Structural changes of thin MgAl films during hydrogen desorption

We used neutron reflectometry (NR) to study the structural changes of thin Pd-capped Mg0.7Al0.3 and Mg0.6Al0.4 alloy films after hydrogen absorption and during hydrogen desorption. NR enabled us to determine the hydrogen content and hydrogen distribution in these thin MgAl alloy films along with the structural changes associated with the desorption process. The thin films expand by about 20% during the hydrogen absorption and the hydrogen is stored only in the MgAl layer with no hydrogen content in the Pd layer. The Mg0.7Al0.3 films are fully desorbed at 448 K, whereas for the Mg0.6Al0.4 films a temperature of 473 K is needed to fully desorb the hydrogen. Our NR measurements show that the higher annealing temperature needed to desorb the hydrogen from the Mg0.6Al0.4 films led to an interdiffusion of the Pd layer into the MgAl layer. This Pd interdiffusion was also observed in a Mg0.7Al0.3 film after a 9 h annealing at 473 K. So, the Pd interdiffusion into a MgAl film that has been charged with hydrogen is a common feature of the Pd/Mg0.7Al0.3 and Pd/Mg0.6Al0.4 alloy system. In contrast, for the as-prepared hydrogen-free Pd/Mg0.7Al0.3 film the Pd layer stays intact and only a small interdiffusion zone occurs at the Pd/MgAl interface.