Exploring the structural stability, optoelectronic characteristics, and H2 storage efficiency of ZBaGaH6 (Z = K, Rb, Cs) complex hydrides: A first-principles insight

Abstract

This work uses density functional theory (DFT) computations to investigate the structural, electronic, optical, mechanical, thermodynamic, and hydrogen storage features of the novel ZBaGaH₆ (Z = K, Rb, Cs) complex hydrides. All hydrides are dynamically stable, according to phonon dispersion studies, and crystallize in the cubic F4̅3m space group. Additionally, AIMD simulations at room temperature verify thermal robustness without structural deterioration. The impact of alkali metal substitution on band alignment and semiconducting character is shown by the electronic band structures, which display indirect band gaps. The VBM in ZBaGaH₆ (Z = K, Rb, Cs) shows flat bands with heavy holes (-2.6 to -3.0 mₑ) from H-localization, while the CBM displays dispersive Ba-d/Ga-s hybridization (44-58% d-orbital). Alkali metals tune properties: Cs widens the gap most (2.4-3.6 eV) via relativistic effects, whereas K minimizes distortion. These materials combine stable H-bonding (VBM) with mobile electrons (CBM), making Cs variants ideal for storage and K versions better for fast cycling, outperforming traditional hydrides in tunability and band separation. The mechanical analysis confirms that all the compounds adhere to the Born stability criteria, indicating a reduction in stiffness as one moves from K to Cs. The evaluation of optical properties reveals high dielectric constants, strong absorption in the UV range, and tunable refractive indices; among the studied compounds, KBaGaH₆ exhibits the highest optical activity. Based on the calculations, the hydrogen storage capacities are determined to be 2.40 wt% for KBaGaH₆, 2.03 wt% for RbBaGaH₆, and 1.75 wt% for CsBaGaH₆. Our computational study demonstrates that ZBaGaH₆ hydrides are thermally robust, with decomposition enthalpies of ∼75 kJ/mol H₂ yielding desorption temperatures of 302–309°C. This stability is a key asset for their potential use in safe, solid-state hydrogen storage systems.