Charge density measurement in MgH2 by synchrotron X-ray diffraction

Abstract

Magnesium and its alloys are considered one of the most promising materials for reversible hydrogen storage. However, their high thermodynamic stability is unfavorable for dehydrogenation processes. In order to improve their properties, understanding of the bonding nature of Mg and H is essential. In this study, we reveal the precise experimental charge density distribution in MgH2. X-ray powder diffraction data were obtained using the synchrotron radiation. The charge density distribution was calculated by the MEM (maximum entropy method)/Rietveld analysis. At room temperature, the charge density distribution around Mg is spherical, whereas the lower charge density distribution around H is non-spherical and slightly spread in the direction of the nearest neighbor Mg and H atoms. The number of electrons within the sphere around the Mg and the H atoms were estimated from the obtained distribution. As the result, Mg is almost fully ionized as Mg2+, whereas hydrogen is very weakly ionized. The ionic charge of hydrogen is lower than the theoretical value.