Nuclear magnetic resonance study of atomic motion in the mixed Borohydride-amide Na₂(BH₄)(NH₂)

To study the reorientational motion of the anions and the translational diffusion of Na⁺ cations in the mixed borohydride-amide Na ₂(BH₄)(NH₂) showing fast-ion conduction, we have measured the ¹H, ¹¹B, and ²³Na NMR spectra and spin-lattice relaxation rates in this compound over broad ranges of temperature (6-480 K) and the resonance frequency (14-132.3 MHz). At low temperatures (T < 190 K), the proton spin-lattice relaxation rate is governed by fast reorientations of BH₄ groups. This reorientational process can be satisfactorily described in terms of a Gaussian distribution of the activation energies with the average E_a value of 74 ± 7 meV. Above 250 K, the measured proton spin-lattice relaxation rate originates from two other (slower) reorientational jump processes characterized by the average activation energies of 340 ± 9 meV and 559 ± 7 meV. The ²³Na spin-lattice relaxation data in the high-temperature region are governed by translational diffusion of Na⁺ ions, which is consistent with high Na-ion conductivity in Na₂(BH₄)(NH₂).