First-principles calculations to investigate structural, mechanical, electronic and optical properties of Na 2 CuSbF 6 for sustainable device applications

The pursuit of environmentally benign and structurally robust alternatives to lead-based perovskites has motivated the investigation of eco-friendly halide double perovskites for next-generation optoelectronic applications. In this work, the structural, mechanical, electronic, and optical properties of the fluoride-based double perovskite Na2CuSbF6 are systematically examined using first-principles density functional theory within the CASTEP framework, employing GGA-PBE, HSE06, and spin–orbit coupling (SOC) approaches. Structural optimization confirms a stable cubic Fm-3m phase with a lattice constant of 8.70 Å. The material satisfies the Born mechanical stability criteria and exhibits ductile behavior with notable elastic anisotropy. Electronic structure analysis reveals an indirect bandgap of 0.471[Formula: see text]eV (GGA-PBE), 0.653[Formula: see text]eV (HSE06), and 0.323 eV (SOC), indicating suitability for infrared optoelectronic applications. Density of states results show that Cu-3d, Sb-5p, and F-3p orbitals dominate near the Fermi level. Optical properties demonstrate strong absorption, a moderate static dielectric constant (–6.5), low reflectivity (–0.06), and a high refractive index (–2.5), highlighting Na2CuSbF6 as a promising lead-free perovskite for photovoltaic and energy-conversion devices.