Atomic motion in the complex hydride Li₃(NH₂)₂I: ⁷Li and ¹H nuclear magnetic resonance studies

To study the dynamical properties of the novel complex hydride Li₃(NH₂)₂I showing fast-ion conduction, we have measured the ⁷Li and ¹H NMR spectra and spin-lattice relaxation rates in this compound over broad ranges of temperature (98-488 K) and the resonance frequency (14-90 MHz). Our measurements have revealed two jump processes with different characteristic rates. The faster process corresponds to the three-dimensional translational diffusion of Li⁺ ions; the characteristic jump rate for this motion reaches ∼10⁸ s^-1 at 310 K. This Li⁺ diffusion process can be satisfactorily described in terms of a Gaussian distribution of the activation energies with the average Ead value of 0.38 eV. Comparison of the ⁷Li and ¹H NMR data with the results of dipolar second moment calculations indicates that high Li⁺ mobility in Li₃(NH₂)₂I is not related to the effects of NH₂ reorientations. On the other hand, specific structural features of the Li-site sublattice in Li₃(NH₂)₂I can facilitate fast Li⁺ diffusion. The slower jump process has been identified as the translational diffusion of intact NH₂ groups. However, the characteristic jump rate for this process remains far below 10⁸ s^-1 up to 488 K.