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 <i>E</i>_a^d 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