Reorientational Dynamics of NaBH4 and KBH4

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• 作者：Nina Verdal ; Michael R. Hartman ; Timothy Jenkins ; Daniel J. DeVries ; John J. Rush ; Terrence J. Udovic
• 刊名：Journal of Physical Chemistry C
• 出版年：2010
• 出版时间：June 3, 2010
• 年：2010
• 卷：114
• 期：21
• 页码：10027-10033
• 全文大小：266K
• 年卷期：v.114,no.21(June 3, 2010)
• ISSN：1932-7455

文摘

The details of the rotational dynamics of borohydride ions in both the ordered and disordered crystal phases of NaBH₄ and in the disordered crystal phase of KBH₄ were determined by quasielastic neutron scattering (QNS). Model fits of the QNS data indicate that the BH₄⁻ tetrahedron in the ordered phase of NaBH₄ rotates with a combination of 2-site and 3-site reorientations that preserve its crystallographic orientation. The QNS results for the disordered phases are well described by a model assuming nearest-neighbor BH₄⁻ jumps from one corner to another of a cube formed by eight hydrogen positions of half occupancy. Distinguishing between likely mechanisms for reorientation was made possible by collecting data at sufficiently high momentum transfer. The activation energies derived for the low-temperature and high-temperature phases of NaBH₄ are 13.4 ± 0.8 and 11.9 ± 0.5 kJ/mol, respectively. We find an activation energy for KBH₄ of 14.6 ± 0.5 kJ/mol in the high-temperature phase. The torsional vibration bands of both borohydrides were also measured by inelastic neutron scattering.