Nuclear Magnetic Resonance Study of the Rotational Motion and the Phase Transition in LiBH4

To study the rotational motion of BH 4 tetrahedra in LiBH 4, we have measured the 1 H and 11 B nuclear magnetic resonance spectra and spin−lattice relaxation rates in this compound over wide ranges of temperature (92−424 K) and resonance frequency (14−90 MHz for 1 H and 14−28 MHz for 11 B). In the low-temperature (orthorhombic) phase of LiBH 4, our spin−lattice relaxation results are consistent with a coexistence of two types of the rotational motion of BH 4 tetrahedra with the activation energies of 0.182 ± 0.003 eV and 0.251 ± 0.004 eV. For both types of motions, the jump rates of the reorientations reach the values of the order of 10 11 s −1 near the upper limit of the temperature range of the orthorhombic phase stability ( T 0 ≈ 381 K). In the high-temperature (hexagonal) phase, both the 1 H and 11 B spin−lattice relaxation rates are governed by an additional low-frequency fluctuation process (with the characteristic rate of the order of 10 7 s −1 just above T 0 ) due to the translational diffusion of Li ions.