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

First‐principles density functional theory calculations using CASTEP were performed to examine X 2 ScH 6 (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 Li 2 ScH 6 to K 2 ScH 6 , 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 H 2 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 H 2 L −1 , without implying system‐level compliance. For decomposition into 2XH + Sc + 2H 2 , 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.