Structure and Dynamics of High-Temperature Strontium Aluminosilicate Melts

We report the study of high-temperature melts (1600–2300 °C) and related glasses in the SrO–Al₂O₃–SiO₂ phase diagram considering three series: (i) depolymerized ([SrO]/[Al₂O₃] = 3); (ii) fully polymerized ([SrO]/[Al₂O₃] = 1); and (iii) per-aluminous ([SrO]/[Al₂O₃] < 1). By considering the results from high-temperature ²⁷Al NMR and high-temperature neutron diffraction, we demonstrate that the structure of the polymerized melts is controlled by a close-to-random distribution of Al and Si in the tetrahedral sites, while the depolymerized melts show smaller rings with a possible loss of non-bridging oxygens on AlO₄ units during cooling for high-silica compositions. A few five-fold coordinated ^VAl sites are present in all compositions, except per-aluminous ones where high amounts of high-coordinated aluminium are found in glasses and melts with complex temperature dependence. In high-temperature melts, strontium has a coordination number of 8 or less, i.e. less than in the corresponding glasses. The dynamics of high-temperature melts were studied from ²⁷Al NMR relaxation and compared to macroscopic shear viscosity data. These methods provide correlation times in close agreement. At very high temperatures, the NMR correlation times can be related to the oxygen self-diffusion coefficient, and we show a decrease of the latter with increasing Si/(Al + Si) ratios for polymerized melts with no compositional dependence for depolymerized ones. The dominant parameter controlling the temperature dependence of the aluminum environment of all melts is the distribution of Al–(OSi)_p(OAl)_(4-p) units.