Structural and Dynamical Properties of NaBH4 and KBH4: NMR and Synchrotron X-ray Diffraction Studies

The temperature dependence of the structural parameters of NaBH4 has been investigated by synchrotron X-ray diffraction over the range of 80-500 K. On the basis of the diffraction data, we revise the structure of the low-temperature (tetragonal) phase of NaBH4 and discuss the changes in the lattice parameters and the patterns of H · · ·H contacts in the low-T and high-T phases. To study the atomic motion in NaBH4 and KBH4, we have performed nuclear magnetic resonance (NMR) measurements of the 1H and 11B spin-lattice relaxation rates in these compounds over wide ranges of temperature (82-424 K) and resonance frequency (14-90 MHz). For both compounds, the NMR results are governed by the thermally activated reorientational motion of the BH4 groups, and the changes in the jump rates of reorientations are traced over the range of 8 orders of magnitude. The activation energies of the reorientational motion are found to be equal to 0.151 ( 0.002 eV (low-T phase of NaBH4), 0.126 ( 0.003 eV (high-T phase of NaBH4), and 0.161 ( 0.002 eV (high-T phase of KBH4). Structural changes in the series of alkali metal borohydrides are analyzed in connection with the changes in the phase-transition temperature, the compressibility, and the barrier of the reorientational BH4 motion. On the basis of this analysis, we discuss the role of different interatomic interactions in determining the physical properties of alkali metal borohydrides