First-principles and AIMD study of structural, magnetic, and optoelectronic properties of vacancy-ordered double perovskites Ag2BCl6 (B = Tc, Re) for advanced spintronic and optoelectronic applications

With the rapid demand for advanced spintronic and optoelectronic materials, vacancy-ordered double perovskites (VODPs), especially halide-based compounds, have gained attention due to their flexible structure, tunable bandgaps, and diverse electronic properties. Using density functional theory (DFT) within the GGA+$U$ framework, we investigate the structural, magnetic, and optoelectronic properties of vacancy-ordered double perovskites Ag${}_2$BCl${}_6$ (B = Tc, Re) for spintronic and optoelectronic applications. Both compounds exhibit thermodynamic and mechanical stability, confirmed by negative formation energies, elastic constants satisfying the Born criteria, and ab initio molecular dynamics simulations. Electronic structure analysis reveals spin-polarized semiconducting behavior with direct band gaps at the $\Gamma$-point (Ag${}_2$TcCl${}_6$: 1.60 eV spin-up, 1.74 eV spin-down; Ag${}_2$ReCl${}_6$: 0.91 eV spin-up, 1.77 eV spin-down). Strong ferromagnetic ordering, driven by superexchange via Cl-mediated Tc/Re $d$-orbital interactions, yields a magnetic moment of 3 ${\mu }_B$ per formula unit. Optical properties show high absorption and low reflectivity in the visible and ultraviolet ranges, making these materials ideal for photovoltaic and optoelectronic devices. The tunability of band gaps and magnetic properties through chemical substitution highlights their potential for next-generation spintronic and optoelectronic technologies.