Band Gap Engineering and Optical Response of Mixed-Halide Cs2TeX6– x ′Xx″ (X′, X″ = Cl, Br, I, x = 0–6) Double Perovskites

Lead-free halide perovskites are promising candidates for sustainable energy and optoelectronic applications. In this report, we investigate the systematic halide mixing in the Cs2TeX6–x′Xx″ (X′, X″ = Cl/Br, Br/I, Cl/I; x = 0–6) series, using a first-principles approach. Each series exhibits a noticeable nonmonotonic curtailment in band gap, with a minimum at the equimolar composition (x = 3): 2.54 eV (Cl/Br), 1.36 eV (Br/I), and 1.30 eV (Cl/I). This originates mainly from the interplay between chemical effects and structural distortion, whereas local symmetry breaking in intermediate compositions induces the lowering of band gap. These mixed-halide systems exhibit strong light absorption (up to 1.87 × 106 cm–1), high refractive indices (up to 3.63), and tunable plasmonic responses (8.6–0.3 eV). Halide mixing in Cs2TeX6 enables precise control of electronic and optical responses across UV to near-IR regions, positioning these materials as strong candidates for optoelectronic applications.