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 (D_q/B ≈ 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.