First-principles study of band gap engineering and thermoelectric performance in K₂AgSbX₆ (X = Cl, F, I) double...

This study presents a comprehensive density functional theory (DFT) investigation of the double perovskite compounds K₂AgSbX₆ (X = Cl, F, I) using the generalized gradient approximation (GGA) and modified Becke-Johnson (mBJ) exchange-correlation functionals. The electronic, optical, elastic, and thermoelectric properties of these materials have been systematically analyzed to evaluate their potential for photovoltaic and thermoelectric applications. The novelty of this work lies in the comparative band-gap engineering of K₂AgSbX₆ through halide substitution, together with a combined assessment of optical absorption, mechanical stability, thermoelectric performance, spin-orbit coupling effects, and thermodynamic behavior. The calculated lattice constants, formation energies, and structural stability parameters confirm the thermodynamic stability of all three compounds. Electronic band structure calculations reveal semiconductor behavior with band gaps ranging from 0.373 eV (K₂AgSb₆) to 2.07 eV (K₂AgSbF₆) using GGA, and 1.041 eV to 4.11 eV using mBJ approximation. Including spin-orbit coupling slightly reduces the mBJ band gaps to 0.97 eV for K₂AgSbI₆, 2.42 eV for K₂AgSbCl₆, and 4.017 eV for K₂AgSbF₆, with the strongest SOC influence observed for the iodine-based compound. Effective-mass calculations further show that K₂AgSbI₆ has the lowest carrier effective mass among the studied compounds, with values of 0.204 m₀, 0.226 m₀, and 0.231 m₀ using GGA, mBJ, and mBJ + SOC, respectively, indicating more favorable carrier transport compared with K₂AgSbCl₆ and K₂AgSbF₆. The optical properties demonstrate excellent absorption characteristics in the visible and near-infrared regions, making these materials promising candidates for solar cell applications. In particular, K₂AgSbI₆ exhibits low-energy absorption starting near ~ 1 eV, supporting its relevance for visible/near-infrared optoelectronic response. Elastic property analysis indicates mechanical stability and ductile behavior for all compounds. Phonon-dispersion calculations show the absence of imaginary frequencies along the investigated high-symmetry directions, confirming the dynamical stability of K₂AgSbCl₆, K₂AgSbF₆, and K₂AgSbI₆. The thermoelectric properties show significant potential for energy harvesting applications, particularly at elevated temperatures. The highest thermoelectric figure of merit is obtained for K₂AgSbI₆, with ZT exceeding 1.0 at high temperature, whereas K₂AgSbF₆ shows the lowest ZT because of its wider band gap and reduced carrier activity. These results identify halide substitution as an effective route to tune the electronic, optical, and thermoelectric response of K₂AgSbX₆ double perovskites.