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

Lanthanide-doped lead-free double perovskites 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 Yb3+-doped Cs2Ag1-xNaxBiCl6 double perovskites 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 Yb3+ centers. The optimized composition Cs2Ag0.1Na0.9BiCl6:0.5mmol% Yb3+ exhibits strong NIR emission centered at ~1000 nm with a high photoluminescence quantum yield (PLQY) of 64% in the 900-1100 nm range. The photoluminescence decay and photoluminescence spectra confirmed the effective energy transfer from STE to Yb3+, while density functional theory calculations and XRD 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 double perovskites, providing new insights for optical materials in solar energy conversion and photonic applications.