Evidence of Intermediate-Range Order Heterogeneity in Calcium Aluminosilicate Glasses

The intermediate-range order in peralkaline calcium aluminosilicate glasses is explored by two different models using solid-state 29 Si MAS NMR spectra as the main experimental basis. The two modeling approaches describe the spatial arrangement of the tetrahedral SiO 4 and AlO 4 units, that is, the intermediate-range order (IRO), assuming either a random distribution of structural units or a hierarchy in the IRO. The hierarchy creates a quasi-heterogeneous distribution of the structural units. Two series of calcium aluminosilicate glasses (five glasses for each) are chosen for the present study, compositions of which vary along the join between the anorthite-wollastonite-gehlenite eutectic point and anorthite-wollastonite-tridymite point, and parallel to the join. The validity of the two modeling approaches is examined by simulation of the frequency and intensity distributions observed in the 29 Si MAS NMR spectra of the 10 glasses using a sum of resonances predicted for the different types of structural units. The results clearly reveal that the 29 Si MAS NMR spectra support the quasi-heterogeneous IRO for the 10 glasses, since this approach gives a satisfactory fit to the experimental spectra. In contrast, simulations based on the model of random IRO cannot reproduce the 29 Si NMR spectra in a satisfactory manner. The observed hierarchy in the IRO may also explain the compositional dependence of both viscous behavior of the melts and the stability of the 10 glasses.