Stability and hydrogen storage characteristics of X2CaH6 (X = Li, Na, K) vacancy-ordered double perovskite hydrides

초록

Vacancy-ordered double perovskite hydrides offer tunable hydrogen-rich frameworks for solid-state storage. Here, first-principles calculations are used to investigate cubic X2CaH6 (X = Li, Na, K) hydrides within the Fm 3m structure. All compositions show negative formation energies (-2.91 to -2.41 eV/f.u.), satisfy Born mechanical stability criteria, and exhibit phonon spectra without imaginary modes, confirming energetic, mechanical, and dynamical stability. Ab initio molecular dynamics over 50 ps further supports short-time thermal robustness of the CaH6 framework. The calculated hydrogen capacities are 10.08, 6.57, and 4.86 wt%, with volumetric densities of 115.9, 73.5, and 53.0 g H2 L- 1 for Li2CaH6, Na2CaH6, and K2CaH6, respectively. HSE06 band structures, total DOS, elemental DOS, and PDOS reveal metallic behavior originating mainly from hybridized Ca-H states, with hydrogen-derived states dominating near the Fermi level. Overall, alkali substitution effectively tunes stability, storage density, and approximate hydrogen-release tendency.

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