Crystal structure and charge density analysis of Li 2NH by synchrotron X-ray diffraction

Abstract

Complex hydrides, such as lithium amide (LiNH 2) and lithium imide (Li 2NH), have recently been noticed as one of the most promising materials for reversible hydrogen storage. In this paper, we reveal the bonding nature of hydrogen in Li 2NH crystal by synchrotron powder X-ray diffraction measurement at room temperature. The crystal structure was refined by Rietveld method and the charge density distribution was analyzed by maximum entropy method (MEM). The Li 2NH crystal is anti-fluorite type structure (space group F m 3 ¯ m ) consisting of Li and NH. Hydrogen atom occupies randomly the 48h (Wyckoff notation) sites around N atom. The refined lattice constant is a = 5.0742(2) Å. The charge density distribution around NH anion in Li 2NH is almost spherical. The number of electrons within the sphere around the Li and NH is estimated from the obtained charge density distribution. As the result, the ionic charge is expressed as [Li 0.99+] 2[NH] 1.21−. Therefore, it is confirmed experimentally that Li 2NH is ionically bonded.