Discovering a new MgH 2 metastable phase.

Formation of a new metastable fcc-MgH 2 nanocrystalline phase upon mechanically-induced plastic deformation of MgH 2 powders is reported. Our results have shown that cold rolling of mechanically reacted MgH 2 powders for 200 passes introduced severe plastic deformation of the powders and led to formation of micro-lathes consisting of γ- and β-MgH 2 phases. The cold rolled powders were subjected to different types of defects, exemplified by dislocations, stacking faults, and twinning upon high-energy ball milling. Long term ball milling (50 hours) destabilized β-MgH 2 (the most stable phase) and γ-MgH 2 (the metastable phase), leading to the formation of a new phase of face centered cubic structure (fcc). The lattice parameter of fcc-MgH 2 phase was calculated and found to be 0.4436 nm. This discovered phase possessed high hydrogen storage capacity (6.6 wt%) and revealed excellent desorption kinetics (7 min) at 275 °C. We also demonstrated a cyclic-phase-transformation conducted between these three phases upon changing the ball milling time to 200 hours.