Sustainable lead-free Na2AgSbX6 (X = Cl, F) double perovskites with tunable optoelectronic and thermoelectric properties: A first-principles study

Lead-free halide double perovskites have emerged as promising candidates for sustainable energy technologies due to their low toxicity and structural stability. In this work, we investigate the structural, electronic, optical, and thermoelectric properties of Na 2 AgSbX 6 (X = Cl, F) using density functional theory. Both compounds crystallize in a stable cubic Fm-3m structure, with tolerance factors (0.808–0.829) confirming their structural feasibility. The calculated indirect band gaps of 2.398 eV (Cl) and 4.11 eV (F) reveal tunable optoelectronic behavior spanning the visible to ultraviolet regions. Optical analysis demonstrates strong light absorption with low reflectivity, highlighting their suitability for photovoltaic applications. Furthermore, the compounds exhibit favorable Seebeck coefficients and promising thermoelectric performance with ZT values approaching technologically relevant ranges. The combination of environmental compatibility, tunable electronic structure, and multifunctional energy conversion capabilities positions Na 2 AgSbX 6 double perovskites as attractive candidates for next-generation sustainable photovoltaic and thermoelectric devices.