The effect of Al3+, Fe3+, and Ti4+ on the configurational heat capacities of sodium silicate liquids

The heat capacities of 29 glasses and supercooled liquids in the Na₂O-SiO₂, Na₂O-Al₂O₃-SiO₂, Na₂O-(FeO)-Fe₂O₃-SiO₂, and Na₂O-TiO₂-SiO₂ systems were measured in air from 328 to 998 K with a differential scanning calorimeter. The reproducibility of the data determined from multiple heat capacity runs on a single crystal MgO standard is within ± 1% of the accepted values at temperatures ≤ 800 K and within ± 1.5% between 800 and 1000 K. Within the resolution of the data, the heat capacities of sodium silicate and sodium aluminosilicate liquids are temperature independent. Heat capacity data in the supercooled liquid region for the sodium silicates and sodium aluminosilicates were combined and modelled assuming a linear compositional dependence. The derived values for the partial molar heat capacities of Na₂O, Al₂O₃, and SiO₂ are 112.35 ± 0.42, 153.16 ± 0.82, and 76.38 ± 0.20 J/gfw · K respectively. The partial molar heat capacities of Fe₂O₃ and TiO₂ could not be determined in the same manner because the heat capacities of the Fe₂O₃- and TiO₂-bearing sodium silicate melts showed varying degrees of negative temperature dependence. The negative temperature dependence to the configurational C _P may be related to the occurrence of sub-microscopic domains (relatively polymerized and depolymerized) that break down to a more homogeneous melt structure with increasing temperature. Such an interpretation is consistent with data from in situ Raman, Mössbauer, and X-ray absorption fine structure (XAFS) spectroscopic studies on similar melts.