Formation and dissociation of MgH2 in epitaxial Mg thin films

We have studied hydride formation and dissociation in epitaxial and textured Mg films capped with Pd. We grew epitaxial Mg in the (001) orientation on (001) Al2O3 and in the (110) orientation on (320) LiGaO2; and as a reference, textured (001) Mg on glass. Using x-ray diffraction techniques, we studied the formation of the hydride at 373 K in 0.6 MPa of H-2 gas, and the dissociation of the hydride in air at 355 K. We show that the hydride forms epitaxially relative to the Mg; we find the epitaxial relationship to be MgH2(110)[001]//Mg(001)[100] on Al2O3 and MgH2(200)[001]//Mg(110)[(1) over bar 11] on LiGaO2. We provide evidence showing that during hydride dissociation a portion of Mg recrystallization begins at the Mg/MgH2 interface, as well as at the MgH2/Pd interface. Finally, we find that the kinetics of hydrogen absorption and desorption depends on the surface orientation and degree of crystallinity of the original Mg, and that hydrogen diffusion is an important factor governing desorption kinetics. (c) 2007 American Institute of Physics.