First-principles insights into structural, electronic, optical, thermal, and hydrogen storage properties of double perovskite hydrides Cs₂BGaH₆ (BK/Rb)

The development of sustainable energy technologies relies critically on advanced materials capable of efficient energy conversion and storage. Double perovskite hydrides, with their structural versatility and tunable properties, have recently attracted considerable attention as promising candidates for solid-state hydrogen storage and multifunctional energy applications. Here, we present a comprehensive first-principles investigation of the structural, electronic, optical, thermal, and hydrogen storage properties of the novel double perovskite hydrides Cs₂BGaH₆ (BK/Rb). Both compounds are found to crystallize in a stable cubic phase (space group Fm-3m), supported by negative formation energies, ideal Goldschmidt tolerance factors and Phonon durability. Electronic structure calculations using the TB-mBJ potential reveal an indirect band gap of 3.52 eV for Cs₂KGaH₆ and 3.81 eV for Cs₂RbGaH₆. Mechanical characteristics of the investigated compounds exhibits ductile nature ensure with Pug ratio (B/G) and Poisson ration ((Formula presented) ) while Share and Young modulus conform the high regidity of the investigated compounds. Optical characterization demonstrates strong absorption in the ultraviolet region, with peak absorption coefficients of 1.26 × 104 cm−1 at 6.68 eV and 1.16 × 104 cm−1 at 8.9 eV for the K and Rb variants, respectively. Moreover, these hydrides exhibit promising thermoelectric performance, achieving figures of merit (ZT) of 0.74 and 0.72 at 800 K. In terms of hydrogen storage, gravimetric capacities of 1.59 wt% and 1.42 wt% are complemented by volumetric capacities of 50.79 g H₂/L and 48.14 g H₂/L, alongside feasible hydrogen desorption temperatures. These multifunctional characteristics underscore the potential of Cs₂BGaH₆ (BK/Rb) hydrides in next-generation optoelectronic, thermoelectric, and hydrogen storage applications.