Sb 3+ ‐Doping Induced Quasi‐2D Single Crystal Growth from 0D Cs 3 BiCl 6 : Modulation of Site‐Specific Cr 3+ Near‐Infrared Emission

Broadband near‐infrared (NIR) emission in low‐dimensional halide perovskites remains challenging because of their intrinsically soft lattice and the accompanying nonradiative losses. In this work, Cr 3+ ‐activated NIR luminescence is achieved in 0D Cs 3 BiCl 6 under green‐light excitation through a site‐preferential doping strategy employing Sb 3 ⁺. The incorporation of Sb 3+ ions selectively occupied strong‐field octahedral sites, effectively directing Cr 3+ dopants into weak‐field environments and thereby promoting radiative transitions. This site engineering simultaneously triggers a structural evolution from isolated 0D units to quasi‐two‐dimensional (2D) single‐crystalline domains with enhanced octahedral connectivity. Photophysical analyses reveal dual‐site Cr 3+ emissions arising from distinct crystal‐field symmetries, while quantum yields of up to 30.68% are obtained under optimized doping conditions. Density functional theory (DFT) calculations and defect formation energy analyses further corroborate the observed site‐occupancy behavior. The co‐engineering of site occupancy and lattice dimensionality thus establishes a design paradigm for efficient NIR emission in soft halide systems, providing new opportunities for perovskite‐based broadband phosphors and integrated light‐emitting devices.