Highly destabilized Mg-Ti-Ni-H system investigated by density functional theory and hydrogenography
C. P. Broedersz, R. Gremaud, B. Dam, R. Griessen, and O. M. L{\o}vvik

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 $\Delta H=-40$~kJ~(mol~H$_2$)$^{-1}$, 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 $\Delta H$ 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 (XRD). Using DFT 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 Mg$_2$Ni and Ti$-$Ni intermetallics in the as-deposited state, which transform into a metastable Ti-doped Mg$_2$NiH$_4$ phase upon hydrogenation. The Ti-doped Mg$_2$NiH$_4$ phase can be considered as a heavily doped semiconductor. PACS numbers:

© 2008 The American Physical Society.