A comparative study of A₂AgInCl₆ (A = Na, K) double perovskites: Stability and suitability for energy technologies

The lead-free halide double perovskites A₂AgInCl₆ [where A = Na and K] present exciting opportunities for optoelectronic and energy conversion applications. Using first-principles computations, this work closely examines their mechanical strength, optoelectronic characteristics, and structure. Both compounds' geometric and thermodynamic stability inside the cubic Fm-3m space group is confirmed by their optimised crystal structures, Goldschmidt tolerance factors, and negative formation enthalpies. The calculated lattice constants are a 0 =b 0 =c 0 = 10.45 Å for Na₂AgInCl₆ and a 0 =b 0 =c 0 = 10.52 Å for K₂AgInCl₆, indicating a slight expansion upon substitution of Na with the larger K ion. Mechanical electronics and elastic stability indicate that the material adheres to the Born stability criteria. According to the DFT calculations, Na₂AgInCl₆ and K₂AgInCl₆ are indirect band gap semiconductors with calculated band gap values of 2.724 eV and 2.701 eV, respectively. The optical spectra, comprising the dielectric function, absorption coefficient, and refractive index, clearly indicate strong optical absorption in the visible region, highlighting the material's potential for optoelectronic applications. • First-principles calculations confirm thermodynamic, geometric, and mechanical stability within the cubic Fm3̅m phase. • Both compounds exhibit indirect band gaps of 2.724 eV (Na) and 2.701 eV (K), suitable for visible light absorption. • Elastic constants satisfy Born stability criteria, confirming mechanical robustness. • Optical properties reveal strong absorption in the visible range, indicating suitability for photovoltaic and optoelectronic devices.