The glass transition temperature and infrared absorption spectra of: (70−x)TeO₂ + 15B₂O₃ + 15P₂O₅ + xLi₂O glasses

Abstract

Glasses of the system: (70−x) TeO₂ + 15B₂O₃ + 15P₂O₅ + xLi₂O, where x = 5, 10, 15, 20, 25 and 30 mol% were prepared by melt quench technique. Dependencies of their glass transition temperatures (T_g) and infrared (IR) absorption spectra on composition were investigated. It is found that the gradual replacement of oxides, TeO₂ by Li₂O, decreases the glass transition temperature and increases the fragility of the glasses. Also, IR spectra revealed broad weak and strong absorption bands in the investigated range of wave numbers from 4000 to 400 cm⁻¹. These bands were assigned to their corresponding bond modes of vibration with relation to the glass structure.

Introduction

Infrared (IR) spectroscopy and differential thermal analysis (DTA) have been used as important tools to study the nature of glasses for the past many years. IR investigations of some phosphate glasses are reviewed [1], where it was found that the absorption frequencies of these spectra are mainly dependent on the nature of the anion rather than the positive ion. IR absorption bands which found in phosphate glasses were reported [1], [2] to occur around the following frequencies: 460, 715, 890, 1095 and 1370 cm⁻¹. These bands attributed as due to the P–O–P harmonic bending, the P–O–P anti-symmetric, the P–O–P symmetric, the ${\mathrm{PO}}_4^{3-}$ υ3-normal tetrahedral ion and PO stretching symmetric vibrational bonds, respectively. Also, IR investigations showed that the phosphate tetrahedral unit, PO₄, dominate the structure of the glasses which contain metal oxides, e.g., CdO [3], V₂O₅ [4], ZnO [5], Co₃O₄ [1], MnO₂ [2] and TeO₂–B₂O₃ [6]. Anyhow, the increasing content of such oxides in the phosphate matrix can change the positions of the obtained absorption bands of different modes of vibrations, i.e., a shift to a higher wave number, indicating an increase in the number of tetrahedral surrounded transition metal cation. It is known [1], [7] that when different cations are added to the P₂O₅ network, either the phosphorus atoms are replaced by atoms of the cation or the cation atoms enter the network interstitially. In the first case, when the cation is relatively heavy compared with oxygen the IR spectrum should remain the same as that of the phosphate, but with gradual shift in its characteristics bond group frequencies. Lighter atom, however, might have more substantial effects on group frequencies, i.e., a weakening of the PO bond groups with this bond being ruptured by network forming cations. In the second case, the cation atom enters the network as an ion, where the network is gradually broken down and new spectral bands may appear corresponding to the vibrational character of those freely charged structural units.

It was reported earlier [8], [9], [10] that pure Tellurite glasses have an absorption band at 640 cm⁻¹which attributed to vibrations of TeO₄ tetragonal pyramids. This band was found to be shifted from 640 to 660 cm⁻¹on adding variable amounts of WO₃ to pure TeO₂ from 0 to 33.8 mol%. On the other hand, this band shifted from 640 to 630 cm⁻¹ on adding ZnCl₂ from 0 to 33.8 mol%.

Pure boron oxide, B₂O₃, glass consists of both six-member boroxol rings constructed from three BO₃ units and of non-ring BO₃ units located outside boroxol rings [11]. The results of B10 of NMR studies of Bray [12], [13], [14], [15] confirmed the predictions of Krogh-Moe [16], [17] that borate glasses would contain a sizable number of at least some of the polyborate groupings which would occur in related crystalline materials. However, three absorption bands were reported [18], [19] in the regions: 1380–1355, 1000–955 and at about 700 cm⁻¹ and assigned to B–O–B stretching, BO₄ units and B–O–B bending bonds, respectively.

Recently, IR transmission spectra of pure Li₂O–MgO–B₂O₃ [20] exhibited absorption bands in the regions: (i) 1600–1200 cm⁻¹ is identified as due to the stretching relaxation of the boron–oxygen bond of the trigonal BO₃ units, (ii) 1050–900 cm^−1, which attributed to the vibration of the BO₄ units and (iii) a band at 715 cm⁻¹ is due to the bending vibrations of the B–O linkages in the borate network [21], [22], [23], [24]. Also, IR spectra of TeO₂–Nb₂O₅–ZnO glasses [25] showed absorption bands in the range, 662–651 cm⁻¹, attributed to TeO₄ trigonal bipyramids (tbp) which moved to a higher frequency with the increase in ZnO content in the glass and another small one found at 754 cm⁻¹ as due to TeO₃ trigonal pyramids [tp] which strengthened upon the addition of ZnO to the glass content in agreement with those observations reported previously [26], [27], [28]. Actually, the TeO₂ chemical belongs to an intermediate class of glass forming oxides. It does not readily form a glass, but it does so, when it is mixed with certain other oxides, such as B₂O₃, P₂O₅, SiO₂, adding a little quantity of alkali oxides as the network modifier in order to obtain a good quality optical glass system [29], [30], [31]. Since the glass transition reflects a change in the coordination number of the network forming atoms and destruction of the network structure brought about by the formation of some non-bridging atoms. Therefore, the present work is aimed to apply infrared spectroscopy and DTA techniques in order to study the influence of the composition of the glass system: (70−x) TeO₂ + 15B₂O₃ + 15P₂O₅ + xLi₂O, where x = 5, 10, 15, 20, 25 and 30 mol% on its absorption bands and glass transition.