Elucidating the multifunctional properties of double perovskite hydrides X₂MnTlH₆ (X = Na, K): A DFT study of half-metallicity, magnetism, hydrogen storage, and optical properties

In this study, the structural, electronic, magnetic, hydrogen storage, mechanical, and optical properties of the double perovskites (DPs) Na₂MnTlH₆ and K₂MnTlH₆ were investigated using first-principles calculations. Both compounds crystallize in a stable cubic structure, with ferromagnetic (FM) ordering being energetically preferred. For Na₂MnTlH₆, the FM state is more stable than the antiferromagnetic (AFM) one by 0.35 eV, while for K₂MnTlH₆, the stability difference is 0.25 eV. The lattice parameters were determined as 8.12 Å for Na₂MnTlH₆ and 8.46 Å for K₂MnTlH₆. The dynamical and thermodynamical stability are confirmed by calculating the phonon dispersion curve, formation energy, and ab-initio molecular dynamics (AIMD) simulation. Electronic structure analysis showed that both materials exhibited half-metallic (HM) behavior, featuring metallic conductivity in the spin-up channel and semiconducting characteristics in the spin-down channel. The HM gap (E_HM) is 0.57 eV for Na₂MnTlH₆ and 1.12 eV for K₂MnTlH₆. Magnetic properties were investigated by calculating the magnetic moment, which is 4.00 μB per formula unit, primarily contributed by Mn atoms. The Curie temperatures are 270 K for Na₂MnTlH₆ and 280 K for K₂MnTlH₆. The gravimetric hydrogen storage capacities are 1.81 wt% for Na₂MnTlH₆ and 1.89 wt% for K₂MnTlH₆. Mechanical stability was confirmed through elastic constant calculations, indicating both materials are mechanically stable. Further mechanical analysis reveals that both materials exhibit a ductile nature. Additionally, the optical properties show high static dielectric constants of 11 for Na₂MnTlH₆ and 25 for K₂MnTlH₆. Overall, these results suggest that Na₂MnTlH₆ and K₂MnTlH₆ are promising candidates for energy storage, spintronic, and optoelectronic applications.