Effects of NaOH in Solid NaH: Solution/Segregation Phase Transition and Diffusion Acceleration

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• 作者：Eric G. Sorte ; E. H. Majzoub ; Tim Ellis-Caleo ; Blake A. Hammann ; Gang Wang ; Dongxue Zhao ; Robert C. Bowman ; Jr. ; Mark S. Conradi
• 刊名：Journal of Physical Chemistry C
• 出版年：2013
• 出版时间：November 14, 2013
• 年：2013
• 卷：117
• 期：45
• 页码：23575-23581
• 全文大小：371K
• 年卷期：v.117,no.45(November 14, 2013)
• ISSN：1932-7455

文摘

The presence of approximately 10 mol % NaOH mixed with NaH powder is shown to result in much more rapid hydrogen motion in the NaH above 150 掳C, as indicated by the onset of hydrogen NMR line narrowing at this temperature. A similar result appears for air-exposed NaH due to the formation of hydroxide from atmospheric H₂O. The NMR line narrowing is too rapid, as a function of temperature, to be described by thermal activation; rather, it is suggestive of a phase transition. Indeed, differential scanning calorimetry finds, after an initial thermal cycle, a reversible thermal anomaly indicating a phase transition near 150 掳C. Powder X-ray diffraction with an excess of NaOH displays a remarkable lattice expansion of the NaH in the temperature range of 100鈥?40 掳C where the (200), (220), and (311) reflections indicate a volume lattice expansion of up to 11%; the expansion is reversible upon cycling. The data thus point to a reversible phase transition in which NaOH enters the NaH structure above 150 掳C and exits the NaH below that temperature. First-principles calculations using OH-substituted NaH supercells indicate significant solubility in the NaH lattice and find a transition temperature and enthalpy change that are in approximate agreement with differential scanning calorimetry (DSC) measurements, confirming the presence of a phase transition.