Highly destabilized Mg-Ti-Ni-H system investigated by density functional theory and hydrogenography

Using hydrogenography, we recently mapped the thermodynamic properties of a large range of compositions in the quaternary Mg-Ti-Ni-H system. The enthalpy of hydride formation of Mg-Ni alloys is significantly altered upon Ti doping. For a small range of compositions, we find a hydrogenation enthalpy $\ensuremath{\Delta}H=\ensuremath{-}40\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}$ ${(\mathrm{mol}\phantom{\rule{0.3em}{0ex}}{\mathrm{H}}_{2})}^{\ensuremath{-}1}$, which is the desired enthalpy for hydrogen storage at moderate temperature and pressure. This enthalpy value is surprising since it is significantly less negative than the $\ensuremath{\Delta}H$ of the Mg-Ni and Mg-Ti hydrides. The nanostructure of the Mg-Ti-Ni-H films hinders a direct determination of the hydride phases involved by x-ray diffraction. Using density functional theory calculations for various hydrogenation reaction paths, we establish that the destabilization of the Mg-Ni-H system by Ti doping is due to the formation of ${\mathrm{Mg}}_{2}\mathrm{Ni}$ and Ti-Ni intermetallics in the as-deposited state, which transform into a metastable Ti-doped ${\mathrm{Mg}}_{2}\mathrm{Ni}{\mathrm{H}}_{4}$ phase upon hydrogenation. The Ti-doped ${\mathrm{Mg}}_{2}\mathrm{Ni}{\mathrm{H}}_{4}$ phase can be considered as a heavily doped semiconductor.