The coordination environment of Ti(IV) in a number of Ti-silicate and Ti-aluminosilicate glasses has been determined by x-ray absorption fine structure (XAFS) spectroscopy at the Ti K-edge at ambient temperature and pressure. These glasses contain 2.7-30.5 wt% TiO2 and varying amounts of Na2O, K2O, or CaO (5.0-38.7 wt%) and Al2O3 (0-11.9 wt%), and have NBO/T ratios ranging from 0.07-0.81. Quantitative analysis of the Ti XANES spectra, based on ab initio multiple-scattering calculations for a variety of Ti-containing clusters, and anharmonic analysis of the normalized XAFS oscillations suggest the presence of three types of atoms around Ti: O first neighbors, (Si, Ti)-second neighbors, and alkali third neighbors. Five-coordinated Ti, Ti, is the dominant Ti species in the glasses most concentrated in Ti (>16 wt% TiO2) and is located in distorted square pyramids (TiO)O4), with one short Ti=O titanyl distance (1.67-1.70 ± 0.03 Å) and four long Ti - O distances (1.94-1.95 ± 0.02 Å). In addition, minor amounts of Ti were detected, the proportion of Ti increasing in the order: Na glasses < K glasses. Ti is the dominant Ti species in the potassic glasses with the lowest TiO2 contents (≈3-6 wt%) and highest NBO/T ratio. The relative amount of Ti increases in the order: Ca glass < K glass. Finally, Ti is a minor species (<20%) when detected in these glasses. The presence of Ti-(Si, Ti) correlations near 3.2-3.4 ± 0.1 Å, as in crystalline Na2(TiO)SiO4, is consistent with TiO5 and SiO4/TiO5 polyhedra sharing corners in these glasses, with Ti - O-(Si, Ti) angles of ≈120°-130° ± 10°. Quantitative analysis of the Ti K-edge XANES for the K-bearing glasses suggests the presence K around Ti, in good agreement with bond-valence predictions, which indicate that Ti is most likely to bond to both nonbridging oxygens (one O in short Ti=O titanyl bonds) and bridging oxygens (four O in long Ti - O bonds), thus can act as a new type of Q4 specie with one additional nonbridging oxygen. Then, we propose Ti to behave simultaneously a network former and a network modifier, with the network former role dominant. Bond valence models explain why the relative proportions of Ti and Ti change when the type of low field strength cation or the type of network-forming cation (Si vs. P) changes in oxide glasses. These models also provide a structural basis for the study of glasses and melts at higher temperatures (see Part III of this study).
ASJC Scopus subject areas
- Geochemistry and Petrology