Thermally Robust Broadband NIR‐II Emission Enabled by Dual‐Site Ni ²⁺ Occupancy in Double Perovskite

Broadband and thermally robust near‐infrared (NIR) emitters are essential for advanced photonic applications from night‐vision imaging to biological sensing and spectroscopy. However, transition‐metal‐activated phosphors face an intrinsic coupling between emission bandwidth and thermal stability, with ultrabroad spectra accompanied by severe thermal quenching. Here, we demonstrate a dual‐site occupancy strategy mitigating this trade‐off by enabling Ni 2+ to occupy two crystallographically distinct octahedral sites in double perovskite Ca 2 AlTaO 6 . The optimized Ca 2 AlTaO 6 :Ni 2+ phosphor exhibits broadband NIR‐II emission (1050–1650 nm) with a 225 nm full width at half maximum, retaining 61% of room‐temperature luminescence intensity at 423 K. Spectral deconvolution, lifetime analysis, and structural characterization reveal the concentration‐dependent emission peak red‐shift (1260 → 1310 nm) originates from dynamic Ni 2+ redistribution between the two sites, with controllable spectral broadening. This site‐selective emission mechanism provides a reliable route for tunable NIR‐II spectral engineering. A phosphor‐converted LED based on a commercial red excitation chip delivers efficient NIR‐II output, validated in night‐vision imaging, biological tissue transillumination, and qualitative chemical identification. These results establish dual‐site occupancy as a viable, generalizable design principle for broadband, thermally stable NIR emitters, with direct relevance to photonic light‐source engineering.