Zero Phonon Line and Near-Infrared Luminescence in Cs₄CdSb_2-CrCl₁₂ 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 Cr³⁺ substitution in Cs₄CdSb₂Cl₁₂ LHDPs. Pristine Cs₄CdSb₂Cl₁₂ 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 Cs₄CdSb_2-CrCl₁₂ (x = 0.05 to 0.3) originating from the d-d transition of Cr³⁺ 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 ⁴T_2g → ⁴A_2g d-d transition from Cr³⁺. ZPL intensity increases with Cr³⁺ substitution. This study demonstrates the promising optical properties of Cr³⁺-substituted LHDPs, indicating their potential for future applications in optoelectronic devices.