A Ti K-edge EXAFS study of the medium range environment around Ti in oxide glasses
Résumé
The local structure around Ti in 6 glasses of ATY2 composition (A = Na, K or Rb; T = Ti and Y = Si or Ge) has been inferred from Ti K-edge measurements of extended X-ray absorption fine structure (EXAFS) spectroscopy at 4.5 K, The cation substitution method is used to help distinguish between network formers (Si vs. Ge), network modifiers (Na vs. K vs. Rb) and Ti in the next nearest neighbor environment of Ti in these glasses. In all glasses, titanium is present mainly as square pyramidal TI(IV)O-5 units. Beyond this oxygen coordination shell, a second contribution is detected near 3 Angstrom. Ab initio calculations suggest that this contribution arises mostly from single-scattering paths of the photoelectron involving ca. 2 Ti-second neighbors at 3.5 +/- 0.1 Angstrom. The double cation substitution method, applied to EXAFS spectroscopy, can efficiently detect the presence of Ti-rich domains in these glasses and estimate their size ( greater than or equal to 20 Angstrom). A double percolative structure is likely to be characteristic of these titanosilicate glasses. This particular structure may be at the origin of some specific thermodynamic properties in oxide melts containing two different types of network formers (titanosilicate, ferrisilicate, etc.). (C) 1999 Elsevier Science B.V. All rights reserved.