Zero Phonon Line and Near-Infrared Luminescence in Cs₄CdSb_2–<i>x</i>Cr_<i>x</i>Cl₁₂ 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-<i>x</i>Cr_<i>x</i>Cl₁₂ (<i>x</i> = 0.05 to 0.3) originating from the <i>d</i>-<i>d</i> 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 ⁴<i>T</i>_2g → ⁴<i>A</i>_2g <i>d</i>-<i>d</i> 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.