From Near‐Metallic to Wide‐Gap Semiconductors: Elastic Resilience and Record Hydrogen Densities in Tetragonal A ₂ BH ₄ (A = Li, Na; B = Be, Mg) From DFT and Molecular Dynamics

Tetragonal A 2 BH 4 (A = Li, Na; B = Be, Mg) hydrides adopting the P4 2 /mnm space group were systematically investigated using full‐potential density functional theory within GGA‐WC and HSE06 frameworks, complemented by phonon dispersion analysis and ab initio molecular dynamics simulations, to evaluate their structural stability, mechanical resilience, electronic character, and hydrogen storage performance. All four compounds satisfy Born–Huang elastic stability criteria and exhibit universal ductile behavior with Pugh ratios of 1.757–1.962, positive Cauchy pressures of 14.69–29.05 GPa, and near‐perfect elastic isotropy, collectively ensuring structural integrity under cyclic hydrogen loading. In marked contrast to boron‐centered analogues, the divalent alkaline‐earth B‐site chemistry produces a compositionally tunable semiconducting series with HSE06 band gaps spanning 0.19 eV for Li 2 BeH 4 to 2.15 eV for Na 2 MgH 4 , driven by systematic A‐site lattice expansion and B‐site covalency reduction. Formation enthalpies of −42.61 to −46.82 kJ/mol H 2 and desorption temperatures of 316–347 K confirm thermodynamically optimal near‐ambient hydrogen release for all compositions. Li 2 BeH 4 achieves an outstanding gravimetric capacity of 14.97 wt% exceeding the DOE target by 130% alongside a volumetric density of 81.59 kg H 2 /m 3 . All four compounds simultaneously satisfy DOE volumetric benchmarks, positioning this family as exceptional multifunctional candidates for next‐generation solid‐state hydrogen storage.