Near‐Unity Internal Quantum Efficiency in Na ₂ CaHf ₂ Ge ₃ O ₁₂ : Fe ³⁺ Phosphors via Non‐Stoichiometric Na ⁺ Engineering

ABSTRACT Fe 3+ ‐activated near‐infrared (NIR) phosphors hold great promise for environmentally friendly light sources, but their efficiency is constrained by spin‐forbidden transitions and defect‐induced quenching. Herein, a novel Fe 3+ ‐doped Na 2 CaHf 2 Ge 3 O 12 (NCHGO) phosphor was developed. Density functional theory calculations and structural analyses confirmed the preferential occupation of Fe 3+ at tetrahedral [GeO 4 ] sites. A targeted non‐stoichiometric Na + engineering strategy was proposed, where excess Na + acts as both a reactive flux to enhance crystallinity and a charge compensator to passivate oxygen vacancies. This dual‐functional approach effectively suppresses non‐radiative recombination, leading to a dramatic 323% enhancement in emission intensity at 770 nm and a near‐unity internal quantum efficiency of 99.81%, compared to 66.65% for the stoichiometric sample. Thermal stability was simultaneously improved, with intensity retention at 423 K rising from 20.91% to 30.41%. Beyond luminescence, the optimized phosphor exhibits sensitive and selective Cu 2+ detection via fluorescence quenching, with a quenching efficiency of approximately 44%. A prototype NIR phosphor‐converted light‐emitting diode fabricated with this phosphor shows stable electroluminescence and clear imaging performance for night‐vision imaging and non‐destructive detection. This work provides an effective defect‐suppression strategy for Fe 3+ ‐activated NIR phosphors, paving the way for high‐performance multifunctional NIR photonic materials.