Formation of a new metastable fcc-MgH2 nanocrystalline phase upon mechanically-induced plastic deformation of MgH2 powders is reported. Our results have shown that cold rolling of mechanically reacted MgH2 powders for 200 passes introduced severe plastic deformation of the powders and led to formation of micro-lathes consisting of γ- and β-MgH2 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-MgH2 (the most stable phase) and γ-MgH2 (the metastable phase), leading to the formation of a new phase of face centered cubic structure (fcc). The lattice parameter of fcc-MgH2 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.
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