Coordination chemistry of Ti(IV) in silicate glasses and melts: III. Glasses and melts from ambient to high temperatures

The local structural environment of Ti in five Na-, K-, and Ca-titanosilicate glass/melts with TiO₂ concentrations ranging from 2.7-30.5 wt% has been determined by in situ Ti K-edge x-ray absorption fine structure (XAFS) spectroscopy at temperatures ranging from 293-1650 K. In parallel, two Ti-model compounds (Ni_2.6Ti_0.7O₄ spinel and TiO₂ rutile) were studied under the same conditions to better understand the effects of temperature (anharmonicity) on the XAFS spectra. Temperature-induced anharmonicity was found to vary, largely as a function of the Ti-coordination, and increases significantly around Ti with increasing temperature when present as ^[6]Ti. In contrast, anharmonicity appears negligible around ^[4]Ti at temperatures below 1200 K. We predict that anharmonicity should be weak around ^[5]Ti as well. No clear evidence was found for a significant change in the average nearest-neighbor coordination environment of Ti in the Na-and K-titanosilicate glasses and melts that exhibit anomalous heat capacities variations just above their glass transition temperatures, T_g (860-930 K). The small (predicted and measured) linear thermal expansion of the (^[5]TiO²⁺) - O bond in these systems at high temperature is expected to have an insignificant effect on the local environment of ^[5]Ti during the glass-to-supercooled liquid transition. In the most dilute Ti-glass studied (KS1; 2.7 wt% TiO₂), the local environment around ^[4]Ti (especially the second-neighbor alkalis) is relatively ordered at ambient temperature, but this order decreases dramatically above T_g. Lower quench rates appear to favor ^[4]Ti over ^[5]1Ti. The origin of the observed anomalous positive variations in heat capacities of these melts may be related to significant changes in the medium-range environment around Ti above T_g including the disappearance of percolation domains involving interfaces between alkali-rich and network-former rich regions during structural relaxation at T_g; these percolation domains are related to the dual structural role of Ti in silicate glass/melts (acting simultaneously as network former and network modifier).