Zero Phonon Line and Near-Infrared Luminescence in Cs₄CdSb_2–xCr_xCl₁₂ Quadruple-Layered Halide Double Perovskite

There is growing interest in understanding the optoelectronic properties of inorganic lead-free layered halide double perovskites (LHDPs). Here, we report a distinctive sharp zero phonon line (ZPL) emission at 727 nm and an intense, broad near-infrared (NIR) emission at 757 nm from the photoluminescence (PL) spectra measured at temperatures below 217 K, resulting from Cr 3+ substitution in Cs 4 CdSb 2 Cl 12 LHDPs. Pristine Cs 4 CdSb 2 Cl 12 exhibits only a weak and broad self-trapped excitonic emission centered around 590 nm. On the contrary, an NIR emission (fwhm = 72.5 nm) at 757 nm is also observed at 300 K in Cs 4 CdSb 2– x Cr x Cl 12 ( x = 0.05 to 0.3) originating from the d – d transition of Cr 3+ ions. This broad mode evolves into 2 distinct emissions: a ZPL (at 727 nm) and an intense, broad NIR (at 746 nm) sideband below 217 K. The ZPL originates from the spin-allowed 4 T 2g → 4 A 2g d – d transition from Cr 3+ . ZPL intensity increases with Cr 3+ substitution. This study demonstrates the promising optical properties of Cr 3+ -substituted LHDPs, indicating their potential for future applications in optoelectronic devices.