PHOSPHORUS SPECIATION CHANGES ACROSS THE GLASS-TRANSITION IN HIGHLY POLYMERIZED ALKALI SILICATE-GLASSES AND MELTS

In situ, high-temperature Raman spectroscopy of Na2O . 9SiO(2) glasses and melts with and without 2 mol% P2O5 has been conducted to 1491 K ( near the liquidus temperature). In the temperature range from 298 to s imilar to 1000 K (below the glass-transition temperature range), ortho phosphate (PO43-) complexes appear to be the dominant phosphate specie s present. As temperature increases above similar to 1000 K, the ortho phosphate complex is partially replaced by pyrophosphate complexes (P2 O74-) so that at similar to 1200 K >70% of the P exists in the latter form. The proportion of pyrophosphate species does not change signific antly as the temperature further increases. This change in phosphate s peciation is accompanied by an increase in the Q(3)/Q(4) silicate spec ies abundance ratio. At the highest temperatures there is also evidenc e for Q(2) species in the melts. The transformation from orthophosphat e to pyrophosphate implies depolymerization of the silicate network be cause the P/O ratio in the P2O74- complex is smaller than in PO44-. Th is depolymerization is consistent with the change in Q(3)/Q(4) ratio i n the silicate. Therefore, use of P speciation data from glasses would lead to overestimation of the degree of polymerization of P-bearing s ilicate melts.