Achieving High Quantum Efficiency in Fe 3+ ‐Doped Garnet for Multifunctional NIR Applications

Near‐infrared (NIR) luminescent materials are urgently required in night‐vision monitoring, biochemical sensing, and structural diagnostics fields. However, their practical applications are mainly hindered by low quantum efficiency. This study proposes a rational‐design strategy that combines stiffness‐oriented matrix selection and symmetry‐driven efficiency tailoring to achieve extremely efficient Fe 3+ ‐activated phosphors. Via the evaluation of Debye temperature and band gap, Ca 3 Sc 2 Si 3 O 12 garnet has been chosen as the matrix for Fe 3+ . Defect formation energy calculation helps to predict the optimal doping position of Fe in CSSO. Introducing to the Sc site in the robust Ca 3 Sc 2 Si 3 O 12 host, Fe 3 + achieves a NIR emission at 745 nm ( 4 T 1 ( 4 G)→ 6 A 1 ( 6 S) transition of Fe 3+ in ScO 6 octahedra) with an internal quantum efficiency (IQE = 96.1%) and external quantum efficiency (EQE = 53.7%). Structural and spectral analyses reveal that this remarkable performance originates from high‐symmetry [ScO 6 ] sites and extreme lattice rigidity ( Θ D = 664.09 K). Furthermore, [Y–Mg] co‐substitution at [Ca–Sc] sites enhances crystal field strength through crystal field engineering, enabling tunable emission from 745 to 810 nm. A prototype NIR lighting diode fabricated with Ca 2.7 Y 0.3 Sc 1.7 Mg 0.3 Si 3 O 12 :0.01Fe 3+ phosphor demonstrates viability for multifunctional applications of plant lighting, bio‐imaging, and night‐vision applications.