Low-Temperature 23Na MAS NMR Reveals Dynamic Effects and Compositions for the Large and Small Channels in the Zeolite-Like Ge-Framework of Na1–xGe3+z Materials

Abstract

The motional dynamics of labile Na atoms within the large channels (Na6 sites) of the zeolite-like Ge-framework in Na1–xGe3+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 (−135 °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 ± 0.02 from PXRD, we obtain a composition Na1–0.28Ge3+0.13 (or Na0.72Ge3.13) 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 Na1–xGe3+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 ± 0.1 obtained only after reducing the sample temperature to −135 °C.