Theoretical İnvestigations on Structural, Electronic, Mechanical, Optical, Dynamic, Thermal, and Hydrogen Storage Properties of Mgah4 (M = Li, Na, K, Rb, Cs)

Abstract

This study presents a comprehensive first-principles investigation of the structural, electronic, mechanical, optical, dynamical, thermal, and hydrogen storage properties of MGaH4 (M = Li, Na, K, Rb, Cs) hydrides using density functional theory. The optimized crystal structures reveal that LiGaH4 and NaGaH4 crystallize in the Cmcm space group, while KGaH4, RbGaH4, and CsGaH4 crystallize in the Pnma space group. All compounds exhibit thermodynamic stability with negative formation enthalpies ranging from − 0.154 eV/atom (LiGaH4) to − 0.248 eV/atom (CsGaH4). Cohesive energies vary between 3.110 and 3.173 eV/atom, confirming strong internal bonding. Mechanical analysis demonstrates mechanical stability for all compounds, with elastic constants satisfying Born–Huang criteria. The polycrystalline bulk moduli (8.26–16.33 GPa), shear moduli (5.47–11.46 GPa), and Young’s moduli (13.44–27.54 GPa) indicate relatively soft and brittle behavior (B/G < 1.75). Hardness calculations yield average Vickers hardness values of 0.88–2.87 GPa, confirming their soft nature. Phonon dispersion spectra show no imaginary frequencies, confirming dynamical stability. Electronic band structures reveal wide band gaps, confirming semiconducting behavior: 4.57 eV (LiGaH4), 4.68 eV (NaGaH4), 5.00 eV (KGaH4), 5.00 eV (RbGaH4), and 4.93 eV (CsGaH4). Optical analysis demonstrates high ultraviolet absorption (maximum absorption coefficient up to 13.1 × 105 cm−1 for CsGaH4), high reflectivity in the UV region (maximum 36% for NaGaH4), and optical transparency in the visible range. Thermal properties reveal Debye temperatures between 168.26 K (CsGaH4) and 352.04 K (LiGaH4), while melting temperatures range from 428.38 to 553.64 K. Minimum thermal conductivities are found between 0.29 and 0.72 W/m·K. Hydrogen storage analysis shows that LiGaH4 possesses the highest gravimetric (5.00 wt%) and volumetric (92.81 gH2L−1) storage capacities, approaching the U.S. DOE 2025 gravimetric target (5.5 wt%) and exceeding the volumetric target (40 gH2L−1). The desorption temperatures range from 113.70 K (LiGaH4) to 183.01 K (CsGaH4), suggesting relatively low hydrogen release energy barriers. Among the studied compounds, LiGaH4 demonstrates superior performance owing to its high storage capacity, strong bonding (Ecoh = 3.173 eV/atom), high Debye temperature (352.04 K), and wide band gap (4.57 eV). Overall, the results highlight LiGaH4 as the most promising candidate for lightweight solid-state hydrogen storage, while providing systematic insights into the role of alkali cation size on the stability, mechanical robustness, electronic band gap, optical absorption, and storage performance of MGaH4 hydrides.