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

Potassium nido-(carba)hydroborates K-7-CB₁₀H₁₃, K-7,8-C₂B₉H₁₂ and K-7,9-C₂B₉H₁₂ 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 ¹H and ³⁹K 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-C₂B₉H₁₂.