First-principles study of Be-based double perovskite hydrides Li2BeMgH6, Li2BeCaH6, and Na2BeMgH6: Hydrogen storage and optoelectronic properties

Many hydrogen-storage materials struggle to balance high capacity with suitable thermodynamic stability, motivating growing interest in perovskite-type hydrides with tunable structures and potentially high hydrogen contents. This study employs first-principles calculations to investigate the hydrogen-storage and optoelectronic properties of three beryllium-based double perovskite hydrides: Li 2 BeMgH 6 , Li 2 BeCaH 6 , and Na 2 BeMgH 6 . All three compounds exhibit positive cohesive energies (3.026–3.257 eV atom −1 ) and negative formation energies (−0.313 to −0.226 eV atom −1 ), although convex-hull analysis indicates that they are metastable with respect to phase separation at 0 K, with decomposition potentially hindered by long-range diffusion; phonon and elastic calculations confirm their dynamical and mechanical stability. Their gravimetric hydrogen capacities range from 7.09 to 11.36 wt% and volumetric hydrogen densities from 106.8 to 127.1 g H 2 L −1 , exceeding the DOE 2025 system-level targets. For the complete-dehydrogenation pathway R1, the estimated desorption temperatures of Li 2 BeMgH 6 , Li 2 BeCaH 6 , and Na 2 BeMgH 6 are 440, 657, and 682 K, respectively. Hydrogen-migration calculations indicate that the neighboring-vacancy pathway is kinetically more favorable than the distant-vacancy pathway. The compounds are brittle and elastically anisotropic, with Young's moduli ranging from 45.8 to 83.4 GPa. They are indirect-band-gap semiconductors, with fundamental indirect band gaps (HSE06) of 2.684–4.228 eV and minimum direct band gaps of 3.16–4.67 eV. Their optical responses are predominantly concentrated in the ultraviolet region. These findings offer theoretical insights into beryllium-based double perovskite hydrides as candidate systems for high-capacity hydrogen storage and tunable optoelectronic applications.