Structural characterization of glass from the inversion of 23Na and 27Al 3Q-MAS NMR spectra

A spectrum-inversion approach to extract information from MQ-MAS NMR spectra in glasses is presented. This allows the reconstruction of the underlying two-dimensional distribution of the isotropic chemical shift correlated to the quadrupolar interaction. The dependency upon the quadrupolar interaction and the RF field strength of coherence transfers involved in the MQ-MAS experiment are taken into account in the present approach. The performance of the inversion procedure is examined. Thereafter, we attempt to correlate the distributions of each interaction to structural local information. Two complex glasses (a borosilicate and a basaltic-like glass) have been studied using Al-17 and Na-23 3Q-MAS NMR. These two nuclei allowed us to investigate their local environment. The interpretation of the different site distributions is discussed in terms of topological disorder, i.e., the distribution of specific geometrical parameters such as bond distances and angles. Using the semiempirical relationships previously established with crystalline silicate compounds, the distributions of the Na-O distance and the Al-O-Si bond angle have been determined from the extracted distributions of the isotropic chemical shift. The limits of the interpretation of quadrupolar interaction distributions are also discussed.