Evidence of Intermediate-Range Order Heterogeneity in Calcium Aluminosilicate Glasses

Abstract

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 anorthitewollastonite-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.

Synthesis of stoichiometrically controlled reactive aluminosilicate and calcium-aluminosilicate powders

“…The spectrum of each sample can be deconvoluted into resonances at -78 ppm, -82.7 ppm, -88 ppm and -94 ppm, as well as smaller contributions to the overall spectra at -70.5 ppm, -74.5 ppm, -101.5 ppm and -110 ppm. Network modifying cations alter the chemical shifts exhibited by Q n (mAl) species, particularly for more strongly polarising cations such as calcium [58,59], which will cause additional overlapping of the individual Q n (mAl) environments. The presence of Al(V) and Al(VI) species further complicates peak assignment in the 29 Si MAS NMR spectra, as discussed above.…”

Elucidation of the Al/Si Ordering in Gehlenite Ca2Al2SiO7 by Combined 29Si and 27Al NMR Spectroscopy/Quantum Chemical Calculations

“…17 The recovered relative intensities of each Q n (mAl) species can be compared to those derived from various models for the distribution of Al and Si among the tetrahedral sites present in the compound. 17−20 Although greatly complicated by the increased 29 Si NMR line width, this approach has also been performed with aluminosilicate glasses 21,22 and gels. 23 17 O NMR also played a key role in investigating this structural aspect of aluminosilicates either in their crystalline or amorphous forms and has now well established that if the distribution of (Al,Si) cations is not fully random this ordering is incomplete.…”

High magnetic field solid‐state NMR analyses by combining MAS, MQ‐MAS, homo‐nuclear and hetero‐nuclear correlation experiments

“…These structures include the Al–O–Al bonding. Even though there is Al–O–Al site avoidance principle owing to the energetically less favor, as many resent studies have concerned, it is not probably obeyed in this modeling approach because of the disordered structures. Therefore, the potential Al–O–Al structure would become available for linking randomly in this study.…”

“…Over the past two or three decades, solid‐state NMR spectroscopy has improved and become recognized as an integral analytical approach for quantitative structural characterizations of less‐ordered substances. Solid‐state NMR provides a unique view of local T‐site environments, including the distribution of framework atoms, reactive sites, and connectivity …”