The relation between the hydrogen motion and the phase transition in the β phase of the Mg2NiHx system

A phase transition between a monoclinic and a cubic structure takes place in the β phase of Mg2NiH4. The relation between the hydrogen motion and the phase transition in β‐Mg2NiH4 was studied by 1H NMR. FT spectra and spin‐lattice relaxation times were measured in the temperature range across the phase transition. The hydrogen motion is mainly an in‐plane rotation of the NiH4 planar unit in the low temperature form, while it is a hybrid motion of an isotropic rotation of the NiH4 unit and a translational diffusion of the hydrogen atoms beyond the Mg cube in the high temperature form. A good correlation was obtained between the hydrogen motion and the crystal structure, and the origin of the phase transition was discussed.