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

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• 作者：Alexander V. Skripov ; Olga A. Babanova ; Alexei V. Soloninin ; Vitalie Stavila ; Nina Verdal ; Terrence J. Udovic ; John J. Rush
• 刊名：Journal of Physical Chemistry C
• 出版年：2013
• 出版时间：December 12, 2013
• 年：2013
• 卷：117
• 期：49
• 页码：25961-25968
• 全文大小：358K
• 年卷期：v.117,no.49(December 12, 2013)
• ISSN：1932-7455

文摘

To study the reorientational motion of icosahedral [B₁₂H₁₂]^{2鈥?/sup> anions in A₂B₁₂H₁₂ (A = Na, K, Rb, Cs) and the translational diffusion of Na+ cations in Na₂B₁₂H₁₂, we have measured the 1H, 11B, and 23Na NMR spectra and spin鈥搇attice relaxation rates in these compounds over the temperature range of 170鈥?80 K. For cubic compounds K₂B₁₂H₁₂, Rb₂B₁₂H₁₂, and Cs₂B₁₂H₁₂, the measured 1H and 11B spin鈥搇attice relaxation rates are governed by thermally activated reorientations of the [B₁₂H₁₂]2鈥?/sup> 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 23Na NMR spectra and spin鈥搇attice 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鈥搇attice relaxation data is 2 脳 108 s鈥?, and the corresponding activation energy for Na+ diffusion is about 410 meV.}