Coordination chemistry of Ti(IV) in silicate glasses and melts .2. Glasses at ambient temperature and pressure

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-[5], is the dominant Ti species in the glasses most concentrated in Ti (>16 wt% TiO2) and is located in distorted square pyramids ((TiO)-Ti-[5])O-4), with one short Ti=O titanyl distance (1.67-1.70 +/- 0.03 Angstrom) and four long Ti-O distances (1.94-1.95 +/- 0.02 Angstrom). In addition, minor amounts of Ti-[4] were detected, the proportion of Ti-[4] increasing in the order: Na glasses < K glasses. Ti-[4] is the dominant Ti species in the potassic glasses with the lowest TiO2 contents (approximate to 3-6 wt%) and highest NBO/T ratio. The relative amount of Ti-[4] increases in the order: Ca glass < K glass. Finally, Ti-[6] is a minor species (<20%) when detected in these glasses. The presence of Ti-(Si, Ti) correlations near 3.2-3.4 +/- 0.1 Angstrom, as in crystalline Na-2((TiO)-Ti-[5])SiO4, is consistent with (TiO5)-Ti-[5] and SiO4/TiO5 polyhedra sharing corners in these glasses, with Ti-O-(Si, Ti) angles of approximate to 120 degrees-130 degrees +/- 10 degrees, 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-[5] 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 Q(4) specie with one additional nonbridging oxygen. Then, we propose [5]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-[4] and Ti-[5] 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).