Heat capacities of TiO₂-bearing silicate liquids: Evidence for anomalous changes in configurational entropy with temperature

The heat capacities of several TiO₂-bearing silicate glasses and liquids containing Cs₂O, Rb₂O, Na₂O, K₂O, CaO, MgO, or BaO have been measured to 1100 K using a differential scanning calorimeter and to 1800 K using a Setaram HT-1500 calorimeter in step-scanning mode. The results for liquids of M₂O-TiO₂-2SiO₂ composition (M - Na, K, Cs) are compared to those for liquids of M₂O-3SiO₂ composition. The presence of TiO₂ has a profound influence on the heat capacity of simple three-component silicate liquids over the temperature range 900-1300 K. Specifically, replacement of Si⁴⁺ by Ti⁴⁺ leads to doubling of the magnitude of the jump in C_p at the glass transion (T_g); this is followed by a progressive decrease in liquid C_p for over 400 K, until C_p eventually becomes constant and similar to that in Ti-free systems. The large heat capacity step at T_g in the TiO₂-bearing melts suggests significant configurational rearrangements in the liquid that are not available to TiO₂-free silicates. In addition, these "extra" configurational changes apparently saturate as temperature increases, implying the completion of whatever process is responsible for them, or the attainment of a random distribution of structural states. Above 1400 K, however, where the heat capacities of TiO₂-bearing and TiO₂-free alkali silicate liquids are similar, their configurational entropies differ by ~3.5 J/g.f.w.-K. The larger configurational entropy of the TiO₂-bearing alkali silicate liquids relative to the TiO₂-free liquids is energetically equivalent to raising the liquid temperature by more than 300 degrees. This result clearly demonstrates the energetic magnitude of the configurational changes apparent in the supercooled liquid region and their impact on the thermodynamic properties of the stable liquid. Consideration of both density measurements on liquids and spectroscopic data on quenched glasses (from the literature) suggests that the anomalous configurational rearrangements may involve the breakdown of alkali and alkaline earth titanate complexes and changes in Ti⁴⁺ coordination.