[01] Gomaa El-Damrawi, Glass Research Group, Physics Department, Faculty of Science, Mansoura University, Mansoura, Egypt. [02] Amal Behairy, Basic Science Department, The High Institute of Engineering and Technology, New Damietta, Egypt. [03] Riham Atef, Chemical Engineering Department, The High Institute of Engineering and Technology, New Damietta, Egypt.

Glasses in the system x (Na₂O)(100-x) SiO₂, (40≤ x ≤ 65 mol%) were prepared using ordinary cooling technique. The presence in the silicate network of relatively high concentrations of Na₂O has been found to have a significant influence on its microstructure. Structural changes in the studied glass network were followed by DSC (Differential Scanning Calorimeter), XRD (X-Ray Diffraction), TEM (Transmission Electron Microscopy) and NMR (Nuclear Magnetic Resonance). Addition of up to 50 mol% Na₂O to SiO₂ results in breaking some of Si-O-Si bonds and as a direct result non-bridging oxygen atoms (NBO) are the well-formed species. But in the modifier-rich silicate glasses (Na₂O = 50 and 65 mol%), beside formation of NBO ions, a relatively high portion from Na₂O was consumed to form crystalline clusters species. In such the case, Na⁺ ions were proved to have six and seven coordination numbers with oxygen ions as a direct first neighbor. These types of coordinated sodium are evidenced (from ²³Na NMR spectroscopy) to be dominant in the modifier rich glasses. The coordination number of Na is increased and the coordination of silicon atoms is decreased with increasing Na₂O content. TEM, (EDP) and XRD spectra in correlation with NMR results have confirmed the presence of crystalline cluster species enriched with Na+ ions.

Na Coordination Sites, Clustered Species, Morphological Investigations

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