Thermodynamic modeling and Raman spectroscopy study of Na₂O-TiO₂-SiO₂ glasses

Abstract

The glasses with composition xTiO₂-(100-x)[40Na₂O-60SiO₂] (0 ≤ x ≤ 30 mol%) were studied using Raman spectroscopy and thermodynamic modeling. According to the thermodynamic modeling results, the chemical structure of studied glasses is defined by seven chemical groups (TiO₂, SiO₂, Na₂O⋅2SiO₂, Na₂O⋅SiO₂, Na₂O⋅3TiO₂, Na₂O⋅TiO₂, Na₂O⋅TiO₂⋅SiO₂), whereas the short-range order (SRO) structures are TiO₅, TiO₆, Q⁴, Q³ and Q² species. Existence of all these SRO structures was confirmed by the analysis of measured Raman spectra. The equilibrium molar amounts of SRO groups calculated in the framework of the Shakhmatkin-Vedishcheva thermodynamic model and those derived from spectroscopic data exhibit a positive correlation.

Introduction

The main oxidation state of titanium in oxide glassy matrix prepared by conventional melt quenching is Ti⁴⁺, whereas the coordination number of the titanium ions can be 4, 5 or 6. When the coordination number is 4, Ti⁴⁺ ion is surrounded by oxygen tetrahedron. Five-fold coordinated Ti⁴⁺ ion occurs inside a square-based pyramid, where four oxygen atoms at an average distance of ∼1.9−2.0 Å from titanium form the base of the pyramid and fifth oxygen is at the apex of the pyramid at the shorter distance from titanium, ∼1.7 Å, (so-called titanyl bond) [1,2]. Finally, six-fold coordination of Ti⁴⁺ corresponds to an octahedral oxygen environment around titanium. Since an addition of TiO₂ to oxide glasses significantly affects the physical and chemical glass properties (density, viscosity, heat capacity, thermal expansion, etc.), much attention has been paid to the study of the coordination state of titanium as a function of glass composition and temperature in different glass-forming systems, in particular to the Na₂O-TiO₂-SiO₂ system [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. There is still no consensus, however, on the structure of Ti-containing glasses and structural role of Ti⁴⁺ ions in melts.

Raman spectroscopy is one of the powerful tools for structural characterizations of various (including non-crystalline) materials. In case of non-crystalline materials, structural assignment of Raman bands is usually based on comparison of spectra of glassy materials with those of crystalline compounds with well-known structure, which are formed in the system under consideration. In other words, the Raman spectra of crystalline materials are used as a benchmark for the identification of specific atomic groups in glassy material. This approach, however, does not necessarily yield an unambiguous result with respect to titanium coordination in silicotitanates [14]. This is due to the fact that the characteristic bands of TiO₆ groups can be observed (or not be observed at all) at different frequency ranges of Raman spectra depending on the local bonding configurations around titanium. Therefore, as it was mentioned in Ref. [14], Raman spectroscopy does not represent a “stand-alone” technique for determining structure of silicotitanates. The more detail study of the structure of Ti-containing glasses requires the application of other experimental or theoretical methods along with Raman technique.

We previously reported the results of studying of xTiO₂-(100-x)[40Na₂O-60SiO₂] (0 ≤ x ≤ 30 mol%) glasses based on qualitative analysis of the Raman spectra [13]. In this paper, more in-depth analysis of these glasses was carried out using deconvolution of the measured spectra in combination with a thermodynamic approach to modeling the structure of oxide glasses developed by Shahkmatkin and Vedishcheva [15,16].