Structural investigations of TiO 2SiO 2 glassy and glass-ceramic materials prepared by the sol-gel method

Abstract

In the present paper we will show by both extended X-ray absorption fine structure (EXAFS) and near-edge (XANES) spectroscopies that the coordination of Ti 4+ ions is only tetrahedral in TiO 2SiO 2 very homogeneous glasses obtained by the sol-gel method. The coordination of Ti 4+ is also tetrahedral in the essicated gels. An “intermediate” ordered phase, not yet well defined, has been observed at the beginning of the crystallization process. Moreover, the thermal treatment causes a progressive ordering of the T d tetrahedral units going from the essicated gels to the more and more dense glasses. The presence of six-fold Ti-sites has been clearly observed by both EXAFS and near-edge spectroscopies only in a partially crystallized glass containing the highest TiO 2 content (19 wt%), at 1200°C. On the contrary, in all the glasses having the lowest TiO 2 content (4.5 wt%) a four-fold Ti coordination has been observed until 1200°C. The crystalline phase, which appears at high temperatures, is anatase having a crystalline size in the range 50–100 Å for thermal treatments in the range 1000–1200°C for the sample richer in TiO 2. An appearance of crystallinity at higher temperatures for lower TiO 2 concentrations has been observed by X-ray diffraction (XRD) analysis, in agreement with other authors.