Low-Temperature 23Na MAS NMR Reveals Dynamic Effects and Compositions for the Large and Small Channels in the Zeolite-Like Ge-Framework of Na1鈥?i>xGe3+z Mat

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• 作者：Hans J. Jakobsen ; Henrik Bilds酶e ; Matt Beekman ; Stevce Stefanoski ; George S. Nolas ; Clifford R. Bowers
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：December 11, 2014
• 年：2014
• 卷：118
• 期：49
• 页码：28890-28897
• 全文大小：345K
• ISSN：1932-7455

文摘

The motional dynamics of labile Na atoms within the large channels (Na6 sites) of the zeolite-like Ge-framework in Na_1鈥?i>xGe_3+z prevent a standard relative quantification of the two different Na sites (Na6 and Na5) by solid-state NMR at room-temperature (RT). Using low-temperature (鈭?35 掳C) ²³Na solid-state MAS NMR, we demonstrate that the dynamics can be effectively frozen out, facilitating NMR characterization of the electronic environment of these sites as well as quantification of the Na6/Na5 ratio. By combining the ²³Na NMR data with the atomic ratio Na/Ge = 0.25 卤 0.02 from PXRD, we obtain a composition Na_1鈥?.28Ge_3+0.13 (or Na_0.72Ge_3.13) for the specimen studied. The results illustrate a potential pitfall of quantitative analysis based on MAS NMR of quadrupolar nuclei in solids when significant atomic motion is present. Specifically, the absence of satellite transition (ST) spinning sidebands (ssbs) in the MAS NMR spectrum does not necessarily validate the assumption that the ST intensity is fully averaged into the central transition (CT). For the case of the Na6 sites for the present Na_1鈥?i>xGe_3+z sample, reorientational motion at RT results in loss of ST ssbs; however, the motion is not sufficiently fast to fully average the ST intensity into the CT, and quantitative analysis based on such an assumption can lead to erroneous conclusions. The complete Na6 ssb manifold could be observed and the correct ratio Na6/Na5 = 1.9 卤 0.1 obtained only after reducing the sample temperature to 鈭?35 掳C.