordered fractal double perovskite (OFDP) Cs 2 NaHoBr 6:Yb 3+ via in situ solution crystallization. The OFDP exhibited significantly enhanced luminescence properties at high temperatures, with a fluorescence intensity 105 times greater than that at room temperature, and the peak temperature for thermal enhancement was increased by approximately 100 K compared to the DDP. Furthermore, the relative sensitivity was improved from 8.58% K –1 in DDP to 11.54% K –1 in OFDP. This work not only elucidates the critical role of crystal order in regulating the thermal stability of rare-earth ion luminescence but also provides a universal strategy for designing optical materials with superior thermal stability and high-performance temperature-sensing capabilities.