Cr³⁺-Induced Structure Modulation and Near-Infrared Luminescence in Double Perovskite

Cr³⁺-doped lead-free halide double perovskites (DPs) have recently emerged as promising near-infrared (NIR) phosphors, owing to their tunable optical properties. Herein, we report a Cr³⁺-doped Cs₂Ag_0.6Na_0.4In_0.9Bi_0.1Cl₆ system exhibiting dual-band luminescence under 405 nm excitation and NIR-only luminescence under 808 nm excitation. Increasing Cr³⁺ concentration induces a redshift and attenuation in self-trapped excitons (STEs) luminescence, while Cr³⁺ luminescence first increases and then decreases. The Tanabe-Sugano diagram (<i>D</i>_q/<i>B</i> ≈ 2.20) confirms the weak crystal field environment and spin-allowed ⁴T₂ → ⁴A₂ NIR transition. Multivariate regression reveals that Cr³⁺ luminescence partly arises from the self-absorption process. First-principles calculations indicate that Cr significantly modifies the conduction band minimum, enhancing electron localization and reducing electron-hole wave function overlap, which in turn suppresses STEs luminescence. Combining experimental observations with first-principles calculations elucidates the luminescence pathways of Cr³⁺ and STEs, offering insights into their distinct behaviors and interaction dynamics.