Anion and cation dynamics in potassium nido-carbaborates: 1 H and 39 K NMR studies

Abstract

Potassium nido-(carba)hydroborates K-7-CB 10 H 13 , K-7,8-C 2 B 9 H 12 and K-7,9-C 2 B 9 H 12 belong to the family of alkali-metal hydroborate salts which are considered as promising solid electrolytes for battery applications. To study the dynamical properties of these compounds at the microscopic level, we have measured the 1 H and 39 K nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 198 -434 K. Our measurements have revealed that the order-disorder phase transitions occurring in these compounds in the range of 345 -400 K are accompanied by dramatic changes in both the reorientational jump rates of complex anions and the diffusive jump rates of potassium cations. For all the studied compounds, the activation energies for anion reorientations in the disordered (high-T) phases are lower than those in the corresponding ordered (low-T) phases.The same is true for the activation energies for diffusive K + jumps in the disordered and ordered phases of the studied compounds. The lowest activation energy (~0.26 eV) is found for K + diffusion in the disordered phase of K-7,8-C 2 B 9 H 12 .