First‐Principles Study of Double Perovskite Hydrides Cs ₂ RbAlH ₆ and Rb ₂ CaNiH ₆ for Combined Hydrogen Storage and Semiconducting Applications

ABSTRACT We report a first‐principles investigation of the structural, mechanical, electronic, and optical properties of the double perovskite hydrides Cs 2 RbAlH 6 and Rb 2 CaNiH 6 . Density functional theory calculations were performed within the GGA‐PBE framework, with an on‐site Hubbard correction applied to Ni‐3d states, using the FP‐LAPW method as implemented in the Wien2k code and plane‐wave calculations in CASTEP. Both compounds are found to crystallize in the cubic Fm‐3m (No. 225) structure and satisfy the mechanical stability criteria, exhibiting brittle behavior according to Pugh's and Poisson's ratios. The calculated hydrogen storage capacities amount to 1.55 and 2.14 wt.% for Cs 2 RbAlH 6 and Rb 2 CaNiH 6 , respectively, with corresponding volumetric densities of 56.16 and 73.36 g H 2 /L, comparable to those reported for related hydride perovskites. Electronic structure calculations indicate that Cs 2 RbAlH 6 is an indirect‐gap semiconductor with a bandgap of 2.35 eV, while Rb 2 CaNiH 6 exhibits an indirect bandgap of 1.10 eV after inclusion of the Hubbard U. The optical response is characterized by pronounced absorption in the ultraviolet region, finite optical conductivity, and distinct plasmonic features. These results establish Cs 2 RbAlH 6 and Rb 2 CaNiH 6 as representative model systems for exploring structure‐property relationships in double perovskite hydrides relevant to hydrogen‐based energy applications.