Charged defects on MgH2 doped with Nb: DFT Study

A. JUAN; F. GAZTAÑAGA; CARLA R. LUNA; M. G. SANDOVAL; V. ORAZI; P. JASEN

Aarhus

Conferencia; HyDEM 2016 Hydrides as Energy Materials Conference; 2016

Magnesium hydride (MgH2) is considered a promising candidate as material for hydrogen storage due to its low cost, availability and non-toxicity, among others. However, the MgH2 activation energy for hydrogen kinetic desorption is relatively high [1]. One alternative for this inconvenient is the addition of transition metals (TM) or defects like vacancies[2]. The goal of this study is to elucidate the chemical and physical changes by DFT calculations due to Nb and charged vacancies incorporation in MgH2. Particularly, were considered a H vacancy (VH), or a Mg vacancy (VMg) or a di-vacancy H-Mg (VH-Mg) in MgH2 pure and MgH2+Nb. For each vacancy were taken into account three possible charge states (q = +1, 0 and −1). The results show that the MgH2 doped with Nb has a band gap reduction of around 68 % (from 3.79 eV to 1.20 eV), nevertheless the p-semiconductor behaviour remains [3]. Furthermore, the Nb induces a magnetic moment of 2.10 μB. Regarding to vacancy incorporation it can be seen that the changes more significant are in the band gap zone, the valence and conduction band practically no change. Depending of charge states of them there is a band gap reduction and appears a magnetic moment. For example in pure MgH2 only the VMg+ induces a magnetic moment (~0.79 μB), whereas in MgH2+Nb, is the only one that no induces magnetism. Moreover, the VH in MgH2 or MgH2+Nb generate additional states in the band gap zone. For example, this fact leads the system to a semiconductor behaviour for the spin up contribution, and to a metallic behaviour for the spin down contribution. Finally, it can be concluded from the results that both, the incorporation of Nb and charged vacancies, improve the H dissociation and conduce to interesting electronic properties.