Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 268

The ionic radius of Ti⁴⁺ is nearly twice that of Si⁴⁺. It is not surprising, therefore, that Ti⁴⁺ in crystalline materials commonly occupies sixfold coordination, whereas Si⁴⁺ is in tetrahedral coordination. In glasses and melts, on the other hand, the structural behavior of Ti⁴⁺ is more complex. From partial molar volume of TiO₂, ‐values near 30 cm³/mol in alkali silicate and to be ≤25 cm³/mol in alkaline earth silicate point to different structural behavior governed by the nature of the metal cations [8].

Raman and XANES spectroscopic data of SiO₂─TiO₂ glasses suggest Ti⁴⁺ in five‐ and sixfold coordination with oxygen at low concentrations (<3 mol % TiO₂), whereas in more concentrated solution, Ti⁴⁺ is surrounded by four oxygens. However, it also has been suggested from X‐ray and neutron diffraction data that fourfold coordination dominates at low TiO₂ concentrations in alkali silicate glasses, whereas fivefold coordination is more important at higher concentrations. The latter conclusions are in qualitative agreement with inferences drawn from Raman spectra of alkali aluminosilicate glasses. In aluminosilicate glasses, the Al/(Al + Si) ratio also affects the oxygen coordination number around Ti⁴⁺.