Lattice distortion-induced enhancement of near-infrared emission in Na/Yb co-doped Cs 2 AgBiCl 6 double perovskites

Abstract Lanthanide-doped lead-free double perovskites (DPs) have attracted significant attention for near-infrared (NIR) optoelectronic applications; however, their luminescence efficiency remains limited by inefficient energy transfer and parity-forbidden 4f–4f transitions. Here, we report a lattice distortion engineering strategy to enhance NIR emission in Yb 3+ -doped Cs 2 Ag 1−x Na x BiCl 6 DPs synthesized via a hydrothermal method. Systematic Na + substitution induces local symmetry breaking and band structure modulation, which promotes self-trapped exciton (STE) formation and improves energy transfer to Yb 3+ centers. The optimized composition Cs 2 Ag 0.1 Na 0.9 BiCl 6 :0.5 mmol% Yb 3+ exhibits strong NIR emission centered at ∼1000 nm with a high photoluminescence quantum yield of 64% in the 900–1100 nm range. The photoluminescence decay and photoluminescence spectra confirmed the effective energy transfer from STE to Yb 3+ , while density functional theory calculations and X-ray diffraction Rietveld refinement, Williamson Hall analysis showed that sodium induced lattice distortion altered the electron density of states and reduced non radiative recombination pathways. This work demonstrates a defect- and lattice-engineering approach for achieving high-efficiency NIR emission in lead-free DPs, providing new insights for optical materials in solar energy conversion and photonic applications.