Efficient Mn²⁺-Doped Red Luminescence in Lead-Free Cs₂Ag_0.5Na_0.5InCl₆ Double Perovskite for X-ray Scintillators

Lead-free double perovskite (DPs) materials have attracted considerable attention due to their outstanding performance in X-ray imaging applications. In this study, a series of Cs2Ag0.5Na0.5InCl6:xMn2+ DPs were successfully synthesized via a hydrothermal method. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) spectra confirmed the divalent oxidation state of the manganese ions. Bond energy theory and density functional theory (DFT) calculations revealed that Mn2+ preferentially substitutes Ag+ sites, accompanied by the formation of Ag vacancies. Under different UV excitations, the DPs exhibit excitation-dependent dual emission: yellow emission centered at 584 nm under 356 nm excitation, attributed to self-trapped excitons (STEs), and red emission at 622 nm under 254 nm excitation, assigned to the 4T1 → 6A1 transition of Mn2+. Notably, Cs2Ag0.5Na0.5InCl6:0.075Mn2+ achieved a photoluminescence quantum yield (PLQY) of 30.97% and a decay lifetime of 5.61 μs under 356 nm excitation, while the Mn2+ emission under 254 nm excitation exhibited a PLQY of 17.66% and a lifetime of 4.52 μs. Furthermore, X-ray scintillation performance tests demonstrated that (1) the undoped Cs2Ag0.5Na0.5InCl6 scintillator emitted yellow light with excellent dose-rate linearity, an ultralow detection limit (469 nGy s–1), and high stability, affording a spatial resolution of 9.67 lp mm–1 in the fabricated scintillation screen; (2) the Mn2+-activated Cs2Ag0.5Na0.5InCl6 emitted red emission with comparable linearity and stability, achieving a resolution of 5.5 lp mm–1 and a detection limit of 85.2 nGy s–1. These results underscore the potential of lead-free DPs for high-performance X-ray imaging applications.