Crystal structure of the novel Mg3MnNi2D3−x interstitial deuteride

► Mg3MnNi2D3−x deuteride was synthesised by direct interaction of Mg3MnNi2 with D2 gas. ► Crystal structure was determined by powder neutron diffraction. ► This compound is an example of interstitial-type magnesium-based hydride. ► Storage in air provokes hydrogen desorption and transformation into α-solid solution.