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

Published

November 22, 2013

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

Pub Type

Journals

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Keywords

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

Citation

Skripov, A. , Babanova, O. , Soloninin, A. , Stavila, V. , Verdal, K. , Udovic, T. and Rush, J. (2013), Nuclear Magnetic Resonance Study of Atomic Motions in A₂B₁₂H₁₂ (A = Na, K, Rb, Cs): Anion Reorientations and Na^{+} Mobility, Journal of Physical Chemistry C, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=914975 (Accessed April 26, 2024)