Understanding of the structural stability and bonding nature of MgH2 is considered as essential for improving its hydrogen storage properties. Here, we present the sequence of phase transformation and bonding nature by first-principles calculation. The calculations indicate that there exist a phase sequence of alpha -> gamma -> beta -> is an element of, delta-MgH2 phase is not stable under high pressure. All phases of MgH2 have a strong ionic character with the analysis of the charge-density distribution and density of states. Making its ionic bonding weaker is considered to improve the dehydrogenation performance of MgH2.
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