First-Principles Investigation of Structural, Mechanical, Electronic, Optical, and Hydrogen Storage Properties of A2CaAlH6 (A = K, Rb, Cs)

Using density functional theory within CASTEP, we investigated the structural, thermodynamic, hydrogen storage, mechanical, electronic, and optical properties of K2CaAlH6, Rb2CaAlH6, and Cs2CaAlH6 double perovskite hydrides. All compounds crystallize in stable cubic structures with lattice constants of 8.34–8.75 Å and negative formation energies (–0.84 to –0.55 eV/atom). Phonon dispersion confirms their dynamic stability. Hydrogen storage analysis reveals gravimetric capacities of 5.39 wt.% (K2CaAlH6), 3.81 wt.% (Rb2CaAlH6), and 1.78 wt.% (Cs2CaAlH6), with K2CaAlH6 showing the highest storage efficiency and Cs2CaAlH6 desorbing at the lowest temperature. Mechanical results satisfy the Born criteria, while optical analysis demonstrates strong absorption, high dielectric constants, and notable refractive indices, indicating multifunctional potential in optoelectronics. Bonding analysis shows mixed ionic–covalent character, supporting flexible hydrogen interactions. Overall, these findings identify A2CaAlH6 (A = K, Rb, Cs) as promising hydrogen storage candidates for sustainable energy technologies.