EFFECT OF TEMPERATURE AND TIO2 CONTENT ON THE STRUCTURE OF NA2SI2O5-NA2TI2O5 MELTS AND GLASSES

Abstract

The interaction of Ti4+ with the anionic structure of glasses and melts along the join Na2Si2O5-Na2Ti2O5 has been examined in situ at temperatures between 25 and 1316°C with microRaman spectroscopy. In the Ti-free endmember system the expression, (1) 2Q3 # Q2 + Q4 describes the anionic equilibria adequately. Solution of Ti4+ stabilizes an additional unit, Q1. This structural change requires an additional expression, (2) 2Q2 # Q3 + Q1, to represent the equilibria. The ΔH1x for reaction 1 is ~24 kJ/mol, whereas that for reaction 2 is ~ -40 kJ/mol. These values differ from those in the analogous Na2Si2O5-Na2(NaAl)2O5 system (Mysen and Frantz, 1994a), where ΔH1x for Al-bearing melts in -11--14 kJ/mol, and that of reaction 2 is 17-33 kJ/mol, depending on the Al(Al + Si).Increasing TiO2 concentration is positively correlated with the abundance of Q2 and Q1 structural units, whereas those of Q3 and Q4 are negatively correlated. In the equivalent Al-system (Mysen and Frantz, 1994a), the Q2, Q1, and Q4 are positively correlated and the Q3 abundance is negatively correlated with increasing Al2O3.The Raman spectra of Ti-bearing glasses and melts are consistent with Ti4+ in at least three different structural positions (Si4+ # Ti4+ substitution, clusters perhaps of TiO2 type, and Ti4+ as a network-modifying cation). This behavior depends on both temperature and TiO2 concentration. The fraction of tetrahedrally coordinated Ti4+ increases from near 0% for =< 3.5 mol% TiO2 to nearly 80% in glasses and melts with 20 mol% TiO2. Increasing temperature above that of the glass transformation interval (400-500°C) results in a 10-30% decrease in the fraction Ti4+ in tetrahedral coordination. As the temperature is raised beyond 800-900°C, this trend reverses. It is suggested that the temperature- and composition-dependent structural behavior of Ti4+ in the melts might be reflected in unusual behavior of melt properties such as viscosity, thermal expansion, and thermodynamic properties.