First-Principles Study of Structural, Elastic, Electronic, Optical, Thermodynamic, Phonon, and Hydrogen Storage Properties Of Xmg2h5 (X =Li, Na, K)

Abstract

First-principles density functional theory calculations were performed to investigate the structural, elastic, electronic, optical, thermodynamic, and hydrogen storage properties of LiMg2H5, NaMg2H5, and KMg2H5 compounds. All structures crystallize in the orthorhombic Pmmn phase and exhibit negative formation energies (−0.207, −0.183, and −0.242 eV), indicating thermodynamic stability. The calculated bulk and shear moduli are 45.27 and 35.22 GPa for LiMg2H5, 22.48 and 19.53 GPa for NaMg2H5, and 36.63 and 26.67 GPa for KMg2H5, revealing strong interatomic bonding and mechanical stability. The corresponding Poisson's ratios (0.19–0.24) confirm brittle elastic behavior. The electronic band gaps are 2.97 eV for LiMg2H5, 3.07 eV for NaMg2H5, and 2.76 eV for KMg2H5, verifying their semiconducting character, which is suitable for hydrogen-related applications. Optical analysis indicates pronounced interband transitions in the ultraviolet region with plasmon peaks around 9–12 eV. The Debye temperatures obtained from thermodynamic analysis are 807.62 K, 547.98 K, and 588.16 K, respectively, implying stable lattice dynamics. The theoretical gravimetric hydrogen capacities are 8.32 wt%, 6.58 wt%, and 5.43 wt%, with corresponding desorption temperatures of 152.47 K, 135.14 K, and 178.59 K. These results provide detailed insight into the structural stability, mechanical hardness, electronic nature, and hydrogen storage potential of Mg-based hydrides.