X-ray photoelectron spectroscopy (XPS) was employed to investigate Mg ₈₀Ti ₂₀ thin film samples prepared by magnetron sputtering and density functional theory (DFT) calculations were performed on atomistic models with similar stoichiometry. As Ti is known to be immiscible in Mg, the microstructure and atomic distribution of Mg-Ti thin films are not fully understood. In this work, it was shown by DFT calculations that the density of states (DOS) depends strongly on whether Ti is arranged in nano-clusters or if it is distributed quasi-randomly. The calculated DOS was compared to valence band spectra measured by XPS, as a new way of indirectly probing short-range order of such thin films. The XPS results of Mg ₈₀Ti ₂₀ were found to correspond best with the DOS calculated for the nano-cluster model, supporting the view that Ti forms small clusters in such sputtered thin films.

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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 ΔH=-40 kJ (mol H2) -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 ΔH 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 Mg2 Ni and Ti-Ni intermetallics in the as-deposited state, which transform into a metastable Ti-doped Mg2 Ni H4 phase upon hydrogenation. The Ti-doped Mg2 Ni H4 phase can be considered as a heavily doped semiconductor.@en