Computational design of double perovskites X2YAlH6 (X = Rb, Cs; Y = Na, Li, K) for hydrogen storage: a renewable energy perspective

• The hydrides X 2 YAlH 6 (X = Rb, Cs; Y = Li, Na, K) were systematically examined using density functional theory calculations. • Thermodynamic analyses and elastic property evaluations confirm that all the hydrides are both thermodynamically and mechanically stable. • The absence of imaginary frequencies in the phonon dispersion spectra verifies the dynamical stability of the hydrides. • Mechanical property analysis indicates that all investigated hydrides exhibit brittle behavior. • Electronic and optical studies reveal that each hydride is a semiconducting material with pronounced absorption in the ultraviolet region. This study investigates the structural, electronic, optical, mechanical, thermodynamic, and hydrogen storage properties of X 2 YAlH 6 (X = Rb, Cs; Y = Li, Na, K) perovskites using the first-principles method. Within the Fm 3 ¯ m space group, all six compounds demonstrate thermodynamic stability, as is verified by their calculated formation energies. The phonon dispersion analysis reveals that five hydrides - Rb 2 LiAlH 6 , Rb 2 NaAlH 6 , Rb 2 KAlH 6 , Cs 2 NaAlH 6 , and Rb 2 KAlH 6 - demonstrate dynamic stability, while Cs 2 LiAlH 6 shows the presence of soft modes. Ab initio molecular dynamics (AIMD) simulations were conducted to evaluate the thermal stability at room temperature. The exchange–correlation is approached using two different approximations - PBE-GGA, and HSE06 to analyze and compare the electronic properties of X 2 YAlH 6 . The calculated elastic constants fulfilled Born’s stability criteria, confirming the mechanical stability of the investigated hydrides. The optical properties showed that X 2 YAlH 6 compounds exhibit peak absorption in the ultraviolet range. The gravimetric hydrogen storage capacity has reached 2.87 wt% for Rb 2 LiAlH 6 , 2.67 wt% for Rb 2 NaAlH 6 , 2.49 wt% for Rb 2 KAlH 6 , 1.98 wt% for Cs 2 LiAlH 6 , 1.88 wt% for Cs 2 NaAlH 6 , and 1.79 wt% for Cs 2 KAlH 6 . Rb 2 LiAlH 6 perovskite is found to have the superior performance out of the materials studied, rendering it a suitable and reliable option for sustained hydrogen storage as a fuel.