Nuclear Magnetic Resonance Study of Atomic Motion in the Mixed Borohydride–Amide Na2(BH4)(NH2)

Abstract

To study the reorientational motion of the anions and the translational diffusion of Na+ cations in the mixed borohydride–amide Na2(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 Ea 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 cond...