Structural And Boding Feature Of Ae₆Mg₇H₂₆（Ae=Sr,Ba） And （Mg, Al, Y）H₂:First Principle Calculations

The energy crisis has increasingly become one of the greatly concerning problems. Hydrogen is a promising energy carrier due to its abundance, cleanliness, renewal and efficiency. In order to realize the scale application of hydrogen energy, to find an effective way of hydrogen storage is particularly critical and challengeable. Magnesium hydride MgH2is a typical and attractive hydrogen storage material with high hydrogen capacity (7.66wt.%) and low cost. However, it is still not applied widely due to its high thermodynamic stability and slow desorption kinetics. A large number of studies have shown that doping the foreign elements into MgH2is an efficient way to decrease dehydrogenation temperature and expedite kinetics of MgH2. In the present work, the structural feature and electronic structure of both Ae6Mg7H26(Ae=Ba, Sr) and (Mg, Al, Y)H2are studied from first principles calculations. The research results reveal the destabilizing mechanism. Hence, it will provide helpful theoretical basis to further study the doped MgH2with the foreign elements. The main contents of the dissertation are as the following:Firstly, structural and bonding features of hydride Ae6Mg7H26(Ae=Sr, Ba) were investigated by first principles calculations. The Ba-H distance in26is longer than the Sr-H in Sr6Mg7H26, while the Ba-containing polyhedron with higher symmetry and high coordination numbers possesses high stability reflected by the more negative formation enthalpy obtained for Ba6Mg7H26. Hydrogen release from Sr6Mg7H26would be easier. The electronic structures indicate that the Ae6Mg7H26(Ae=Sr, Ba) exhibits non-metallic properties and the valence band is primarily dominated by H-s electrons. The chemical bonds in Ae6Mg7H26(Ae=Sr, Ba) exhibit primary covalent feature with definite ionic component, the covalent feature of Ba-H is more obvious than that of Sr-H. These electronic structures reveal the underlying mechanism for structural and bonding features of the hydride Ae6Mg7H26(Ae=Sr, Ba).Secondly, First-principles calculations based on density functional theory (DFT) were performed to study the destabilizing mechanism of co-doped MgH2with Al and Y. From the minimization of total electronic energy, the preferential positions of dopants are determined. The calculated formation enthalpy and substitution enthalpy show that incorporation of Al combined with Y atoms into MgH2is energetically favorable relative to Al doping alone. Due to strong interaction of the dopant Y with Mg and Al, the hydrogen dissociation energy and the dehydrogenation enthalpy are both reduced, indicating that the synergetic effect of Al and Y on destabilizing the MgH2is superior to that of Al doping. The electronic structures show that the breakage of Mg-H bond is much easier in co-doped case, because of the conduction band shift below the Fermi level and the hybridization of dopants with Mg atoms, which effectively decrease the hybridization between Mg and H.