Stability of transition metals on the Mg-terminated $MgB_2$ (0001) surface and their effects on hydrogen dissociation

The re-hydrogenation of $MgB_2$ is a critical step in the reversibility of several key hydrogen storage reactions. Two main activated processes affect the kinetics of hydrogen absorption by $MgB_2$: the dissociation of the $H_2$ molecule and the diffusion of atomic H into the bulk. In order to have fast absorption kinetics both activated processes need to have a low barrier. Using first-principles calculations, we investigate the dissociation of $H_2$ on the Mg-terminated $MgB_2$ (0001) surfaces. We investigate both ideal surfaces as well as surfaces with vacancies, and transition-metal-dopants (TM=Sc$\sim$Zn,Y$\sim$Cd,Au,Pt). Our calculations show that the late TMs more favorably substitute for the Mg atoms in the outermost layer of the Mg-terminated surface, rather than for those in the sub-layers. We find the dissociation barrier for $H_2$ on the clean Mg-terminated $MgB_2$ (0001) surface is 0.46eV. The TM dopants have only a small effect on dissociation barrier when they are incorporated into the sub-layers. However, when doped in the outermost layer, we find examples of dopants that significantly decrease the activation barrier for the dissociation of $H_2$. We also investigate the diffusivity of H in $MgB_2$ and find strong anisotropy in the diffusion pathways.