COORDINATION CHEMISTRY OF TI(IV) IN SILICATE-GLASSES AND MELTS .3. GLASSES AND MELTS FROM AMBIENT TO HIGH-TEMPERATURES

The local structural environment of Ti in five Na-, K-, and Ca-titanos ilicate 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 struct ure (XAFS) spectroscopy at temperatures ranging from 293-1650 K. In pa rallel, two Ti-model compounds (Ni2.6Ti0.7O4 spinel and TiO2 rutile) w ere studied under the same conditions to better understand the effects of temperature (anharmonicity) on the XAFS spectra. Temperature-induc ed anharmonicity was found to vary, largely as a function of the Ti-co ordination, and increases significantly around Ti with increasing temp erature when present as Ti-[6]. In contrast, anharmonicity appears neg ligible around Ti-[4] at temperatures below 1200 K. We predict that an harmonicity should be weak around Ti-[5] as well. No clear evidence wa s found for a significant change in the average nearest-neighbor coord ination environment of Ti in the Na- and K-titanosilicate glasses and melts that exhibit anomalous heat capacities variations just above the ir glass transition temperatures, T-g (860-930 K). The small (predicte d and measured) linear thermal expansion of the ((TiO2+)-Ti-[5])-O bon d in these systems at high temperature is expected to have an insignif icant effect on the local environment of Ti-[5] during the glass-to-su percooled liquid transition. In the most dilute Ti-glass studied (KS1; 2.7 wt% TiO2), the local environment around Ti-[4] (especially the se cond-neighbor alkalis) is relatively ordered at ambient temperature, b ut this order decreases dramatically above T-g. Lower quench rates app ear to favor Ti-[4] over Ti-[5]. The origin of the observed anomalous positive variations in heat capacities of these melts may be related t o significant changes in the medium-range environment around Ti above T-g including the disappearance of percolation domains involving inter faces between alkali-rich and network-former rich regions during struc tural relaxation at T-g; these percolation domains are related to the dual structural role of Ti in silicate glass/melts (acting simultaneou sly as network former and network modifier).