Crystal Structures Of Mg_yti_(100-y) Thin Film Alloys In The As-deposited And Hydrogenated State

摘要

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 10~5 Pa H_2 transforms the unit cell into face-centered cubic for the Mg_(70)Ti_(30) and Mg_(80)Ti_(20) compounds, whereas the unit cell of hydrogenated Mg_(90)Ti_(10) 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.