Boosting Near‐Infrared Luminescence of Lanthanide in Cs2AgBiCl6 Double Perovskites via Breakdown of the Local Site Symmetry

Currently, lanthanide (Ln³⁺ )-doped near-infrared (NIR)-emitting double perovskites (DPs) suffer from low photoluminescence quantum yield (PLQY). Herein, we develop a new class of NIR-emitting DPs based on Ln³⁺ -doped Cs₂ (Na/Ag)BiCl₆ . Benefiting from the Na⁺ -induced breakdown of local site symmetry in the Cs₂ AgBiCl₆ DPs, effective NIR emissions of Ln³⁺ are realized through Bi³⁺ sensitization. Specifically, 7.3-fold and 362.9-fold enhanced NIR emissions of Yb³⁺ and Er³⁺ are achieved in Cs₂ Ag_0.2 Na_0.8 BiCl₆ DPs relative to those in Na-free Cs₂ AgBiCl₆ counterparts, respectively. The optimal absolute NIR PLQYs for Yb³⁺ and Er³⁺ in Cs₂ Ag_0.2 Na_0.8 BiCl₆ DPs are determined to be 19.0 % and 4.3 %, respectively. Raman spectroscopy and first-principles density functional theory calculations verify the sublattice distortion in Cs₂ (Na/Ag)BiCl₆ DPs via Na⁺ doping. These findings provide fundamental insights into the design of efficient NIR-emitting Ln³⁺ -doped DPs for versatile optoelectronic applications.