Halide-driven tuning of structural, electronic, and optical properties in lead-free K2AgSbX6 (X = I, Br, Cl) double perovskites: a DFT study.

CONTEXT: Lead-free double perovskites are actively explored as environmentally benign alternatives to lead-based halide perovskites for optoelectronic applications. In this work, the influence of halide substitution (X = I, Br, Cl) on the structural, mechanical, electronic, and optical properties of cubic K2AgSbX6 double perovskites is systematically investigated. Halide replacement induces a monotonic reduction in lattice parameters and a widening of the electronic band gap, increasing from the iodide to the chloride compound. Hybrid-functional calculations predict indirect band gaps ranging from 0.60 eV for K2AgSbI6 to 1.73 eV for K2AgSbCl6. Mechanical analysis reveals ductile behavior for the iodide and bromide phases, while the chloride phase is significantly stiffer and brittle. Optical calculations indicate strong absorption across the visible range, with K2AgSbBr6 exhibiting an optimal balance between band gap, absorption strength, and mechanical flexibility, making it particularly promising for optoelectronic and photovoltaic applications.

METHODS

First-principles density functional theory calculations were performed using the CASTEP code. Structural optimization and ground-state properties were obtained using the rSCAN meta-GGA functional, while electronic band structures were refined using the HSE06 hybrid functional. Ultrasoft pseudopotentials generated within the on-the-fly scheme were employed with a plane-wave cutoff energy of 500 eV and a Γ-centered 4 × 4 × 4 Monkhorst-Pack k-point mesh. Elastic properties were evaluated via the Voigt-Reuss-Hill approach, phonon dispersions were calculated using density-functional perturbation theory, and frequency-dependent optical properties were derived from the complex dielectric function.