The partial molar volume and thermal expansivity of TiO2, in alkali silicate melts:: Systematic variation with Ti coordination

The densities of seven K2O-TiO2-SiO2 (KTS) liquids and seven Na2O-TiO2-SiO2 (NTS) liquids have been measured between 1236 and 1771 K using the double-bob Archimedean method. In addition, the low-temperature density of these liquids at their limiting fictive temperature (T-f'; near the glass transition) were also measured. Compositions range from 15 to 39 mol % K2O and Na2O, 10 to 35 mol % TiO2, and 33 to 61 mol % SiO2. Plots of molar volume vs. temperature are linear for all samples in the stable liquid region, but there is an increase in the slope (partial derivativeV/partial derivativeT, with decreasing temperature in the supercooled liquid region, analogous to that observed for the heat capacity of similar alkali titanosilicate liquids. Derived values of (V) over bar (TiO2), (at 1373 K) decrease systematically from 32.5 to 26.5 cm(3)/mol between 39 and 15 mol% K2O and from 29.6 to 25.7 cm(3)/mol between 39 and 25 mol% Na2O land are nearly constant at similar to 25 +/- 0.5 cm(3)/mol between 25 and 15 mol% Na2O). Values of (V) over bar (TiO2) do not appear to be a function of the TiO2 concentration, but instead depend strongly on the nature of the alkali present (larger by similar to3 cm(3)/mol in KTS vs. NTS liquids) and the degree of polymerization (increase with NBO/T). By comparing our values of (V) over bar (TiO2) at 1373 K to published Ti coordination numbers on samples of similar composition, we calculate that the average Ti coordination number varies from similar to4.0 to 5.1 in the KTS liquids and from similar to4.6 to 5.5 in the NTS liquids. The value of partial derivative(V) over bar (TiO2)/partial derivativeT in the stable liquid region correlates strongly with (V) over bar (TiO2) at 1373 K land therefore with Ti coordination). It is near zero when the average coordination is close to four, but reaches a maximum of similar to6 to 8 X 10(-3) cm(3)/mol-K when the average Ti coordination number is close to five. In all corresponding glasses, the value of partial derivative(V) over bar (TiO2)/partial derivativeT zero. Together, these features require that in the liquid is associated with the presence of Ti-[5] and involves structural changes not available to solids. Copyright (C) 2001 Elsevier Science Ltd.