Exploring BaMg2XH8 (X = V, Mn, Fe, and Co) hydrides for solid-state hydrogen storage: Mechanical, electronic, optical, and phonon perspectives

The development of safe, efficient solid-state hydrogen storage media remains a critical hindrance in the transition to a hydrogen economy. In this study, we employed Density Functional Theory (DFT) with a Hubbard U correction to systematically investigate the structural, mechanical, electronic, and optical properties of the complex metal hydrides BaMg 2 XH 8 (X = V, Mn, Fe, and Co). Structural optimization reveals that these hydrides crystallize in a trigonal P 3 ¯ m 1 space group, exhibiting thermodynamic stability consistent with their negative formation energies. Mechanical assessment confirms that all variants satisfy the Born stability criteria, displaying a brittle nature. Electronic structure analysis distinguishes BaMg 2 VH 8 , BaMg 2 MnH 8 , and BaMg 2 FeH 8 as semiconductors with band gaps of 1.931, 1.606, and 2.364 eV, respectively, whereas BaMg 2 CoH 8 exhibits metallic character. Crucially, phonon dispersion analysis highlights a divergence in dynamic stability; while the Fe- and Co-based hydrides are dynamically stable, the V- and Mn-based analogues exhibit imaginary vibrational modes, indicating intrinsic instability at 0 K. Consequently, we identify BaMg 2 FeH 8 and BaMg 2 CoH 8 as the primary viable candidates for experimental synthesis, offering gravimetric capacities of 3.22 wt% and 3.18 wt%, respectively, alongside promising volumetric hydrogen densities of 110.0 and 108.8 kg H 2 /m 3 . Furthermore, the strong ultraviolet absorption and optical conductivity observed in these stable compounds suggest their potential for photo-enhanced hydrogen desorption, making them promising candidates for stationary storage systems integrated with solar-thermal management technologies.