Tuning volumetric hydrogen storage in A2BH6 vacancy-ordered perovskites hydrides via B-site substitution (Zr to Hf): A DFT study of electronic, optical, and mechanical properties

The computation of Density functional theory (DFT) is employed to evaluate volumetric hydrogen storage capacities and the mechanical, structural, and optoelectronic properties of vacancy-ordered double perovskite hydrides A 2 BH 6 (A = Rb, Cs, and B Zr, Hf) for sustainable energy applications. This study presents a systematic first-principles examination within the GGA-PBE, TB-mBJ, and HSE06 framework. The negative values of formation energies for Rb 2 ZrH 6 , Rb 2 HfH 6 , Cs 2 ZrH 6 , and Cs 2 HfH 6 are −2.57, −2.28, −2.47, and −1.95 eV/atom, and variation of total energy during the simulation time derived from ab initio molecular dynamics (AIMD), confirm that the systems are energetically stable. The band gap between valence band and conduction bands, provides clear evidence of the semiconducting nature of compounds. Moreover, electronic band gaps calculations reveal that the compounds exhibit a tunable bandgap ranging from 1.27 to 1.90 eV (GGA-PBE), 2.75-3.96 eV (TB-mBJ), and 2.79-3.34 eV (HSE06), of indirect and direct characteristics on substitution of Rb to Cs. The interplay between atomic mass and ionic radius upon substituting hafnium with zirconium results in a 25% decrease in gravimetric capacity and 4% increase in volumetric capacity. The reduced ionic radius of Hf 4+ promotes lattice contraction, thereby improving the volumetric storage capacity to 66.6 and 62.2 g.H 2 /L. Furthermore, climbing-image nudged elastic band (CI-NEB) computations reveal that Hf substitution lowers the hydrogen diffusion barriers to 0.369 and 0.32 eV, facilitating hydrogen migration and release. The mechanical stability was confirmed using Reuss-Voigt-Hill method, and the Born stability criteria, indicating a brittle characteristic (B/G < 1.75). This, combined with robust refractive indices, and optical absorption, establishes the studied hydrides are versatile materials for integrated energy harvesting and optoelectronic applications. These findings offer a strong theoretical framework supporting the use of Zr-Hf based perovskites as promising materials for optoelectronics and solid-state hydrogen storage devices.