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

Abstract

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 (Ni2.6Ti0.7O4 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, Tg (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 Tg. Lower quench rates appear to favor [4]Ti over [5]Ti.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 Tg including the disappearance of percolation domains involving interfaces between alkali-rich and network-former rich regions during structural relaxation at Tg; these percolation domains are related to the dual structural role of Ti in silicate glass/melts (acting simultaneously as network former and network modifier).