Divacancies and the hydrogenation of Mg-Ti films with short range chemical order

We obtained evidence for the partial chemical segregation of as-deposited and hydrogenated $Mg1−yTiy$ films $(0≤y≤0.30)$ into nanoscale Ti and Mg domains using positron Doppler-broadening. We exclusively monitor the hydrogenation of Mg domains, owing to the large difference in positron affinity for Mg and Ti. The electron momentum distribution broadens significantly upon transformation to the $MgH2$ phase over the whole compositional range. This reveals the similarity of the metal-insulator transition for rutile and fluorite $MgH2$⁠. Positron lifetime studies show the presence of divacancies in the as-deposited and hydrogenated Mg-Ti metal films. In conjunction with the relatively large local lattice relaxations we deduce to be present in fluorite $MgH2$⁠, these may be responsible for the fast hydrogen sorption kinetics in this $MgH2$ phase.