Ab initiostudy of hydrogen insertion in ultrathin transition metal doped V films: Structural and electronic properties

Abstract

We investigate the insertion of hydrogen in ultrathin V films with three monolayers in which we consider a substitutional doping impurity of Ca, Sc, Ti, V, Cr, Mn, and Fe. Calculations are conducted within the density-functional theory, as implemented in the SIESTA code, to analyze the impact of the different doping impurities on the structural shape and, in particular, on the different local environments available for hydrogen insertion, as well as on the electronic structure. The impurities located on the left side of vanadium in the periodic table induce a volume expansion in their neighborhood while elements located on the right side induce a contraction. The embedding energy of the H atom indicates that the impurities at the left act as trapping centers. In general, H prefers tetrahedral interstitial sites in the presence of substitutional impurities at the left of vanadium, whereas octahedral interstitial sites are favored when the substitutional element is at the right. In addition, an increase in the density of states is obtained at the Fermi level when H sits on a tetrahedral site. This H effect can be related with experimental observations of transport properties. Finally, a bonding state is obtained at an energy position that depends on both the substitutional impurity and the symmetry of the interstitial site.