Al and Sr environment in tectosilicate glasses and melts: Viscosity, Raman and NMR investigation

Structure and properties of SrO-Al2O3-SiO2 glasses and melts were investigated along the tectosilicate join (SrO/Al2O3 = 1), varying the amount of silica. The structure of the glasses was studied by means of various spectroscopic techniques: Raman, 27Al NMR and XAS at the Sr K-edge. Raman spectroscopy revealed that the fraction of high-membered tetrahedral rings diminishes upon substitution of SiO2 by SrAl2O4 favouring the formation of low-membered rings. Sr K-edge XANES shows that the strontium coordination number is around 9, based on the spectrum similarity with the one obtained for crystalline strontianite. 27Al NMR spectroscopy indicates the presence of four- and five-coordinated aluminium the latter being found in small quantities (< 5%), i.e. smaller than for analogous Mg- and Ca-based aluminosilicate glasses. A minimum in Tg is found when the AlO5 content is maximum, both in Sr and Ca aluminosilicates. This fact indicates the importance of minor species such as five-fold aluminium in activating viscous flow, similarly to what has been proposed for five-fold silicon by Farnan and Stebbins (1994). Increase of Tg at low silica content was correlated to a decrease in AlO5 content as well as to a decrease of a number of different structural units and, as a consequence, an ordering of the system.