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 Cs2Ag0.6Na0.4In0.9Bi0.1Cl6 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 (Dq/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.