1H NMR study of hydrogen motion in the β phase of the Mg2NiHx system

Abstract

The motion of hydrogen atoms in the β phase of Mg2NiHx was studied by the 1H pulsed NMR spectroscopy. Fourier-transformed spectra and spin-lattice relaxation times in the laboratory frame (T1) and in the rotating frame (T1ρ) for Mg2NiH3.85 were measured in the temperature range between 170 and 460 K. The 1H magnetization in the hydride relaxes by the dipolar modulation caused by the hydrogen motion and paramagnetic impurities. It was confirmed that all that hydrogen atoms in the hydride are equivalent with respect to their motion. The correlation frequencies were determined by the temperature dependences of the linewidth and the two relaxation times T1 and T1ρ, and they are in good agreement among them, giving a single activation energy 12 kcal/mol in the temperature range between 210 and 460 K. The 1H–1H dipole–dipole interaction exists in the motional narrowing region, which supports that the hydrogen motion is not a diffusion but a kind of rotation.