In situ electrochemical XRD study of (de)hydrogenation of MgyTi100−y thin films

Abstract

X-Ray diffraction and electrochemical (de)hydrogenation were performed in situ to monitor the symmetry of the unit cells of Mg_yTi_100−y thin film alloys (70 ≤ y ≤ 90 at.% Mg) along the pressure–composition isotherms at room temperature. The diffraction patterns show that the crystal structures of all as-deposited alloys have a hexagonal closed packed symmetry. Inserting hydrogen transforms the structure to a body centered tetragonal structure for Mg₉₀Ti₁₀, whereas the unit cells of Mg₇₀Ti₃₀ and Mg₈₀Ti₂₀ transform into a face centered cubic symmetry. The structural change of the hydrides along with the ability to rapidly (de)hydrogenate the films emphasize the influence of the symmetry of the host lattice on the hydrogen transport properties. The lattice spacings of the unit cell of Mg₉₀Ti₁₀H_x as a function of hydrogen content do not change much in the phase transformation region, indicating that only the fractions of the phases change. Remarkably, the lattice spacings found for Mg₇₀Ti₃₀H_x in the two phase coexistence region reveal that not only the fractions but also the Mg-to-Ti ratio of both phases continuously change. Evidently, there is a large spread in the thermodynamic stability of the available sites for hydrogen. Since the X-ray diffraction patterns rule out large scale segregation, the results imply a nanostructured alloy with Ti-poor and Ti-rich regions and illustrate that the Mg and Ti atoms in Mg₇₀Ti₃₀ are not randomly distributed.