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 Ti. In contrast, anharmonicity appears negligible around Ti at temperatures below 1200 K. We predict that anharmonicity should be weak around 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 (TiO2+) - O bond in these systems at high temperature is expected to have an insignificant effect on the local environment of Ti during the glass-to-supercooled liquid transition. In the most dilute Ti-glass studied (KS1; 2.7 wt% TiO2), the local environment around 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 Ti over 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 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).
ASJC Scopus subject areas
- Geochemistry and Petrology