First Principles Study of Hydrogen Storage in Scandium-Based Vacancy Ordered Hydrides

First-principles density functional theory calculations using CASTEP were performed to examine X₂ScH₆ (X = Li, Na, K) vacancy-ordered hydrides. The Perdew-Burke-Ernzerhof generalized gradient approximation was used for ground-state calculations, with additional HSE06 electronic-structure analysis. The examined cubic phases satisfy the elastic stability criteria, while their Pugh and Poisson ratios suggest ductile behavior. Bulk and shear moduli decrease from Li₂ScH₆ to K₂ScH₆, indicating reduced lattice stiffness. The displayed phonon dispersions contain no imaginary branches along the sampled paths, and the electronic results indicate metallic behavior with composition-dependent Sc and H contributions near the Fermi level. The theoretical total hydrogen contents are 9.32, 6.24, and 4.68 wt%, with intrinsic volumetric contents of 109.85, 85.04, and 64.94 g H₂ L^-1, respectively. These material-level quantities are compared with the commonly cited Department of Energy system benchmarks of 5.5 wt% and 40 g H₂ L^-1, without implying system-level compliance. For decomposition into 2XH + Sc + 2H₂, the stoichiometric released-hydrogen fractions are instead 6.21, 4.16, and 3.12 wt%. The calculated migration barriers decrease from 0.46 to 0.32 eV across the series. These results suggest that alkali substitution can balance intrinsic hydrogen content and migration energetics; competing-phase stability, reaction thermodynamics, and reversible cycling require further assessment.