First-principles insights into lead-free double perovskites X2SbAgBr6 (X = Li, Na, K) for sustainable optoelectronic applications

Lead-free halide double perovskites are promising alternatives to toxic Pb-based absorbers, yet the impact of alkali-metal substitution on lattice stability and optoelectronic response remains unclear. Here, first-principles calculations are employed to investigate the structural, lattice-dynamical, electronic, optical, and photovoltaic properties of cubic X 2 SbAgBr 6 (X = Li, Na, K). All compounds satisfy the Born mechanical stability criteria, whereas phonon dispersions reveal that only Li 2 SbAgBr 6 is dynamically stable at 0 K, while Na 2 SbAgBr 6 and K 2 SbAgBr 6 exhibit soft modes associated with octahedral tilting. Indirect band gaps in the range 0.75–0.81 eV and strong near-infrared absorption are obtained, with visible absorption arising from higher-energy interband transitions. These results identify Li 2 SbAgBr 6 as the most robust cubic phase and demonstrate that A-site cation engineering provides an effective route to tune lattice stability and optoelectronic properties in lead-free double perovskites.