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The local structure of glasses in the system Na2O−B2O3−Al2O3 is studied by multinuclear magic-angle spinning (MAS) solid-state NMR spectroscopy at 7.0 and 11.7 T. The ²³Na isotropic chemical shifts vary linearly with Na2O concentration, indicating that the sodium ions are homogeneously distributed over the network and not clustered. The ¹¹B MAS NMR spectra reveal the presence of trigonal BO3/2 units, tetrahedral BO4/2^- sites, and three-coordinate BO2/2O^- species containing nonbridging oxygen. The quantitative contributions of these three types of sites are obtained by detailed computer simulations of these spectra. High-resolution ²⁷Al satellite transition spectroscopy indicates that aluminum is predominantly present as AlO4/2^- sites. The structure of these glasses is discussed in terms of various melt reaction schemes, in which the network former oxides B2O3 and Al2O3 react with O^2- introduced by the network modifier Na2O. While the data suggest that the formation of AlO4/2^- units takes priority over boron conversion, a detailed analysis of the oxide balance arising from these reactions indicates that small amounts of five- and six-coordinate aluminum must be present in nearly all of the samples; this conclusion is further confirmed by ²⁷Al 2-D triple-quantum NMR experiments. Although these high-resolution solid-state NMR spectra primarily inform about nearest-neighbor environments, they also allow inferences about the connectivities of these sites. Thus, a detailed inspection of compositional isotropic chemical shift trends suggests that the framework sites present in these glasses are not interlinked statistically, but rather that the tetrahedral BO4/2^- and AlO4/2^- sites are preferentially surrounded by three-coordinate boron. On the basis of this concept, it is also possible to explain the compositional dependence of the glass transition temperature on a structural basis in terms of an average framework site connectivity.

The structures of the ternary Chevrel phases CdMo6Se8 and Cd2Mo6Se8 have been studied by several complementary ¹¹¹Cd NMR spectroscopic techniques. Specifically, cadmium mobility and bonding properties are probed by temperature and frequency dependent measurements of static line shapes, magic angle spinning (MAS) NMR spectra, and spin−lattice relaxation rates. Furthermore, models for the spatial cadmium distribution are tested on the basis of ¹¹¹Cd−¹¹¹Cd dipole−dipole interactions, measured by spin−echo decay spectroscopy on isotopically labeled materials (97% ¹¹¹Cd). CdMo6Se8 undergoes a phase transition near 130 K; the cadmium ions are static on the NMR time scale over the whole temperature range in both phases. The spatial cation distribution is close to homogeneous, and it specifically excludes the presence of Cd−Cd dimers. For the high-temperature phase, the ¹¹¹Cd spectra indicate a large degree of static disorder. In addition, the large ¹¹¹Cd chemical shift temperature coefficient and fast spin−lattice relaxation reveal strong interactions between the intercalated cadmium atoms and the conduction band wave functions of the Mo6Se8 matrix. This behavior is typical for charge-transfer intercalation compounds in which the conduction band is only partially filled. Cd2Mo6Se8, which crystallizes in the rhombohedral R&thremacr; structure, shows the typical NMR signature of a rigid, semiconducting compound. The intercalated metal species show no apparent interaction with conduction electron wave functions. ¹¹¹Cd MAS and spin−echo decay data suggest a disordered atomic distribution of Cd²⁺ ions with a minimum Cd−Cd internuclear distance of 258 pm.