Charge Density Analysis in Magnesium Hydride

Magnesium is considered one of the most promising materials for reversible hydrogen storage, because it has high storage capacity. However, the high thermodynamic stability of magnesium hydride is unfavorable for dehydrogenation processes. Understanding the bonding nature of Mg and H is essential for improving its dehydrogenation performance. Therefore, the charge density distribution in MgH2 was measured. Charge density is typically investigated by X-ray diffraction, but the diffraction intensity from hydrogen atoms is very weak. So far, analyzing the hydrogen in metal hydrides by X-ray diffraction has been difficult. We have overcome this difficulty with precise powder diffraction measurement by synchrotron radiation, which is a highly-brilliant X-ray source. The charge density was analyzed by the MEM/Rietveld method from the measurement data. The results show weak covalent bonds between Mg and H as well as between H and H. The charge density in the interstitial region is extremely low, which denies the existence of metallic bonding. As a result of estimation of the number of electrons within the sphere around the Mg and the H atoms, the ionic charge in MgH2 was represented as Mg H. We experimentally revealed that the crystal of MgH2 is stabilized by ionic and weak covalent bonding. We consider that its ionic bonding must be made weaker in order to improve the dehydrogenation performance of MgH2.