Designing and unlocking the potential of double perovskites X 2 NaVCl 6 (X = Rb, K) for green energy applications

The vanadium-based halide double perovskites Rb2NaVCl6 and K2NaVCl6 are studied systematically in terms of various physical aspects like structural, mechanical, thermodynamic, electronic, thermoelectric performance and optical efficiency. The studied compounds crystallize in a face-centered cubic structure (space group#225, [Formula: see text]) with lattice constants of 10.41 Å and 10.36 Å, respectively. Structural, mechanical and dynamical stability was confirmed through Goldschmidt tolerance factors, Born–Haun criteria and phonon dispersion spectra. Mechanical analysis indicates ductile behavior in Rb2NaVCl6 and brittleness in K2NaVCl6, with both exhibiting positive Cauchy pressures and notable incompressibility and stiffness. Direct electronic band gaps of 2.86 eV (Rb2NaVCl6) and 2.81[Formula: see text]eV (K2NaVCl6) were observed. Optical studies revealed significant absorption ([Formula: see text][Formula: see text]cm[Formula: see text] in the visible range and static refractive indices above 1.6. Thermoelectric performance was promising, with high Seebeck coefficients ([Formula: see text]150–250[Formula: see text] [Formula: see text]V/K), power factors over [Formula: see text][Formula: see text]W/mK 2 s and figure-of-merit (ZT) values approaching 1.47 at room temperature. These findings suggest that Rb2NaVCl6 and K2NaVCl6 are stable, eco-friendly, and efficient to be used for optoelectronic and thermoelectric industrial applications.