Quantitative studies on the microstructure of ternary CaO-Al₂O₃-SiO₂ glasses by Raman spectroscopy, ²⁷Al MAS NMR and quantum chemistry ab initio calculation

To investigate the structural behaviors of Al3+ in aluminosilicate systems, the microstructural characteristics of CaO-SiO2-based glassy samples with various Al2O3 contents were examined quantitatively by Raman spectroscopy and 27Al MAS NMR. A sequence of multiple model clusters of aluminosilicate systems modified with Ca2+ and Na + cations was designed. Then quantum chemistry (QC) ab initio calculations were performed for geometric optimization, and Raman spectral simulations were carried out. The functional relationship between the Raman scattering cross section (RSCS) and stress index of silicon-oxygen tetrahedron (SIT) for aluminosilicates was established, which was successfully applied to calibrate experimental Raman spectra. Some fivefold coordinated aluminum (AlV, approximately 5%) and less than 2% sixfold coordinated aluminum (AlVI) were detected by 27Al MAS NMR, while most aluminum remained in tetrahedral sites (AlIV). The ever-finer quantitative results of Raman spectroscopy and NMR showed a gradual production of AlIV with the addition of Al2O3, along with a significant adjustment in Qi species distribution. Specifically, Q1, Q2 decreased, fully polymerized Q4 increased and Q3 showed a nonmonotonic variation and obtained the maximum at Al2O3 = 12 mol%. Furthermore, the effects of aluminum on bridging oxygen bond types (T-Ob, T = Si, Al) and the degree of polymerization were discussed in detail. These structural features related to composition are essential theoretic foundations for understanding the properties of these systems.