Structural, electronic, mechanical, hydrogen storage and optical properties of A2MgCaH6 (A=K and Rb) double hydride perovskites

In this work, we performed density functional theory (DFT) calculations to investigate various properties of the double hydrides A 2 MgCaH 6 (A = K and Rb). The structural properties show a stable cubic structure with lattice parameters of 8.472 and 8.642 Å for K 2 MgCaH 6 and Rb 2 MgCaH 6 , respectively. The electronic properties calculation suggests semiconductor behavior with indirect band gaps of 3.26 eV and 3.33 eV for K 2 MgCaH 6 and Rb 2 MgCaH 6 , respectively. Furthermore, the phonon dispersion and AIMD simulations confirm the dynamical and thermal stability of both compounds, whereas elastic constants indicate mechanical stability. Moreover, the optical properties show remarkable absorption in the UV region. The hydrogen storage properties show promising volumetric storage capabilities for K 2 MgCaH 6 and Rb 2 MgCaH 6 are 66.14 and 62.37 g.H 2 /L, which exceed the target criterion (40 g.H 2 /L) for volumetric storage capacities stated by the U.S DOE (2025). The double hydrides of K 2 MgCaH 6 and Rb 2 MgCaH 6 were found to have gravimetric capacity values of 4.07 and 2.51 wt%, respectively. Thus, the results highlight the prospects of A 2 MgCaH 6 (A = K and Rb) in hydrogen storage systems.