Electronic Structure, Optical and Thermoelectric Properties of Li\(_2\)AgSbX\(_6\) (X = F, Cl, Br, I) Double Perovskites

Halide double perovskites offer wide compositional flexibility and are being explored for energy-related applications. In this work, the structural, electronic, optical, and thermoelectric properties of Li\(_2\)AgSbX\(_6\) (X = F, Cl, Br, I) were investigated using density functional theory (DFT) within the WIEN2k package. Structural optimization shows a systematic increase in lattice parameter from F → I, accompanied by a decrease in bulk modulus, indicating higher compressibility for the heavier-halide compounds. Electronic-structure calculations within PBE-GGA identify all compositions as indirect-gap semiconductors, with band gaps decreasing across the series: 1.599 eV (F), 1.389 eV (Cl), 0.509 eV (Br), and 0.307 eV (I). Density-of-states analysis indicates that the valence band is dominated mainly by Ag and halogen states, while the conduction band is largely governed by Li and Sb contributions. Optical spectra derived from the calculated electronic structure (dielectric function and related optical constants) show strong optical activity and absorption spanning the visible-to-UV range, with the response shifting to lower photon energies for heavier halides. Thermoelectric transport coefficients were evaluated using BoltzTraP within the constant relaxation-time framework (\(\sigma / \tau\), \(\kappa_e / \tau\), S, and \(S^2 \sigma / \tau\)), and the reported ZT trend indicates that Li\(_2\)AgSbCl\(_6\) exhibits the most favorable and temperature-stable thermoelectric performance in the presented dataset (\(ZT \approx 1\)), whereas the I-based compound shows a pronounced reduction at elevated temperature. The Seebeck trends indicate p-type behavior for Li\(_2\)AgSbF\(_6\) and Li\(_2\)AgSbBr\(_6\) and n-type behavior for Li\(_2\)AgSbCl\(_6\) and Li\(_2\)AgSbI\(_6\) under the adopted transport conditions.