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

The interaction of Ti4+ with the anionic structure of glasses and melt s along the join Na2Si2O5-Na2Ti2O5 has been examined in situ at temper atures between 25 and 1316-degrees-C with microRaman spectroscopy. In the Ti-free endmember system the expression, (1) 2Q3 double-line arrow pointing left and right Q2 + Q4 describes the anionic equilibria adeq uately. Solution of Ti4+ stabilizes an additional unit, Q1. This struc tural change requires an additional expression, (2) 2Q2 double-line ar row pointing left and right Q3 + Q1, to represent the equilibira. The DELTAH(x)1 for reaction 1 is approximately 24 kJ/mol, whereas that for reaction 2 is approximately -40 kJ/mol. These values differ from thos e in the analogous Na2Si2O5-Na2(NaAl)2O5 system (Mysen and Frantz, 199 4a), where DELTAH(x)1 for Al-bearing melts is -11- -14 kJ/mol, and tha t of reaction 2 is 17-33 kJ/mol, depending on the Al/(Al + Si). Increa sing 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), th e Q2, Q1, and Q4 are positively correlated and the Q3 abundance is neg atively correlated with increasing Al2O3. The Raman spectra of Ti-bear ing glasses and melts are consistent with Ti4+ in at least three diffe rent structural positions (Si4+ double-line arrow pointing left and ri ght Ti4+ substitution, clusters perhaps of TiO2 type, and Ti4+ as a ne twork-modifying cation). This behavior depends on both temperature and TiO2 concentration. The fraction of tetrahedrally coordinated Ti4+ in creases from near 0% for less-than-or-equal-to 3.5 mol% TiO2 to nearly 80% in glasses and melts with 20 mol% TiO2. Increasing temperature ab ove that of the glass transformation interval (400-500-degrees-C) resu lts in a 10-30% decrease in the fraction Ti4+ in tetrahedral coordinat ion. As the temperature is raised beyond 800-900-degrees-C, this trend reverses. It is suggested that the temperature- and composition-depen dent structural behavior of Ti4+ in the melts might be reflected in un usual behavior of melt properties such as viscosity, thermal expansion , and thermodynamic properties.