Future Insights into Double Perovskites Ba₂AlTMO₆ (TM = W, Re, and Os) for Sustainable and Clean Energy Production: A DFT Investigation Using GGA, TB-mBJ, and HSE06 Methods

Ab-initio simulations based on density functional theory (DFT) were employed to investigate the structural, electronic, magnetic, mechanical, optical, and thermoelectric properties of the 5d-based double Perovskite oxides Ba₂AlTMO₆ (TM = W, Re, and Os). The results reveal that all three compounds exhibit a stable ferromagnetic (FM) ground state within the cubic Fm-3m symmetry. Structural and thermodynamic stabilities were confirmed through comprehensive assessments using the Goldschmidt tolerance factor (t_G), octahedral factor (μ), modified tolerance factor (τ), and formation energies (ΔE_f). The absence of imaginary frequencies in phonon spectra and compliance with elastic constant criteria further substantiate the stability of these materials. Mechanical property evaluations suggest ductility and pronounced anisotropy in all three compounds. Band structure analyses, performed using the Tran-Blaha modified Becke-Johnson (TB-mBJ) exchange potential and the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional, confirm their half-metallic ferromagnetic (HMF) nature. Optical investigations reveal significant absorption, refractive index, extinction coefficient, and reflectivity features in the energy range of 0–13 eV, indicating strong interactions in the UV region. Thermoelectric properties, assessed using the BoltzTraP code, indicate low thermal conductivities and figure-of-merit (ZT) values close to 1 at room temperature, demonstrating excellent thermoelectric performance. The unique combination of half-metallic ferromagnetism, strong UV absorption, and superior thermoelectric quality makes Ba₂AlTMO₆ (TM = W, Re, Os) highly promising for spintronic, optoelectronic, and thermoelectric applications.