The structural, mechanical, phonon and pressure induced thermodynamic properties of X4Mg4H12 (X = Rb and Cs) for hydrogen storage applications via PBE-GGA and TB-mBJ potentials

A comprehensive investigation was conducted on the structural, electronic, thermodynamic, mechanical, phonon, and hydrogen storage properties of orthorhombic X4Mg4H12 compounds, where X represents Rb and Cs. The study utilized the PBE-GGA method within the framework of density functional theory (DFT). Electronic calculations demonstrated that these hydrides exhibit semiconductor behavior, which was further elucidated using the Tran and Blaha modified Becke-Johnson (TB-mBJ) potential and spin orbit coupling (SOC) effect. Additionally, the thermodynamic properties, such as specific heat at constant volume, Debye temperature, and coefficient of thermal expansion, were examined at different pressures from 0 to 25 GPa. The investigation revealed that Rb4Mg4H12 possesses a higher Debye temperature compared to Cs4Mg4H12 at 0 GPa, indicating its greater stability. Furthermore, the calculated gravimetric densities indicated the potential of these hydrides for hydrogen storage applications. The heat of formation and desorption temperature of these hydrides were also determined. The mechanical analysis demonstrated that both compounds are mechanically stable and show brittle nature. Finally, the phonon dispersion curves provided insights into the dynamic stability of these hydrides.