Light‐Element Vacancy‐Ordered Perovskites A 2 TiH₆ (A = Li, Na, Be, Mg): First‐Principles Design of Advanced Titanium Hydrides for Reversible Hydrogen Storage

ABSTRACT Vacancy‐ordered double perovskite hydrides A 2 TiH₆ (A = Li, Na, Be, Mg) were systematically investigated using first‐principles density functional theory to evaluate their structural stability, mechanical performance, electronic properties, and hydrogen storage capabilities for solid‐state energy storage applications. Mechanical characterization establishes all compounds as ductile materials with Pugh's ratios (B/G) of 2.20–2.70, bulk moduli of 32.30–36.77 GPa, and Debye temperatures of 436–559 K, indicating excellent mechanical robustness for practical applications. Electronic structure calculations reveal distinct A‐site‐dependent behavior: Li 2 TiH₆ and Na 2 TiH₆ are indirect‐gap semiconductors with HSE06 band gaps of 2.26 and 2.28 eV, respectively, whereas Be 2 TiH₆ and Mg 2 TiH₆ exhibit intrinsic metallic character. Hydrogen storage analysis demonstrates exceptional performance, with Li 2 TiH₆ and Be 2 TiH₆ achieving outstanding gravimetric capacities of 8.46 and 8.49 wt %, substantially exceeding the U.S. DOE ultimate target of 6.5 wt %. Formation enthalpies span the optimal thermodynamic range (−24.75 to −28.41 kJ/mol·H 2 ), yielding practical desorption temperatures of 335–365 K, well within the DOE operating window. These results establish A 2 TiH₆ vacancy‐ordered perovskites as mechanically robust, thermally stable, and electronically tunable hydrides with exceptional hydrogen storage performance, providing quantitative design guidelines for titanium‐based hydrogen storage materials and positioning Li 2 TiH₆ and Be 2 TiH₆ as priority targets for experimental synthesis and characterization.