Thermal, optical and structural properties of glasses within the TeO₂TiO₂ZnO system

Highlights

• This paper reports on original results on new tellurium oxide-based glasses which are actually very promising glasses in the field of nonlinear optics.

• We present for the first time the determination of a new glassy system and the structure of the glasses has been investigated using Raman spectroscopy which is actually the most adapted method in laboratory to study the local structure of tellurite glasses, a detail linear and non-linear optical study is also presented.

Abstract

A glass-forming domain was evidenced and studied within the TeO₂TiO₂ZnO system. Density, glass transition temperature (T_g) and onset crystallization temperature (T₀) were measured and interpreted as a function of the zinc oxide mole fraction for relevant glasses. It was concluded that the zinc oxide favors the thermal stability of glasses. On the other hand, the impact of TiO₂ addition is even more pronounced on the enhancement of the thermal stability. The optical transmission was recorded for series of glasses in the UV–Visible–NIR range. Refractive index and optical band gap were extracted from these measurements and studied as a function of the ZnO content. Linear refractive indices and optical band gap were found to decrease and increase respectively, with increasing ZnO content. The third-order non-linear susceptibility Re (χ³), measured for two series of glasses (TiO₂ content was fixed either to 5 or 10 mol%), was found to progressively decrease when the ZnO concentration increases. The impact of ZnO modifier on the glass structure was discussed based on Raman spectroscopy data. We evidenced that TiO₂ does not change drastically the glass network, whereas ZnO leads in a first step to the breaking of the TeOTe bridges, inducing network depolymerization. A further addition in ZnO leads to the formation of new TeOZn and ZnOZn linkages.

Introduction

Over the past twenty years, tellurite glasses have been of an increasing interest because of their physical and chemical properties. Such amorphous materials are good candidates for new optical devices because of their excellent optical properties, such as a high refractive index and a high dielectric constant, a wide band infrared transmittance and a large third order non-linear optical susceptibility [1], [2], [3], [4]. Pure TeO₂ cannot form a glass except under extreme conditions [5]. Thus, the incorporation of network modifiers is needed to form more easily tellurite glasses. Several TeO₂-based glass in ternary and binary systems were evidenced and studied in our laboratory for several years [6], [7], [8], [9], [10], [11], [12], [13]. Among them, TeO₂TiO₂ glassy system is of special interest, as it has been evidenced that TiO₂ inhibits structural changes of the Te polyhedra and maintains a continuous glass network thus favoring their high polarizability and hyperpolarizability. A good compromise between high non linear optical properties and ability to synthesize bulk glasses is finally obtained with those glasses [7], [15]. In parallel, it was found that the TiO₂ addition increases the thermal stability of TeO₂-based glasses by replacing TeOTe linkages by more rigid TeOTi ones [6]. Structural properties for some ternary glasses such as (100 − x − y)TeO₂xTiO₂yX (with X = Bi₂O₃, Tl₂O and Nb₂O₅) were studied and published in [6], [7], [14] respectively. However, other studies were focused on the linear and non-linear optical properties of (100 − x − y) TeO₂xTiO₂yX glasses (with X = BaO and Li₂O), results are reported in [15], [16] respectively.

A good glass forming ability was demonstrated in the binary zinc tellurite system [17] and the corresponding glasses were found to be also stable. They were reported as a basic of multicomponent optical glass synthesis as a useful medium for ultra low loss (1 dB/km) optical fibers for wavelengths in the 3.5–4 μm region [18]. Several studies also reported on the structure of the TeO₂ZnO binary glasses, based on Raman spectroscopy data [19], [20], [21], [22], [23]. Therefore, local structure of zinc atoms was performed on compositional series of binary ZnOTeO₂ glasses using neutron and X-ray diffraction the latter technique was advantageous for extracting both TeO and ZnO distances, and it was shown that TeO and ZnO distances are in the same bond length range [22]. Recently, the structure of the binary (100 − x)TeO₂xZnO glass was studied using EXAFS data [24]. The variation of the ZnO bond length and the total coordination number of zinc atoms was studied versus the ZnO content. It was found that the ZnO bond lengths remain constant, in the range 1.94–1.98 Å. However, the Zn coordination number decreases from 6.2 ± 1.6 to 3.9 ± 0.6 as the percentage of ZnO in the glass increase from 15 to 40 mol%.

Some publications reported on the evolution of the refractive index and the optical band gap for binary zinc tellurite glass system [25], [26], and for some ternary glassy systems such as TeO₂Tl₂OZnO [27], TeO₂BaOZnO [28], TeO₂Bi₂O₃ZnO[29] and TeO₂As₂O₃ZnO [18].

Taking into account these points, we turn our interest to the ternary system TeO₂TiO₂ZnO (called TT5Z and TT10Zy′ in this paper for respectively (95 − y)TeO₂5TiO₂yZnO and (90 − y′)TeO₂10TiO₂y′ZnO), to elaborate glasses combining the above mentioned properties. To the best of our knowledge, only some glass compositions in the TeO₂TiO₂ZnO system were previously studied by Kabalci et al. [30], and they focused only on the optical and crystallization aspects. In this paper, we detail glass forming domain, thermal, optical and structural properties for this ternary glass system with the attempt to establish a correlation between the glass structure and the corresponding properties.