Nuclear Magnetic Resonance Study of Atomic Motion in A(2)B(12)H(12) (A = Na, K, Rb, Cs): Anion Reorientations and Na+ Mobility

To study the reorientational motion of icosahedral [B12H12](2-) anions in A(2)B(12)H(12) (A = Na, K, Rb, Cs) and the translational diffusion of Na+ cations in Na2B12H12, we have measured the H-1, B-11, and Na-23 NMR spectra and spin-lattice relaxation rates in these compounds over the temperature range of 170-580 K. For cubic compounds K2B12H12, Rb2B12H12, and Cs2B12H12, the measured H-1 and B-11 spin-lattice relaxation rates are governed by thermally activated reorientations of the [B12H12](2-) anions. The activation energy of this reorientational motion is found to decrease with increasing cation radius, changing from 800 meV for K2B12H12 to 549 meV for Rb2B12H12 and 427 meV for Cs2B12H12. For Na2B12H12, the first-order transition from the low-temperature monoclinic to the high-temperature cubic phase near 520 K is accompanied by a 2 orders of magnitude increase in the reorientational jump rate, and the corresponding activation energy changes from 770 meV for the low-T phase to 270 meV for the high-T phase. Measurements of the Na-23 NMR spectra and spin-lattice relaxation rates show that the transition from the low-T to the high-T phase of Na2B12H12 is also accompanied by the onset of the fast translational diffusion of Na+ ions. Just above the transition point, the lower limit of the Na+ jump rate estimated from the Na-23 spin-lattice relaxation data is 2 x 10(8) s(-1), and the corresponding activation energy for Na+ diffusion is about 410 meV.