Crystal structures of MgyTi100−y thin film alloys in the as-deposited and hydrogenated state

In situ X-ray diffraction was used to identify the crystal structures of as-deposited and hydrogenated Mg_yTi_100-y thin film alloys containing 70, 80 and 90 at.% Mg. The preferred crystallographic orientation of the films in both the as-prepared and hydrogenated state made it difficult to unambiguously identify the crystal structure up to now. In this work, identification of the unit cells was achieved by in situ recording diffraction patterns at various tilt angles. The results reveal a hexagonal closed packed structure for all alloys in the as-deposited state. Hydrogenating the layers under 10⁵ Pa H₂ transforms the unit cell into face-centered cubic for the Mg₇₀Ti₃₀ and Mg₈₀Ti₂₀ compounds, whereas the unit cell of hydrogenated Mg₉₀Ti₁₀ has a body-centered tetragonal symmetry. The (de)hydrogenation kinetics changes along with the crystal structure of the metal hydrides from rapid for fcc-structured hydrides to sluggish for hydrides with a bct symmetry and emphasizes the influence of the crystal structure on the hydrogen transport kinetics.