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 MgyTi100−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 Mg90Ti10Hx 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 Mg70Ti30Hx 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.