| The structural, mechanical, optical, thermo-physical, and electronic properties of the monoclinic XGaH5 (X = Ba, Ca, and Mg) used as hydrogen storage material were investigated in detail using the ab initio technique. Gravimetric hydrogen densities (5.1 wt % for the MgGaH5, 4.4 wt % for the CaGaH5, and 2.38 wt % for the BaGaH5), hydrogen desorption temperatures (29.05 K for the MgGaH5, 175.56 K for the CaGaH5, and 218.36 K for the BaGaH5) and enthalpies of formation (−0.039 eV/atom for the MgGaH5, -0.237 eV/atom for the CaGaH5 and -0.296 eV/atom for the BaGaH5) of these compounds were also calculated. As a result of electronic band structure calculations, band gap values for BaGaH5, CaGaH5, and MgGaH5 were obtained as 3.08 eV, 4.05 eV, and 3.61 eV, respectively. It is clear from the high band gap values that all three materials have insulator character. The second-order independent elastic constant values, which provide information about the hardness and mechanical stability of the materials, were calculated. The elastic constant values showed that XGaH5 is mechanically stable. Hardness parameters such as bulk modulus, shear modulus, B/G ratio (1.19 for the MgGaH5, 1.11 for the CaGaH5, and 1.68 for the BaGaH5), Young's modulus, and Poisson's ratio were also calculated using elastic constant values. According to the B/G ratio, all three structures were found to be brittle materials. From Poisson's ratio (0.17 for the MgGaH5, 0.15 for the CaGaH5, and 0.25 for the BaGaH5), the atoms in XGaH5 compounds are connected by covalent bonds. Besides, the formation enthalpies of the materials were calculated, and it was concluded that all three compounds could be synthesized in the laboratory. In addition, some optical properties of XGaH5 such as dielectric function, conductivity, reflectivity, and absorption were also calculated. Finally, thermo-physical properties were calculated for XGaH5 compounds. |
Çağatay Yamçıçıer
Cihan KÜRKÇÜ
