Boosting internal quantum efficiency to near-unity in Fe3+-doped NIR phosphors: Structural engineering and energy transfer in A2LuSbO6 (A = Ba/Sr/Ca) double perovskites

Fe 3+ is a promising near-infrared (NIR) activator ion due to its tunable emission and non-toxicity, which has attracted growing interest. However, developing high-performance broadband NIR-emitting phosphors based on Fe 3+ remains a significant challenge. In this work, we present Fe 3+ -activated A 2 LuSbO 6 (A = Ba/Sr/Ca) double perovskite phosphors, which exhibit unusually long-wavelength NIR emission (>850 nm). By adopting a cation substitution strategy at the A-site, we designed the gradual replacement of large-radius ions with small-radius ions, achieving controllable tuning of emission from 847 nm to 937 nm. This substitution process simultaneously reduces structural symmetry and induces octahedral distortion, which breaks the spin-forbidden transition of Fe 3+ and enhances luminescence performance. However, the continuous emission redshift leads to a large Stokes shift, which suppresses Fe 3+ luminescence. The combined effect of these two factors results in optimal luminescence efficiency in Sr 2 LuSbO 6 :1.0 %Fe 3+ (SLSO) sample, with internal/external quantum efficiencies (IQE/EQE) of 94.8 %/54.5 %. To further enhance luminescence performance, we introduced the rare-earth Yb 3+ , establishing an efficient Fe 3+ →Yb 3+ energy transfer (ET) channel that reduces non-radiative transition losses. Compared with the Yb 3+ -free sample, the SLSO:1.0 %Fe 3+ , 3.0 %Yb 3+ sample exhibits a broadened FWHM from 115 nm to 180 nm, enhanced IQE and EQE of 98.1 % and 56.4 %, respectively, and 13 % improved thermal stability. Finally, a pc-LED device based on the SLSO:1.0 %Fe 3+ , 3.0 %Yb 3+ phosphor demonstrates potential for applications in NIR spectroscopy analysis, night vision, biomedical imaging, and other fields. This work lays a foundation for further research on strategies to regulation the optical properties of Fe 3+ -activated NIR phosphors.