Anion reorientations and the phase transition in nido -KB 11 H 14 : 1 H and 11 B NMR studies

The potassium nido -hydroborate KB 11 H 14 exhibits high K-ion conductivity above the order–disorder phase transition temperature, ~ 400 K. To study the dynamical properties of this compound at the microscopic level, we have measured the 1 H and 11 B nuclear magnetic resonance spectra and spin–lattice relaxation rates over the temperature range of 148–418 K. Our measurements have revealed the fast reorientational motion of the large cluster-like nido -[B 11 H 14 ] − anions. In the ordered triclinic low- T phase, this reorientational motion is characterized by the activation energy of 0.53(2) eV, and the corresponding reorientational jump rate reaches ~ 10 8 s −1 at 393 K. It has been found that the transition from the ordered triclinic phase to the disordered cubic phase with high ionic conductivity is accompanied by the dramatic two-orders-of-magnitude acceleration of the anion reorientations. This result supports the idea that the anion reorientational motion in alkali-metal hydroborates facilitates the cation diffusive mobility. The activation energy for the anion reorientations in the disordered high- T phase of KB 11 H 14 is estimated to be 0.19(3) eV.