A‐Site Alloying Activates Efficient and Thermally Stable Mo ⁴⁺ 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 Cs 2 HfCl 6 :Mo 4+ metal halides to simultaneously improve luminescence efficiency and thermostability. The optimized (Cs 0.91 Rb 0.09 ) 2 HfCl 6 :7%Mo 4+ 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 Mo 4+ 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.