Influence of Phosphate on Network Connectivity and Glass Transition in Highly Polymerized Aluminosilicate Glasses

Abstract

Melt-derived metaluminous (Al/Na = 1) aluminosilicate glasses in the system SiO 2 −Al 2 O 3 −Na 2 O−P 2 O 5 were prepared with P 2 O 5 and SiO 2 contents varying from 0 to 7.5 and 50 to 70 mol %, respectively. The glass structure was investigated by X-ray absorption near edge structure, far-and medium-infrared, and polarized Raman spectroscopic techniques. The results indicate the incorporation of phosphate into the aluminosilicate network not only as partially depolymerized groups but also as fully polymerized groups charge-balanced by aluminate units in Al−O−P bonds. A new analysis method based on polarized Raman spectra in the bending frequency range indicates a preference of phosphate to reorganize the smallest ring structures. Changes in the glass transition temperature with the increase in phosphate content were found to be consistent with the depolymerization of the network structure shown by spectroscopy. By contrast, increasing the silica content by substituting SiO 4 for AlO 4 tetrahedra, while keeping the phosphate content constant, was found to have a negligible effect on network polymerization. Still, the glass transition temperature decreased and correlated with a far-infrared sodium band shift to higher frequency. This was interpreted as local changes in bond strength caused by complex interactions between the different network formers and sodium ions.

Dispersion, ionic bonding and vibrational shifts in phospho-aluminosilicate glasses

“…17 In previous studies, we found the presence of phosphate to reduce glass transition temperature and elastic moduli, owing to decreases in polymerisation and packing density. 18,19 Additionally, our previous structural analysis by Raman and NMR spectroscopy supports the presence of Al–O–P bonds. 19,20 In the literature, the formation of these bonds has been correlated with a change in refractive index in high-silica (>77 mol% SiO 2 ) SiO 2 –Al 2 O 3 –P 2 O 5 glasses.…”

“…18,19 Additionally, our previous structural analysis by Raman and NMR spectroscopy supports the presence of Al–O–P bonds. 19,20 In the literature, the formation of these bonds has been correlated with a change in refractive index in high-silica (>77 mol% SiO 2 ) SiO 2 –Al 2 O 3 –P 2 O 5 glasses. 21 The relationship between phosphate content and refractive index in lower silica content sodium aluminosilicate glasses still requires investigation.…”

“…This is supported by cross-polarisation analysis of the Raman spectra, as all additional bands revealed by cross-polarisation also displayed shifts to higher wavenumbers with increasing P 2 O 5 and SiO 2 content. 19…”

“…Here the notation Q Si/Al 4,high and Q Si/Al 4,low indicates stretching of Q Si/Al 4 units with larger and smaller intertetrahedral angles, respectively, while T 2s corresponds to the stretching of Q 4 units where two oxygens move toward the tetrahedral centre and the other two move away from it, regardless of their intertetrahedral angle. 19 The two additional bands used for deconvolution of phosphate-containing glasses were assigned to the symmetric and asymmetric stretching of Q P 2 units, ν s (Q P 2 ) and ν as (Q P 2 ), respectively (Fig. S4, ESI†).…”

“…FIR and MIR spectra were merged and spectra of the absorption coefficient were calculated using the Kramers–Kronig transformation as detailed previously. 19…”