Electronic and bonding properties of MgH_2-Nb containing vacancies

摘要

The magnesium hydride stability and bonding have been studied using density functional theory (DFT). To this aim, calculations on the electronic structure were performed. We also modeled the bulk hydride with a Nb atom as a substitutional impurity. Furthermore, both systems were modeled containing different types of vacancies (Mg,H or H-Mg complex). The crystal orbital overlap population for both the metal-metal and metal-hydrogen bonds was also computed. The influence of vacancy-like defects was studied through the calculation of the positron lifetimes in defected MgH_2 and defected MgH_2-Nb. For the pure hydride, the results show an increment in the atom bonds in correlation with an increase of the positron localization reflected in a rise of the positron lifetimes. On the other hand, in all considered cases for Mg or/and H vacancies, the presence of Nb reduces the hydride bond about 36%. This decrease in the hydride stability was associated with a decrease in the probability of the positron localization and a consequently reduction of the positron lifetimes.