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Nuclear Magnetic Resonance Study of Atomic Motions in A2B12H12 (A = Na, K, Rb, Cs): Anion Reorientations and Na{+} Mobility

Published

Author(s)

Alexander V. Skripov, Olga A. Babanova, Alexei V. Soloninin, Vitalie Stavila, Kristina Verdal, Terrence J. Udovic, John J. Rush

Abstract

To study the reorientational motion of icosahdral [B12H12]2- anions and the translational diffusion of alkali-metal (A+) cations in A2^B12H12 (A = Na, K, Rb, Cs), we have measured the 1H, 11B, and 23Na NMR spectra and spin-lattice relaxation rates in these compounds over the temperature range of 170 K to 580 K. For cubic compounds K2B12H12, Rb2B12H12, and Cs2B12H12, the measured 1H and 11B 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 two-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 23Na 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 23Na spin-lattice relaxation data is 2x108 s-1, and the corresponding activation energy for Na+ diffusion is about 410 meV.
Citation
Journal of Physical Chemistry C
Volume
117

Keywords

dodecahydro-closo-dodecaborate, ionic compounds, ion mobility, Na ion diffusion, nuclear magnetic resonance, nuclear spin relaxation, reorientation

Citation

, A. , , O. , , A. , Stavila, V. , , K. , Udovic, T. and , J. (2013), Nuclear Magnetic Resonance Study of Atomic Motions in A<sub>2</sub>B<sub>12</sub>H<sub>12</sub> (A = Na, K, Rb, Cs): Anion Reorientations and Na<sup>{+}</sup> Mobility, Journal of Physical Chemistry C, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=914975 (Accessed May 7, 2021)
Created November 22, 2013, Updated February 19, 2017