Chemical bonding of hydrogen in MgH₂

MgH₂ is one of the promising base materials for hydrogen storage, which is a key technology of clean energy source. In this study, the bonding nature of hydrogen in MgH₂ was fully uncovered by examining the charge density distribution of this substance obtained by the maximum entropy method from the synchrotron radiation powder data. MgH₂ can be expressed as Mg^1.91+ H^0.26-, which is much weaker ionicity than the theoretical expectations. It also shows weak covalence between Mg and H. Though the bonding nature of hydrogen in MgH₂ is rather complex, i.e., the mixture of ionic and covalent bonding, it is certain that hydrogen is weakly bonded to Mg, which must be a big advantage of hydrogenation-dehydrogenation of this substance.