First principle investigation of the structural, elastic, optoelectronics, and thermodynamic properties of barium-based tetragonal complex perovskite hydrides X₂BaH₄ (X= Na, K, and Rb) for hydrogen storage applications

This study utilizes density functional theory (DFT) to explore the structural, hydrogen storage capacity, optical, electronic, and thermo-physical properties of complex metal hydrides X₂BaH₄ (X = Na, K, and Rb) for potential hydrogen storage applications. All compounds possess a stable tetragonal structure and space group 14/mmm. Their dynamic stability is confirmed by positive frequency values of phonon dispersion curves. The compounds satisfy the Born–Huang stability criteria, confirming their dynamic and mechanical stability. Electronic properties evaluation by hybrid HSE06 functional calculations reveals the insulating behavior of these materials, with band gaps of 1.52 eV for Na₂BaH₄, 3.00 eV for K₂BaH₄, and 2.24 eV for Rb₂BaH₄. This insulation character prevents electrical interference during hydrogen absorption and desorption. Na₂BaH₄ possesses the highest hydrogen storage capacity (2.10 wt%). The optical study gives insights into light and matter interactions, relevant to energy applications for these materials. These results highlight X₂BaH₄ perovskite hydrides as stable and potential materials for efficient hydrogen storage technologies.