Abstract

The solubility behavior of phosphorus in glasses and melts in the system Na2O-Al2O3-SiO2-P2O5 has been examined as a function of temperature and Al2O3 content with microRaman spectroscopy. The Al2O3 was added (2, 4, 5, 6, and 8 mol % Al2O3) to melts with 80 mol % SiO2 and ~2 mol % P2O5. The compositions range from peralkaline, via meta-aluminous to peraluminous. Raman spectra were obtained of both the phosphorus-free and phosphorus-bearing glasses and melts between 25° and 1218°C.

The Raman spectrum of Al-free, P-bearing glass exhibits a characteristic strong band near 940 cm-1 assigned to P-O stretching in orthophosphate complexes together with a weaker band near 1000 cm-1 assigned to P2O7 complexes. With increasing Al content, the proportion of P2O7 initially increases relative to PO4 and is joined by AlPO4 complexes that exhibit a characteristic P-O stretch mode slightly above 1100 cm-1. The latter complex appears to dominate in meta-aluminosilicate glass and is the only phosphate complex in peraluminous glasses.

When P-bearing peralkaline silicate and aluminosilicate glasses are transformed to supercooled melts, there is a rapid decrease in PO4/P2O7 so that in the molten state, PO4 units are barely discernible. The P2O7/AlPO4 abundance ratio in peralkaline compositions increases with increasing temperature. This decrease in PO4/P2O7 with increasing temperature results in depolymerization of the silicate melts.

Dissolved P2O5 in peraluminous glass and melts forms AlPO4 complexes only. This solution mechanism has no discernible influence on the aluminosilicate melt structure. There is no effect of temperature on this solution mechanism.