A-Site Alloying Activates Efficient and Thermally Stable Mo⁴⁺ NIR Emission in Zero-Dimensional Metal Halides

Broadband near-infrared (NIR) phosphors with high efficiency and thermal stability are essential for high-power NIR light-emitting diodes (LEDs). Herein, A-site cation alloying strategy is introduced into 0D Cs₂HfCl₆:Mo⁴⁺ metal halides to simultaneously improve luminescence efficiency and thermostability. The optimized (Cs_0.91Rb_0.09)₂HfCl₆:7%Mo⁴⁺ phosphor achieves a broad NIR luminescence covering 800-1400 nm, achieving a record-breaking external quantum efficiency (EQE) of 69.6% and a near-unity internal quantum efficiency (IQE) of 99.2%. Notably, it exhibits negligible thermal quenching at 398 K compared to room temperature. Density functional theory (DFT) calculations and structural analyses reveal that Rb⁺ incorporation induces local symmetry breaking and intensifies lattice distortion, which partially relaxes the parity-forbidden restriction of Mo⁴⁺ d-d transitions and thus boosts optical absorption and radiative transition rates. In-situ characterizations and molecular dynamics simulations further elucidate that A-site alloying enhances lattice rigidity and suppresses thermally activated nonradiative relaxation. Benefiting from these advantages, high-performance NIR phosphor-converted LEDs and ultraviolet photodetectors were fabricated, demonstrating their exceptional potential for imaging through smoke and flame detection. This work establishes A-site alloying as an effective strategy for designing efficient and thermally stable NIR-emitting 0D metal halides.