COORDINATION OF TI4-GLASSES - A HIGH-RESOLUTION XANES SPECTROSCOPY STUDY AT THE TI K EDGE( IN SILICATE)

The coordination environment of Ti in eight Ti-bearing glasses of the Na2Si4O9-Na2Ti4O9 join (NTS) and in six ATY2 glasses (A(2)O . TiO2 . 2 YO(2), with A = Na, K, or Rb and Y = Si or Gel was determined using hi gh-resolution, X-ray absorption near-edge structure (XANES) spectrosco py at the Ti K edge in ambient conditions. Fivefold-coordinated Ti (Ti -[5]) is the dominant Ti species (greater than or equal to 50 +/- 10% of the total Ti) in all the glasses studied. Sixfold-coordinated Ti wa s detected mostly in sodic glasses (NTS, NTS2, NTG2), and it increases with TiO2 content (as high as 40 +/- 10% of the total of Ti in the mo st TiO2-rich NTS glasses) and in the order Si < Ge. Fourfold-coordinat ed Ti was detected only in nonsodic ATY2 glasses, and its content incr eases in the order Na < K < Rb and Ge < Si. Fivefold-coordinated Ti4is probably present as square pyramidal, titanyl-bearing moieties, or (Ti-[5]=O)O-4. A synthesis of Ti4+ coordination for oxide glasses deri ved using direct methods (X-ray absorption and neutron scattering) can be used, for instance, to help in the interpretation of Raman scatter ing spectra collected for Ti-bearing glasses and to estimate NBO/T rat ios better for titanosilicate glasses and melts.