Thermodynamically stable Mg2XH6 (X = Al, Si) double perovskite hydrides for high-capacity hydrogen storage: A DFT and AIMD study

• Mg 2 XH 6 (X = Al/Si) double perovskite hydrides investigated via first-principles DFT. • Mechanical and dynamical stability confirmed by Born criteria and phonon spectra. • Metallic electronic structure with strong dielectric response and optical conductivity. • High gravimetric H 2 capacities of 7.41 wt% (Al) and 7.31 wt% (Si) achieved. • Desorption temperatures of 361.7 K (Al) and 368.0 K (Si) predicted. The escalating worldwide demand for decarbonized energy systems has led to hydrogen being established as a leading candidate owing to its high specific energy and environmentally benign combustion profile. Among the advanced materials for solid-state hydrogen storage, double perovskite hydrides of Mg 2 XH 6 (X = Al/Si) are a promising class of lightweight compounds. Using density functional theory, this study presents a comprehensive computational analysis of the structural, elastic, mechanical, electronic, optical, thermodynamic, and vibrational properties. Moreover, their thermal stability, finite-temperature dynamics via molecular dynamics simulations, and hydrogen-storage performance were evaluated. Tolerance factor calculations revealed that these hydrides adopt stable cubic perovskite configurations, with their negative formation enthalpies providing additional evidence of thermodynamic stability. All hydrides satisfy Born’s mechanical stability criteria and exhibit high stiffness, superior mechanical strength, enhanced hardness, shear resistance, and ductile mechanical characteristics. Electronic band structure calculations demonstrate that both Mg 2 AlH 6 and Mg 2 SiH 6 exhibit metallic behavior using the generalized gradient approximation (GGA) based on the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional. Optical analysis revealed a strong dielectric response, pronounced light–matter interaction, and high optical conductivity, along with superior absorption and excellent photonic performance. Phonon dispersion analysis confirmed the dynamic stability of these compounds. Their thermodynamic parameters and high melting points highlight their overall stability and attractive properties. Furthermore, ab initio molecular dynamics (AIMD) simulations provide compelling evidence of the robust thermal stability of these compounds under realistic conditions. Beyond these attributes, assessments of hydrogen storage capacity exhibit impressive gravimetric contents of 7.41 wt% for Mg 2 AlH 6 and 7.31 wt% for Mg 2 SiH 6 values that exceed the U.S. Department of Energy (DOE) 2025 target of 5.5 wt%. These results highlight the strong potential of these compounds as high-capacity hydrogen-storage materials.