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

To study the rotational motion of BH4 tetrahedra in LiBH4, we have measured the 1H and 11B 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 1H and 14-28 MHz for 11B). In the low-temperature (orthorhombic) phase of LiBH4, our spin-lattice relaxation results are consistent with a coexistence of two types of the rotational motion of BH4 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 1011 s-1 near the upper limit of the temperature range of the orthorhombic phase stability (T0 ≈ 381 K). In the high-temperature (hexagonal) phase, both the 1H and 11B spin-lattice relaxation rates are governed by an additional low-frequency fluctuation process (with the characteristic rate of the order of 107 s-1 just above T0) due to the translational diffusion of Li ions