Theoretical DFT insights into the stability, electronic, mechanical, optical, thermodynamic, and hydrogen storage properties of RbXH3 (X = Si, Ge, Sn) Perovskite Hydrides

Abstract

Hydrogen is a promising alternative to fossil fuels, but its storage remains a challenge. This study investigates the stability and properties of RbXH₃ (X = Si, Ge, Sn) perovskites for hydrogen storage using density functional theory. Geometry optimization and X-ray diffraction confirm the cubic phase, with lattice constants ranging from 3.99 Å to 4.4 Å. Formation energies indicate these materials are synthesizable and thermodynamically stable. Elastic constants and phonon dispersion show mechanical and dynamic stability, while band structure analysis confirms metallic behavior, which supports efficient charge transfer, a property that can facilitate hydrogen diffusion and storage. Additionally, these materials exhibit excellent mechanical properties such as stiffness, high thermal conductivity, and high melting points. Although their gravimetric hydrogen storage capacities (1.46–2.59 wt%) fall short of DOE (The US Department of Energy) targets, the stability, fast kinetics, and mechanical robustness make them promising for niche hydrogen storage applications, such as small-to-medium storage systems.