Hydrogen Storage in Cu-Li-Mg Alloys*

CuMg$_{2}$ has an orthorhombic crystal structure (Fddd) and does not form a hydride. However CuLi$_{x}$Mg$_{2-x}$ (x $\sim $ 0.11) has a hexagonal crystal structure (P6$_{2}$22), just like NiMg$_{2}$, a compound known for its hydrogen storage properties. A comparison between the phase diagrams of the systems Cu-Mg and Ni-Mg shows that these binary systems form compounds with similar stoichiometry. NiMg$_{2}$ is formed by peritectic reaction of the elements at 759 $^{\circ}$C (1032 K) and CuMg$_{2}$ at 568 $^{\circ}$C (841 K) by congruent melting. Since the energy of formation of the hydride is related to that of the primary alloy, it was hypothesized that CuLi$_{x}$Mg$_{2-x}$ might also be a hydrogen storage material similar to NiMg$_{2}$. Presumably, its advantage would be that it would release hydrogen at a lower temperature (possibly close to room temperature). In order to determine the properties of the hydrogenated (and deuterated) CuLi$_{x}$Mg$_{2-x}$ material, absorption/desorption experiments were performed at several temperatures and under different pressures of H$_{2}$ (and D$_{2})$. Neutron diffraction patterns and neutron vibrational spectra were collected to elucidate the behavior of hydrogen in the Li-doped CuMg$_{2}$ intermetallic.