Nuclear magnetic resonance study of Li and H diffusion in the high-temperature solid phase of LiBH 4

Abstract

In order to study the atomic jump motions in the high-temperature solid phase of LiBH 4, we have measured the 1H and 11B nuclear magnetic resonance (NMR) spectra and the 1H, 7Li and 11B spin–lattice relaxation rates in this compound over the resonance frequency range of 14–34.4 MHz. In the temperature range 384–500 K, all the spin–lattice relaxation data are satisfactorily described in terms of a thermally activated jump motion of Li ions with the pre-exponential factor τ 0=1.1×10 −15 s and the activation energy E a =0.56 eV. The observed frequency dependences of the spin–lattice relaxation rates in this temperature range exclude a presence of any distributions of the Li jump rate or any other jump processes on the frequency scale of 10 7–10 10 s −1. The strong narrowing of the 1H and 11B NMR lines above 440 K is consistent with the onset of diffusive motion of the BH 4 tetrahedra.