A Shanwan
email: anwar.shanwan@univ-orleans.fr

C Cruanes

S Allaoui
email: samir.allaoui@univ-orleans.fr

S Méo

M.-P Deffarges

F Lacroix

G Hivet

GFRP MECHANICAL BEHAVIOUR INDUCED BY A MESOSCOPIC SHAPING DEFECT

Keywords: forming process, defects, fatigue, induced behaviour

The aim of this study is to evaluate the effect of the "buckles" mesoscopic defects on the composites mechanical properties. For this purpose, an analysis was made on the kinematics involved during the appearance of these defects on complex shapes. Then, a device and a protocol were developed in order to realize calibrated specimens. "Buckles" defects have been generated on glass plane weave fabric layers taking care to reproduce the amplitudes observed on a complex preforms. Composites plates with seven stacked layers were then injected and specimens were cut in different areas before mechanically testing them under static and fatigue tensile tests. The results showed that the "buckles" defects have an effect on the s static and fatigue behavior of the specimens with defects. The measured strain using DIC method showed the presence of damage in the specimen's areas with defects, which induced a loss of rigidity and a decrease of the ultimate stress. The specimen with transverse defects showed a more significant loss of mechanical performances.

INTRODUCTION

The composites materials are widely used in the industry thanks to their very good mechanical characteristics for a lower mass than metal alloys. To manufacture these composites, especially when dealing with complex geometries, parts LCM processes are among the best candidates. However, these processes might induce defects like during textiles forming.

Those defects can be divided in two groups: macroscopic and mesoscopic. Macroscopic defects (wrinkles), appearing at the fabric scale, are the most common and studied in the literature. Wrinkles appearance highly depends on the fabric behavior and boundary conditions and can lead to bad quality of the parts coupled to a significant decrease of the composite performance [START_REF] Boisse | Simulation of wrinkling during textile composite reinforcement forming. Influence of tensile, in-plane shear and bending stiffness[END_REF][START_REF] Wilems | Forming simulation of a thermoplastic commingled woven textile on a double dome[END_REF][START_REF] Allaoui | Experimental preforming of highly double curved shapes with a case corner using an interlock reinforcement[END_REF][START_REF] Allaoui | Experimental and numerical analyses of textile reinforcement forming of a tetrahedral shape[END_REF][START_REF] Hallander | An experimental study of mechanisms behind wrinkle during forming of composite laminates[END_REF]. The second kind of defects, the mesoscopic defects, appears at the yarn level. Among those, there may be mentioned fibers and/or yarn breakage, "buckles", "weave pattern heterogeneity", "yarn waviness", etc… [START_REF] Allaoui | Experimental preforming of highly double curved shapes with a case corner using an interlock reinforcement[END_REF][START_REF] Potter | Variability, fibre waviness and misalignment in the determination of the properties of composite materials and structures[END_REF][START_REF] Lightfoot | Defects in woven preforms: formation mechanisms and the effects of laminate design and layup protocol[END_REF][START_REF] Härtel | Process parameters studies and comparison of different preform processes with NCF material[END_REF]. There are few studies treating of these mesoscopic defects and their effect the induced behavior of the composite despite the fact that they are more recurring during complex preforms shaping [START_REF] Allaoui | Experimental preforming of highly double curved shapes with a case corner using an interlock reinforcement[END_REF][START_REF] Allaoui | Experimental and numerical analyses of textile reinforcement forming of a tetrahedral shape[END_REF].

The aim of this study is to evaluate the effect of the "buckles" defects, which is an out of plane mesoscopic defect (figure 1), on the induced mechanical behavior of a GFRP composite. First, "buckles" have been generated on glass plane weave dry fabric layers using a specific bench designed and developed for this purpose. The plies with calibrated defects have been produced taking care to reproduce the amplitudes observed on a complex preform (see figure 1) used in a precedent study [START_REF] Allaoui | Experimental preforming of highly double curved shapes with a case corner using an interlock reinforcement[END_REF]. Plates, of seven stacked layers, were then performed by injecting epoxy resin using RTM process.

Plates without defects were also performed to be used as a reference. Specimens were cut in the transverse and longitudinal direction of the composite plates to perform static and fatigue tensile tests.

MATERIAL AND METHODS

Mesoscopic defects

This study deals with the "buckle" mesoscopic defect that appears at the yarn level. This defect appears when the yarns are submitted to buckling out of plane of the fabric (Figure 2). This lead locally to a significant overthickness that can achieve several times the initial thickness of the fabric. For example, for the case of only one layer of G1151 interlock, this overthickness reaches 5 mm, while the fabric thickness is only about 0.62 mm. Therefore, this defect may be a cause for disposal of composite parts after their manufacturing for geometric non-compliance. In addition, this defect may have an effect on the behavior of the obtained composite hence the need to evaluate its criticality and understand the involved mechanisms in order to develop shaping strategies for achieving healthy preforms. To achieve this purpose, this study proposes to evaluate the effect of this defect by performing mechanical tests on calibrated specimens.

Material used

The material used in this study is a glass fiber reinforced polymer. VARTM process was used to produce composite plates. The fabric used is a balanced glass plain weave produced by Chomarat Company. It is denoted G-WEAVE 600P and has the following properties: a real weight of 600 g / m2 ± 5%, thickness of 0.55 mm, warp and weft yarns count of 600 Tex. Thermosets epoxy resin LY564 ARALDITE with a suitable hardener 3487 ARADUR was used to inject staking performed according to the protocol described below.

Specimens preparation

To realize these specimens, the kinematics of the apparition of the "buckles" was identified on preforming tests done on tetrahedron punch (Figure 3). Figure 3 shows that this defect appears on a tape with a width of about 3cm located at the center of the face. This area is delimited with two zones where the shear angles are in the same order of magnitude (here 24°), but that occur in two opposite directions: clockwise and counterclockwise [START_REF] Allaoui | Experimental preforming of highly double curved shapes with a case corner using an interlock reinforcement[END_REF][START_REF] Allaoui | Experimental and numerical analyses of textile reinforcement forming of a tetrahedral shape[END_REF]. On the area with defects, one network of yarns (here in blue) are bended, while the yarns of the other network (in red) remain straight and are submitted to a large tension due to severe geometry of the top of the punch which is a triple point. We can conclude that the triple point apply a tension on the straight yarns that carry with them the transverse yarns (due to the fabric cohesion) inducing the two shear areas and the bending of the yarns in blue and consequently their out of plane buckling.

20 ±1 50°-60°2 4 ±1
24.5 ±1.5

21.5 ±0.5

Curved yarn A specific bench was designed and developed in order to study the "buckles" defects and produce calibrated specimens. The device consists of four axes that can be controlled separately in force or displacement. Preliminary work focusing especially on the boundary conditions was carried out during the commissioning of the device.

To produce specimens for the study, two kind of composites plates were manufactured: plates with defect and plates with calibrated defects. In order to produce plates without defects (healthy), the reinforcement was cut into square samples of 250×250 mm². The dimensions were selected to fit with the mold size. For plates with buckles defect, the reinforcement was cut into square samples of 500×500 mm². Then, the sample was placed on the bench and the generation of the "buckle" defects was obtained by applying the optimal kinematic and boundary conditions. After reaching the needed amplitude of defects, the sample is kept fixed and a fixing agent is sprayed on the surface of the fabric sample. When the sprayed layer becomes dry, the calibrated sample can be removed and then cut into a square shape of 250×250 mm². Several plies with calibrated defects have been produced according to this protocol.

After producing plies of fabrics (with and without defects), staking of 7 plies oriented in the same orientation (0°) was performed and placed in the VARTM mold. Then, the epoxy matrix is injected and the mold transferred into a preheated oven at 100°C. After 3 hours, the mold was removed and kept cooling down at ambient temperature for 6 hours. All precautions have been taken in order to have plates without porosities. The final thickness of the obtained composite plates is about 2.5 mm. Test specimens were subsequently cut on the injected composite plates to test theme mechanically. These specimens were divided into three batches according to their origin. The first group is that of healthy specimens cut on the reference plates. The second batch, denoted A, was obtained on the sheared regions of the plates with calibrated defects ((Figure 4). The third group, noted B, consists of specimens cut from the area with the «buckles" defects ((Figure 4). Subsequently uniaxial tensile tests with optical measurements were carried out on these samples and the results are presented in the next section.

After producing composite plates, specimens for mechanical tests were cut out using waterjet cutting.

Those samples were all designed following the ISO 527-4 1B allowing the use of lower capacity load cells and the localization of the damage in a central area of the sample, away from the clamps. Three configuration of samples were produced. The first one is specimens without defects (healthy specimens) that will be considered as reference in the study. As the fabric is balanced, only one configuration of the healthy specimen is considered without differentiating the two direction (weft and warp) of the plates. This assumption ahs been validated with DMA measurements performed on the GFRP which showed that the mechanical characteristics were similar along the warp and weft orientations. These specimens will be noted "ND" which means no defects specimens. For the specimens with defects, two configuration were considered: transverse direction (noted TD) and longitudinal direction (noted LD). Longitudinal defect samples, or « LD », cut along the area of the plates containing the calibrated buckle defects. The defect is placed on the central axis along the sample (figure 4). Transverse defect samples, or « TD », cut in the transverse direction of the plates with calibrated defects. The buckles are oriented at 90° with respect to the central axis of the sample (figure 4) and are present locally in an area on the center of the sample.

TD LD

Experimental tests

In order to investigate the effect of the buckles mesoscopic defects and its orientation on the mechanical behavior of the GFRP, static and fatigue tests were performed. They were both conducted on an Instron 8872 servo-hydraulic fatigue testing machine at room temperature. The static tests were displacement controlled at 2mm/min. The loading cells used were with a capacity of 25kN for the ND and LD samples and 5kN for the TD. The sampling used was 100Hz. The fatigue tests were carried out using the same machine and loading cells. The tests were stress controlled, uniaxial, at 5Hz and with a 0.1 load ratio (defined as the ratio between the lowest and highest stress undergone by the sample during a cycle ) to ensure that the samples would not undergo buckling.

Table1 shows the solicitations investigated for the three types of samples. The tests were carried out until failure of the sample and were chosen to cover from 50% of the ultimate stress , measured during the static test, to 25% of for the ND samples. For the two other types of samples, the tests started also at 50% of and then the maximum strain has been steadily decreased based on the fatigue life of previous solicitations. The lower solicitation for which a failure of the sample occurred was at 40% of for the TD and 25% of for the LD.

Digital Image Correlation (DIC) device was used to measure the strain field during tests. A CCD camera (2448x2048 pixels) and the ARAMIS acquisition software have been used. The samples were prepared by firstly a layer of white paint and secondly a random pattern of dots with black paint in order to get a contrasted surface in front of the cameras. Firstly, the strain was directly measured by the fatigue machine and then corrected with DIC measurements every 20 minutes (or 10 minutes for the shorter tests) to take into account the compliance of the machine. Table 1 -List of the solicitations investigated in fatigue depending of the type of defect and the fatigue life associated. If the latter is in red, it means that the test was stopped before the failure.

RESULTS AND DISCUSSION

Figure 5 illustrates an example of the stress-strain curves of the three configuration of samples (ND, LD and TD). We observe that the ND and LD samples have close evolutions, moduli and ultimate stresses. For the TD specimens, we observe a significant decrease in the mechanical behavior, elastic and failure characteristics. Consequently, the TD sample differs greatly with much lower mechanical characteristics. This is the consequence of the presence defect and the fact that there are no more yarns oriented in the loading direction as these yarns are those who are submitted to out of plane buckling and as a consequence they rotate on the areas adjacent to the defects. The oriented yarns have an effect on the rigidity and the failure character tics while the "buckles" defects contribute to the early failure of the specimens. This effect can correlated to the strain distribution measured with the DIC where we can observe a high strains, which are rather damage presence and propagation, located in the central part of the sample where the buckles are present (figure 6).

The slight decrease in the ultimate decrease of the LD specimen can be attribute to the mesoscopic defect, which cause damage initiation and promote its propagation. This can be seen in the strain distribution of the specimen (figure 6) where the local strains measured are very high in the area where there is buckles. These high strains follow the direction of the buckled yarns. The Wohler curves obtained with fatigue tests for the three configuration of samples are presented on figure 7. The ND samples showed an evolution with a limit between LCF and HCF around 29% of the ultimate stress, at a stress of 132MPa. The stiffness decreased during fatigue tests. Two phases can be identified: a slow decrease followed by a faster one until the failure of the sample. The intensity of the first phase depends on the solicitation: the stronger the maximum stress, the stiffer the slope. The reason for this first slow decrease is that the pattern followed by the yarns (taffeta) generates some local plasticization, implying that the matrix will creep. Then, at some point during the test, cracks appeared and propagated, accelerating the decrease of the stiffness [START_REF] Demers | Tension-tension axial fatigue of E-glass fiber-reinforced polymeric composites: tensile fatigue modulus[END_REF][START_REF] Montesano | Non -destructive assessment of the fatigue strength and damage progression of satin woven fiber reinforced polymer matrix composites[END_REF].

The fatigue characteristics of the LD samples are lower than ND with a LCF/HCF limit between 20% and 22% of the ultimate stress, at a stress around 78MPa. The TD samples were expected to have the lowest fatigue characteristics among the three types of sample given the static tests. The limit between low and high cycle fatigue is close to 45% of the ultimate stress, at a stress of 14MPa. The areas of maximum strain are in the neighborhood of the defect, which is the same as in static.

CONCLUSION

The aim of this work is to contribute to the understanding of the effect of "buckles" mesoscopic defects that are recurrent when dealing with preforming of complex shapes. To achieve this aim, a device was designed and built in order to produce calibrated specimens with calibrated defects. After validating the device, a protocol was established and specimens were realized with VARTM process. The results of uniaxial tensile tests, under static and fatigue loading, showed that the "buckles" defects have major effect on the GFRP mechanical behavior. The results revealed that both defect orientations (transverse and longitudinal) have a significant effect on the fatigue life and on the strain field distribution. Specimens with defects in the transverse direction are the most crippling condition but the longitudinal direction configuration was also very affected by the presence of the mesoscopic defects. Those observations lead to conclude that this mesoscopic out of plane defect has a major negative influence on the fatigue life of such a composite and must be considered when designing technical parts.

Figure 1 :

 1 Figure 1: "Buckles" mesoscopic defects observed on double curved shape with carbon fabric

Figure 2 :

 2 Figure 2: "buckles" mesoscopic defect.

Figure 3 :

 3 Figure 3: Tetrahedron preform realized with interlock dry fabric.

Figure 4 :

 4 Figure 4: Example of composite plate with "buckle" defects

Figure 5 -Figure 6 :

 56 Figure 5 -Evolution of the stress versus the strain for the ND, LD and TD samples (1 test by curve)

Figure 7 -

 7 Figure 7 -Wöhler curve for each sample in a maximum stress/fatigue life chart. The dotted lines represent the fatigue behavior during the LCF phase.

ACKNOWLEDGMENT

The authors wish to thank Centre-Val-de-Loire Region for its financial support in the project IDDEFORM. They also would thank the team from the CERMEL and more particularly Mathieu Venin for his precious help.


Mohammed Reda Zaki

David Hamani
email: david.hamani@unilim.fr

Maggy Dutreilh-Colas

Jean-René Duclère

Olivier Masson

Philippe Thomas

Keywords: Tellurite glasses, glass structure, Raman spectroscopy, linear optical properties

The glass-forming domain and physical properties, namely, the density, thermal, structural and linear optical properties of new glasses within the TeO2-TiO2-WO3 system have been investigated. By means of micro-Raman spectroscopy, the structure of these glasses was examined based on a full-scale spectral decomposition. The optical transmission and refractive index dispersion were measured by UV-Vis-NIR spectroscopy and spectroscopic ellipsometry respectively. Consistent correlations were established between the structural and linear optical properties. Only minor structural modifications of the original TeO2-rich network are taking place upon adding TiO2 or WO3, suggesting the absence of Te-O-Te bond network depolymerization. Our results also suggest that, upon increasing TiO2 content, a phase separation occurs between amorphous TeO2-rich and amorphous TiO2-rich regions. We also argue that the structural trends upon adding WO3 evolve from uniformly dispersed WO6 octahedra (at low WO3 content) to amorphous WO3-like regions (at higher WO3 content) before the crystallization of γ-WO3 at 30 mol.% of WO3. The compositional dependence of the refractive index and several optical constants (Urbach EU and dispersion Ed energies, oxide ion polarizabilities) is highlighted. These glasses collectively exhibit high refractive indices in the order of ~2.2.

Graphical abstract.

Highlights.

 New glass-forming domain within the TeO 2 -TiO 2 -WO 3 system was determined.

 Relevant correlations between the structural and optical properties are established.

 Structural effects of TiO 2 and WO 3 are revealed and discussed in detail.

 The investigated glasses exhibit high refractive indices in the order of ~2.2.

Introduction.

In the past two decades, tellurium oxide-based glasses have attracted a great deal of interest owing to their potential applications as fiber amplifiers and nonlinear optical devices [START_REF] Kim | Linear and nonlinear optical properties of TeO 2 glass[END_REF][START_REF] Dimitrov | Linear and nonlinear optical properties of simple oxides. II[END_REF][START_REF] El-Mallawany | Tellurite glasses handbook: physical properties and data[END_REF].

They are still regarded as considerably promising optical materials thanks to, inter alia, remarkable optical properties, good thermal and chemical stabilities, and low glass transition temperatures [START_REF] El-Mallawany | Tellurite glasses handbook: physical properties and data[END_REF]. The understanding of the origins behind the above set of physical properties is an ongoing subject matter in glass science. Therefore, a great deal of attention is still being devoted to establish the physical properties-structure relationships in these TeO 2 -based glasses.

Routinely, the structural description of TeO 2 -rich glasses is closely influenced by the study of the parent crystalline phases and pure TeO 2 glass. Similarities between the pure TeO 2 glass and the γ-TeO 2 crystal structures were highlighted [START_REF] Champarnaud-Mesjard | Crystal structure, Raman spectrum and lattice dynamics of a new metastable form of tellurium dioxide: γ-TeO 2[END_REF][START_REF] Noguera | Vibrational and structural properties of glass and crystalline phases of TeO 2[END_REF]. The latter is composed of (i) TeO 4 , or more precisely TeO 3+1 distorted disphenoids with one Te-O bond longer (2.197 Å) than the other three ones (1.859, 1.948 and 2.019 Å) and the presence of a lone electron pair for each Te 4+ cation constraining the short-and medium-range structure; (ii) asymmetric Te-O-Te bridges (1.859-2.197 Å) and nearly symmetric ones (1.948-2.019 Å). By comparing the γ-TeO 2 compound and pure TeO 2 glass using Raman spectroscopy (cf. Fig. 3 in [START_REF] Champarnaud-Mesjard | Crystal structure, Raman spectrum and lattice dynamics of a new metastable form of tellurium dioxide: γ-TeO 2[END_REF]), it has been argued that such nearly symmetric bridges constitute one of the main structural features of the TeO 2 glass. Representing the near-medium-range structure, these bridges are responsible for the broad Raman band in the 400-550 cm -1 region.

From the optical properties viewpoint, the high linear and nonlinear optical indices of TeO 2based glasses have been associated with the presence of the Te 4+ lone electron pair [START_REF] Fargin | Optical non-linearity in oxide glasses[END_REF][START_REF] Jeansannetas | Glass structure and optical nonlinearities in thallium(I) tellurium(IV) oxide glasses[END_REF][START_REF] Suehara | Non-linear optical properties of TeO 2 -based glasses: ab initio static finite-field and time-dependent calculations[END_REF][START_REF] Suehara | Localized hyperpolarizability approach to the origin of nonlinear optical properties in TeO 2 -based materials[END_REF][START_REF] Roginskii | Comparative Analysis of the Electronic Structure and Nonlinear Optical Susceptibility of α-TeO 2 and β-TeO 3 Crystals[END_REF] and characteristic Te-O-Te bridges [START_REF] El-Mallawany | Tellurite glasses handbook: physical properties and data[END_REF][START_REF] Mirgorodsky | Ab initio study of the nonlinear optical susceptibility of TeO 2 -based glasses[END_REF][START_REF] Soulis | Local molecular orbitals and hyper-susceptibility of TeO 2 glass[END_REF][START_REF] Smirnov | Quantum mechanical study of pre-dissociation enhancement of linear and nonlinear polarizabilities of (TeO 2 ) n oligomers as a key to understanding the remarkable dielectric properties of TeO 2 glasses[END_REF]. It has been shown that this inherently high optical response can be further improved upon adding transition metal oxides of empty d-orbital cations, such as titanium oxide TiO 2 or tungsten oxide WO 3 [START_REF] Lines | Influence of d orbitals on the nonlinear optical response of transparent transition-metal oxides[END_REF][START_REF] Kim | Nonlinear optical properties of TeO 2 -based glasses: MO x -TeO 2 (M= Sc, Ti, V, Nb, Mo, Ta, and W) binary glasses[END_REF]. Moreover, it is well established that adding modifier oxides to TeO 2 glass induces the structural transformation of TeO 4 disphenoids into TeO 3 trigonal pyramids, and therefore contributes to the short-range modification of the glass network [START_REF] El-Mallawany | Tellurite glasses handbook: physical properties and data[END_REF][START_REF] Sekiya | Raman spectra of MO 1/2 -TeO 2 (M= Li, Na, K, Rb, Cs and Tl) glasses[END_REF][START_REF] Mclaughlin | The structure of alkali tellurite glasses[END_REF]. This transformation is regarded as a "structural depolymerization" since it reduces the network crosslinking density by breaking Te-O-Te bridges. Moreover, it has been demonstrated that this transformation is accompanied with a decrease of the nonlinear optical properties [START_REF] El-Mallawany | Tellurite glasses handbook: physical properties and data[END_REF][START_REF] Berkaïne | Structure and dielectric properties of tellurium oxide-based materials[END_REF]. By means of Raman spectroscopy, some authors claimed that adding WO 3 induces this structural depolymerization in TeO 2 -based glasses [START_REF] Upender | Structure, physical and thermal properties of WO 3 -GeO 2 -TeO 2 glasses[END_REF][START_REF] Fares | Investigations of thermal, structural and optical properties of tellurite glass with WO 3 adding[END_REF], which is not the case upon adding TiO 2 [START_REF] Sabadel | Structural and nonlinear optical characterizations of tellurium oxide-based glasses: TeO 2 -BaO-TiO 2[END_REF][START_REF] Soulis | The role of modifier's cation valence in structural properties of TeO 2 -based glasses[END_REF][START_REF] Udovic | Formation domain and characterization of new glasses within the Tl 2 O-TiO 2 -TeO 2 system[END_REF].

Adding TiO 2 to TeO 2 allows to (i) enhance the glass-forming ability and obtain homogeneous glasses in the (100-x)TeO 2 -xTiO 2 binary system with a glass-forming domain up to x = 18 mol.%, and (ii) maintain the original structural organization of pure TeO 2 glass [START_REF] Soulis | The role of modifier's cation valence in structural properties of TeO 2 -based glasses[END_REF][START_REF] Udovic | Thermal characteristics, Raman spectra and structural properties of new tellurite glasses within the Bi 2 O 3 -TiO 2 -TeO 2 system[END_REF].

Hence, TiO 2 -containing TeO 2 -based glasses are of special interest thanks to the positive role of TiO 2 in conserving the glass network of pure TeO 2 which contributes to the high linear and nonlinear optical properties [START_REF] Kim | Nonlinear optical properties of TeO 2 -based glasses: MO x -TeO 2 (M= Sc, Ti, V, Nb, Mo, Ta, and W) binary glasses[END_REF][START_REF] Berkaïne | Structure and dielectric properties of tellurium oxide-based materials[END_REF][START_REF] Udovic | Formation domain and characterization of new glasses within the Tl 2 O-TiO 2 -TeO 2 system[END_REF][START_REF] Ghribi | Thermal, optical and structural properties of glasses within the TeO 2 -TiO 2 -ZnO system[END_REF]. Most of the preceding studies [START_REF] Sabadel | Structural and nonlinear optical characterizations of tellurium oxide-based glasses: TeO 2 -BaO-TiO 2[END_REF][START_REF] Udovic | Thermal characteristics, Raman spectra and structural properties of new tellurite glasses within the Bi 2 O 3 -TiO 2 -TeO 2 system[END_REF][START_REF] Ghribi | Thermal, optical and structural properties of glasses within the TeO 2 -TiO 2 -ZnO system[END_REF] dealing with the structure of TeO 2 -TiO 2 glasses supported the existence of Te-O-Ti bridges replacing the Te-O-Te ones in the glass network upon increasing TiO 2 content. This idea is based on the unique phase from the TeO 2 -TiO 2 system, TiTe 3 O 8 [START_REF] Meunier | Sur une déformation inédite du reseau de type fluorine. Structure cristalline des phases MTe 3 O 8 (M = Ti, Sn, Hf, Zr)[END_REF], which features interconnected TeO 4 and TiO 6 polyhedra via nearly symmetric Te-O-Ti bridges with the following bond lengths: 1.867 Å and 1.955 Å respectively.

With increasing WO 3 content in binary TeO 2 -WO 3 glasses, the thermal and optical properties are progressively improved [START_REF] Kim | Nonlinear optical properties of TeO 2 -based glasses: MO x -TeO 2 (M= Sc, Ti, V, Nb, Mo, Ta, and W) binary glasses[END_REF][START_REF] Blanchandin | New investigations within the TeO 2 -WO 3 system: phase equilibrium diagram and glass crystallization[END_REF]. These glasses have been extensively studied and the fact that no crystalline phase, i.e. no "parent crystalline model", has ever been identified in the binary TeO 2 -WO 3 system led to considerable efforts for the understanding of their structural features. Most of these studies focused on the tungsten local environment (to demonstrate whether WO 4 or WO 6 polyhedra exist in the glass network) using IR and/or Raman spectroscopy techniques [START_REF] Dimitrov | IR-spectral study of the effect of WO 3 on the structure of tellurite glasses[END_REF][START_REF] Sekiya | Structural study of WO 3 -TeO 2 glasses[END_REF][START_REF] Shaltout | Structural studies of tungstate-tellurite glasses by Raman spectroscopy and differential scanning calorimetry[END_REF][START_REF] Chowdari | Studies on Ag 2 O-M x O y -TeO 2 (M x O y = WO 3 , MoO 3 , P 2 O 5 and B 2 O 3 ) ionic conducting glasses[END_REF]; yet, despite many efforts, this basic information is still obscure and controversial [START_REF] Upender | Structure, physical and thermal properties of WO 3 -GeO 2 -TeO 2 glasses[END_REF][START_REF] Sokolov | Structure of WO 3 -TeO 2 glasses[END_REF][START_REF] Upender | Glass transition temperature-structural studies of tungstate tellurite glasses[END_REF][START_REF] Mirgorodsky | Structural peculiarities and Raman spectra of TeO 2 -WO 3 -based glasses: A fresh look at the problem[END_REF][START_REF] Kaur | Optical, thermal, and structural properties of Nb 2 O 5 -TeO 2 and WO 3 -TeO 2 glasses[END_REF]. In this paper, we will bring to light the local structure around W 6+ cations in WO 3 -containing TeO 2 -based glasses. It was shown from previous works using Raman spectroscopy [START_REF] Sekiya | Structural study of WO 3 -TeO 2 glasses[END_REF][START_REF] Shaltout | Structural studies of tungstate-tellurite glasses by Raman spectroscopy and differential scanning calorimetry[END_REF][START_REF] Sokolov | Structure of WO 3 -TeO 2 glasses[END_REF] that compared with the Raman spectrum of TeO 2 glass, three additional major bands (at ~360, 860 and 925 cm -1 ) appear at the Raman spectra of WO 3 -containing TeO 2 -based glasses and their intensities increase upon adding WO 3 . It is generally agreed that the high-wavenumber band at 925 cm -1 originates from the stretching mode of the shortest W-O bonds. Sekiya et al. [START_REF] Sekiya | Structural study of WO 3 -TeO 2 glasses[END_REF] suggested the growth of clusters of corner-sharing WO 6 octahedra with increasing WO 3 content in TeO 2 -WO 3 glasses. It is important to recall here the existence of corner-sharing distorted WO 6 octahedra in the γ-WO 3 compound with twelve distinct W-O bond lengths ranging from 1.714 to 2.159 Å [START_REF] Loopstra | Neutron diffraction investigation of WO 3[END_REF].

Keeping the above facts in focus, we have embarked upon studying dual-transition metal oxide-containing TeO 2 -based glasses within the (100-x-y)TeO 2 -xTiO 2 -yWO 3 ternary system (labeled TTxWy). To the best of our knowledge, only a handful studies have investigated compositions within the TeO 2 -TiO 2 -WO 3 system. First, Safonov [START_REF] Safonov | Interactions in the TeO 2 -TiO 2 -WO 3 system[END_REF] attempted to determine its equilibrium and non-equilibrium phase diagram; Muñoz-Martín et al. [START_REF] Munoz-Martín | TeO 2 -based film glasses for photonic applications: structural and optical properties[END_REF] prepared and measured the structural and optical properties of the glass composition 80TeO 2 -5TiO 2 -15WO 3 ; and finally, a similar investigation was performed by Fares et al. [START_REF] Fares | Investigations of thermal, structural and optical properties of tellurite glass with WO 3 adding[END_REF] on the following quaternary composition:

(89-x)TeO 2 -10TiO 2 -xWO 3 -1Nd 2 O 3 .
The aim of the present paper is to report the glass-forming domain within the TTxWy system, the glasses' thermal characteristics, their structural features as probed by micro-Raman spectroscopy and finally, their linear optical properties from the optical transmission and spectroscopic ellipsometry measurements. The focus is laid on the evolution of these properties as a function of TiO 2 and WO 3 contents.

Experimental details.

New glasses were prepared within the ternary TTxWy system by the conventional meltingquenching technique using α-TeO 2 (99.99%) and WO 3 (99.8%) both from Alfa Aesar, and anatase TiO 2 (99+%) from Sigma-Aldrich. In appropriate proportions, the raw dried powders have been ground for 40 minutes using agate mortar and pestle, and melted in Pt crucibles at 850 °C for 1h in air atmosphere. Under these experimental conditions, fifteen glass samples in the form of pellets were prepared from batches weighing 2 g, stirred three times during melting (once every 15 minutes, in order to increase their homogeneity) and quenched from 850 °C into a brass ring put over a preheated brass block at 150 °C. After cooling down from 150 °C, glass pellets were systematically annealed at 15 °C below the glass transition temperature (T g ) for 12 h at a rate of 2 °C/min (during both heating and cooling) in order to release the mechanical stresses resulting from fast thermal quenching.

Moreover, to properly interpret the structural properties of TTxWy glasses, we have prepared pure TeO 2 and binary (100-x)TeO 2 -xTiO 2 glasses (labeled TTx) by fast-quenching the melts from 850 °C down to approximately -11 °C (using an ice bath containing NaCl and ethanol) after 1 hour spent inside the furnace during which the melts were stirred three times.

To check the amorphous state of the samples, X-ray diffraction patterns were recorded from powder samples using Bruker D8 Advance diffractometer with Bragg-Brentano geometry (copper K α1 wavelength of 1.5406 Å, angular 2θ range of 10-70° with a step of 0.02° and 1s per step). The glass samples were analyzed by energy dispersive X-ray microanalysis (EDS) using a Quanta FEG-450 scanning electron microscope. The elemental analysis results showed that the experimental glass compositions were in good agreement with the theoretical ones. The characteristic temperatures of glasses, namely the glass transition (T g ) and onset crystallization (T O ) temperatures were measured by heat flux differential scanning calorimetry (DSC) using NETZSCH STA 449 F3 Jupiter equipment. About 25 ± 5 mg of glass fragments were put into Pt pans, and the measurements were performed under N 2 atmosphere between room temperature and 720 °C at a heating rate of 10 °C/min. Densities of glass powder samples (150 ± 20 mg) were measured by helium pycnometry using Accupyc II 1340 pycnometer from Micromeritics.

The Raman spectra of the obtained glass samples were recorded at 514 nm using T64000 Horiba Jobin-Yvon spectrophotometer operating in triple subtractive configuration (1800 gr/mm) associated to a liquid nitrogen-cooled CCD detector. The Raman spectra were recorded in the 15-1100 cm -1 wavenumber range while focusing the laser beam (exposure time of 10 seconds) at a depth of approximately 2 µm from the top surface of the glass pellets.

In each glass sample, three Raman spectra have been recorded from three different spots. The excellent agreement within each set of spectra demonstrates high degrees of topological homogeneity within the samples. As a reliable approach to study the compositional dependence of the Raman spectra, no baseline correction treatments were carried out on any of the measured Raman spectra, and the as-recorded intensities of each spectrum were divided by its total area. Using Focus 1.0 program [START_REF] Sousa Meneses | Software utility for the creation of optical function (FOCUS)[END_REF], the total-area averaged Raman spectra were decomposed in the entire wavenumber range (15-1100 cm -1 ) using a log-normal distribution for the boson peak and twelve Gaussian functions for the rest of the wavenumber range. The calculated spectra are considered to match to the experimental ones with chi-square reliability values less than 0.05.

To evaluate the optical transmission of TTxWy glasses, we measured the UV-Vis-NIR spectra from double side polished pellets of 1.44-1.47 mm in thickness over a wide wavelength range from 300 to 3300 nm using the Varian Cary 5000 spectrophotometer. Using the same samples, the refractive index dispersion was recorded by spectroscopic ellipsometry using a phase-modulated ellipsometer (Horiba Jobin-Yvon UVISEL) with a fixed 60° incidence angle.

Results and discussion.

Glass-forming domain

The amorphous state of TTxWy samples was checked by X-ray powder diffraction in order to determine the glass-forming domain shown in Fig. 1(a). It extends from x = 5 to 15 mol.% in TiO 2 and y = 5 to 25 mol.% in WO 3 . In total, fifteen melt compositions have yielded yellowish-transparent glasses that shifted to honey-brown with increasing WO 3 content and remained practically unchanged upon adding TiO 2 (cf. Fig. 1(b)).

Crystals of γ-WO 3 compound [START_REF] Loopstra | Neutron diffraction investigation of WO 3[END_REF][START_REF] Tanisaki | Crystal structure of monoclinic tungsten trioxide at room temperature[END_REF]

Density and thermal properties

The densities and thermal characteristics of TTxWy glasses are jointly listed in Table 1. Since very close trends of these properties are observed upon adding TiO 2 (or WO 3 ) at different WO 3 (or TiO 2 ) contents, we suggest to consider TTxW5 and TT5Wy sets of samples for the sake of simplicity. The density increases with addition of WO 3 by approximately 6% from TT5W5 (5.586 g.cm -3 ) to TT5W25 (5.925 g.cm -3 ). On the other hand, a density decrease by approximately 2% is recorded upon adding TiO 2 from TT5W5 (5.586 g.cm -3 ) to TT15W5 (5.454 g.cm -3 ). Therefore, the compositional dependence of the density seems to follow the additive density rule: densities of raw oxides TiO 2 , TeO 2 and WO 3 are respectively 3.90, 6.13 and 7.16 g.cm -3 . The density evolutions in TTxWy glasses are logically assigned to the higher molar mass of WO 3 (231.8 g.mol -1 ) and lower molar mass of TiO 2 (79.9 g.mol -1 ) than that of TeO 2 (159.6 g.mol -1 ).

The glass transition temperature T g in TTxWy glasses increases linearly upon adding either TiO 2 or WO 3 (T g(min) = 328 °C and T g(max) = 426 °C). It increases by ~16% from TT5W5 (328 °C) to TT15W5 (380 °C) and by ~15% from TT5W5 (328 °C) to TT5W25 (376 °C). A step increase by 10 mol.% in TiO 2 induced an overall rise of T g that is twice more consequential (increase by ~50 °C) than in the case of WO 3 (increase by ~25 °C).

It is worth pointing out that the thermal stabilities (T= T O -T g ) which reflect the ability of the glass network to resist against devitrification upon heating above T g , are fairly moderate among TTxWy glasses (ΔT min = 31 °C and ΔT max = 57 °C) (cf. Table 1 and Fig. 2). These thermal stabilities might represent a limiting factor for the use of these glasses in the optical fiber technology, for which as an "experimental standard", a minimum value of 100 °C is needed. Adding TiO 2 induces an increase of T from TT5Wy to TT10Wy glasses before a decrease in TT15Wy glass samples. It seems though that T is marginally improved with continuous increase in WO 3 content.

Structural properties

Structural effect of adding TiO 2

According to Dietzel's cationic field strength theory [START_REF] Dietzel | Die kationenfeldstärken und ihre beziehungen zu entglasungsvorgängen, zur verbindungsbildung und zu den schmelzpunkten von silikaten[END_REF][START_REF] Varshneya | Chapter 3 -Glass formation principles[END_REF], one of the classical structural theories of glass formation, the concept of field strength (FS) represents the interaction of forces between cations and anions during the melt cooling. It is expressed as follows: FS = Z c /a 2 , where Z c is the charge number of the cation, and a is the sum of ionic radii of the cation and anion. In binary oxide melts where the two constituent cations have very close FS values, a slow cooling rate (towards crystallization) is expected to lead to a solid solution type of organization in the crystal; whereas faster cooling/quenching rates (towards glass formation) would result in a phase separation between the two pure oxide phases. We calculated the FS values while considering O 2-, Te 4+ and Ti 4+ ions to be two-fold, four-fold and six-fold coordinated respectively and having the following ionic radii: 1.35 Å, 0.66 Å and 0.605 Å [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF]. It is very important to highlight that (i) the FS values for Te 4+ (0.990) and Ti 4+ (1.047) cations are very close, and that (ii) the only reported crystal compound from binary TeO 2 -TiO 2 system, namely TiTe 3 O 8 [START_REF] Meunier | Sur une déformation inédite du reseau de type fluorine. Structure cristalline des phases MTe 3 O 8 (M = Ti, Sn, Hf, Zr)[END_REF], is in fact a solid solution of TiO 2 in TeO 2 with a ratio of 1:3 respectively. Therefore, since the FS theory proves to be valid for the equilibrium TeO 2 -TiO 2 system, one would expect that it would successfully predict the structure of binary TTx glasses as well. In other words, a phase separation within the glass network could be predicted and thus resulting in TeO 2 -rich and TiO 2 -rich regions. The limited experimentally-detected glass forming ability upon adding TiO 2 (maximum of x = 15 mol.% in both TTx and TTxWy glasses) supports this predicted heterogeneous nature of their glass networks.

To elucidate the structural effect of adding TiO 2 , we first examine the TTx system before rushing into the more complicated TTxWy system given that: (i) identical glass forming ability upon adding TiO 2 (limited to 15 mol.%) is found in TTx and TTxWy systems; (ii) the close force constants of Te-O and Ti-O bonds [START_REF] Soulis | The role of modifier's cation valence in structural properties of TeO 2 -based glasses[END_REF] would likely cause strong overlapping of their corresponding bands in the Raman spectra of TiO 2 -containing TeO 2 -based glasses.

 Binary (100-x)TeO 2 -xTiO 2 glasses (TTx glasses)

The recorded Raman spectra from TTx glasses are presented in Fig. 3. They show an intensity increase of bands at ~445, ~615 and ~850 cm -1 upon adding TiO 2 from 2.5 to 15 mol.%, and an intensity decrease of bands at ~660 and ~730 cm -1 . The assignment of these Raman bands, is given in Table 2; particularly, that of Ti-O vibrations is based on the refs [START_REF] Swamy | Size-dependent modifications of the Raman spectrum of rutile TiO 2[END_REF][START_REF] Zhou | Origin of magnetic moments in defective TiO 2 single crystals[END_REF][START_REF] Lukačević | Lattice dynamics and Raman spectrum of rutile TiO 2 : The role of soft phonon modes in pressure induced phase transition[END_REF]. Since the bands at ~660 and ~730 cm -1 are associated with Te-O vibrations in TeO 4 and TeO 3 units respectively, their decreasing number with addition of TiO 2 suggests that no structural depolymerization takes place in TTx glasses (as opposed to a striking depolymerization observed in TeO 2 -Tl 2 O glassy system for example [START_REF] Noguera | Dynamics and crystal chemistry of tellurites. II. Composition-and temperature-dependence of the Raman spectra of x(Tl 2 O)+(1-x) TeO 2 glasses: evidence for a phase separation?[END_REF]). It appears that this decreasing number of TeO 4 and TeO 3 units is compensated by an increasing number of Ti-O-Ti bridges (at ~445 cm -1 and ~615 cm -1 ) and terminal Ti-O bonds (at ~850 cm -1 ). The presence of Ti-O-Ti and Ti-O linkages in the glass network is supported by the features of the Raman spectrum of nano-rutile TiO 2 (cf. inset in Fig. 3). As highlighted in the inset, the shoulder at ~850 cm -1 is only detected from the few-nanometer-sized rutile TiO 2 samples (cf. spectrum in orange) and absent in the bulk sample (spectrum in black). It is associated with the antisymmetric stretching vibrations of Ti-O bonds from distorted TiO 6-x polyhedra at the surface of TiO 2 nanoparticles [START_REF] Swamy | Size-dependent modifications of the Raman spectrum of rutile TiO 2[END_REF][START_REF] Zhou | Origin of magnetic moments in defective TiO 2 single crystals[END_REF]. Therefore, it seems that amorphous "rutile-like" TiO 2 -rich regions (of few nanometers) exist separately from the TeO 2 -rich network and their number increases with increasing TiO 2 content from 2.5 to 15 mol.% in TTx glasses before an extensive growth takes place via the crystallization of rutile TiO 2 at 20 mol.% detected by X-ray powder diffraction.

A thorough search of the relevant literature yielded only a single and unanimously accepted structural description of the TiO 2 -containing TeO 2 -based glasses [START_REF] Sabadel | Structural and nonlinear optical characterizations of tellurium oxide-based glasses: TeO 2 -BaO-TiO 2[END_REF][START_REF] Udovic | Thermal characteristics, Raman spectra and structural properties of new tellurite glasses within the Bi 2 O 3 -TiO 2 -TeO 2 system[END_REF][START_REF] Ghribi | Thermal, optical and structural properties of glasses within the TeO 2 -TiO 2 -ZnO system[END_REF][START_REF] Sukhov | Structure and properties of erbium-titanium-tellurite glass[END_REF][START_REF] Gupta | Thermal characteristics, Raman spectra, optical and structural properties of TiO 2 -Bi 2 O 3 -B 2 O 3 -TeO 2 glasses[END_REF] (and above all, the binary TeO 2 -TiO 2 glasses [START_REF] Sabadel | Structural and nonlinear optical characterizations of tellurium oxide-based glasses: TeO 2 -BaO-TiO 2[END_REF][START_REF] Soulis | The role of modifier's cation valence in structural properties of TeO 2 -based glasses[END_REF][START_REF] Udovic | Thermal characteristics, Raman spectra and structural properties of new tellurite glasses within the Bi 2 O 3 -TiO 2 -TeO 2 system[END_REF]). This description is based on the participation of symmetric Te-O-Ti bridges in the glass network by replacing the original Te-O-Te ones. It was largely supported by the fact that these two bridges are chemically indiscernible, thus explaining the insignificant changes in the Raman spectra upon adding TiO 2 . However, as expressed in the following, we provide here a new viewpoint regarding this description.

Let us consider the symmetric Te-O-Ti bridges. The latter exist in the TiTe 3 O 8 compound, and their symmetric stretching vibrational mode gives rise to the most intense band at ~477 cm -1 (cf. Fig. 3). If those symmetric Te-O-Ti bridges truly constituted the backbone of the structural network in TTx glasses, then we would have recorded a significant intensity increase at ~477 cm -1 upon adding TiO 2 , which is clearly not the case when analyzing the spectra of TTx glasses. Therefore, since the number of Te-O-Ti bridges does not increase with addition of TiO 2 and since the vibrational signature of nano-rutile TiO 2 is observed, we propose that the structure of the glass network is made of "rutile-like" TiO 2 -rich regions in a TeO 2 -rich network as initially predicted using the field strength theory.

 Ternary (100-x-y)TeO 2 -xTiO 2 -yWO 3 glasses (TTxWy glasses)

The Raman spectra of TTxWy glasses upon adding TiO 2 are given in Fig. 4. A similar spectral evolution, i.e. intensity increase at ~445, ~615 and ~850 cm -1 is observed from TTxWy glasses. Because of two overlapping bands at ~860 and ~925 cm -1 due to vibrating W-O bonds, the intensity increase at ~850 cm -1 is less noticeable than from TTx glasses. It is important to recall here that, same as in binary TTx, the glass-forming domain in TTxWy is limited to only 15 mol.% of TiO 2 . Hence, it seems that the influence of TiO 2 on a network of either pure TeO 2 (TTx system) or WO 3 -containing TeO 2 (TTxWy system) is indifferent.

From the above discussion, we argue that a phase separation between amorphous TeO 2 -rich and amorphous "rutile-like" TiO 2 -rich regions occurs in TiO 2 -containing TeO 2 -based glasses. This interpretation is shared with a previous study [START_REF] Noguera | Propriétés structurales, vibrationnelles et diélectriques de matériaux à base d'oxyde de tellure[END_REF] that predicted phase separation in the 75TeO 2 -25TiO 2 glass composition.

Structural effect of adding WO 3

The evolving features of the Raman spectra of TTxWy glasses with increasing WO 3 content (Fig. 5) are in agreement with the reported studies on WO 3 -containing TeO 2 -based glasses by Raman spectroscopy [START_REF] Upender | Structure, physical and thermal properties of WO 3 -GeO 2 -TeO 2 glasses[END_REF][START_REF] Sekiya | Structural study of WO 3 -TeO 2 glasses[END_REF][START_REF] Kaur | Optical, thermal, and structural properties of Nb 2 O 5 -TeO 2 and WO 3 -TeO 2 glasses[END_REF]. These studies have shown that three additional bands (at ~360, ~860 and ~925 cm -1 ) emerge at the Raman spectra and increase in intensity upon adding WO 3 .

Since the same structural evolutions were observed from TT5Wy, TT10Wy and TT15Wy samples, we suggest to focus on the set of TT5Wy for the sake of simplicity (Fig. 6). The assignment of the Raman bands is given in Table 2. The origin of bands E-J (425-772 cm -1 ) is supported by previous pioneer works on the vibrational and structural properties of the three TeO 2 polymorphs (α [START_REF] Mirgorodsky | Dynamics and structure of TeO 2 polymorphs: model treatment of paratellurite and tellurite; Raman scattering evidence for new γand δ-phases[END_REF], β [START_REF] Mirgorodsky | Dynamics and structure of TeO 2 polymorphs: model treatment of paratellurite and tellurite; Raman scattering evidence for new γand δ-phases[END_REF] and γ [START_REF] Champarnaud-Mesjard | Crystal structure, Raman spectrum and lattice dynamics of a new metastable form of tellurium dioxide: γ-TeO 2[END_REF]). The rest of the identified Raman bands (C-D extending from 230 to 351 cm -1 and K-M from 820 to ~934 cm -1 ) are associated with the vibrational response of distorted WO 6 octahedra. The basis of the previous assignment is a bilateral analysis of the vibrational (Raman spectra) vs. structural features that we conducted on a large number of tungsten-containing oxides (metal tungstates for the most part). A similar approach has been previously undertaken on few tungstates by Sekiya et al. [START_REF] Sekiya | Structural study of WO 3 -TeO 2 glasses[END_REF]. In the low-wavenumber region, the band B is attributed to intra-chain vibrations of Te-Te bonds (as in amorphous metallic trigonal Te [START_REF] Vasileiadis | Photo-induced oxidation and amorphization of trigonal tellurium: A means to engineer hybrid nanostructures and explore glass structure under spatial confinement[END_REF]) and the band A (boson peak) is linked to the glass network at medium-range scale [START_REF] Denisov | Boson Peak and Medium-Range Order Structure of Alkali Borate Glasses[END_REF][START_REF] Stavrou | Raman scattering boson peak and differential scanning calorimetry studies of the glass transition in tellurium-zinc oxide glasses[END_REF].

For the spectral decomposition, we used a conventional fitting technique based on the addition of bands of which the position (wavenumber), width (more precisely the full width at half maximum, FWHM) and intensity can be either assigned fixed values or free (unconstrained) in order to consistently investigate all the glass spectra. In wavenumber regions where band shift and/or broadening are observed with increasing WO 3 content (at constant TiO 2 ), we kept the introduced function's wavenumber and/or FWHM respectively parameter-free. Thus, the bands A, H and M were fully unconstrained, while bands F and L were introduced with fixed FWHM values and bands D and J with fixed wavenumbers values.

The remaining bands B, C, E, G, I and K were introduced with fixed wavenumber and FWHM values.

As can be seen in Fig. 6 content implies that less and less short W-O bonds (shorter than 1.80 Å) exist in the network. In other words, less and less uniformly dispersed WO 6 octahedra are directly connected to the glass network. In fact, the normalized intensity increase of the band K suggests a possible increase of the number of corner-sharing WO 6 octahedra as in γ-WO 3 (also supported by Sekiya et al. [START_REF] Sekiya | Structural study of WO 3 -TeO 2 glasses[END_REF] in TeO 2 -WO 3 glasses). It is important to stress that given the small size and minor contribution of the band K, the previous discussion on its evolution is more of a hypothetical nature. Therefore, it seems that the intensity increase of the large shoulder within the 700-820 cm -1 range (Fig. 5) arises from vibrating W-O-W bridges in the continuously growing amorphous WO 3 -like regions upon increasing WO 3 content. It is important to recall that within this wavenumber range, symmetric and asymmetric stretching vibrational modes of W-O-W bridges are expected in -WO 3 at ~715 and 820 cm -1 respectively (cf. Raman spectrum in Fig. 5). Moreover, the weak intensity increase in the 200-300 cm -1 range is probably due to bending vibrational modes of WO 6 octahedra as observed in -WO 3 . These findings are in agreement with XPS results [START_REF] Charton | TeO 2 -WO 3 glasses: Infrared, XPS and XANES structural characterizations[END_REF] showing a decrease of O1s binding energy in (100-x)TeO 2 -xWO 3 glasses from x = 5 to 30 mol.%, which indicates that oxide ions are more likely to participate within the glass network as bridging ones.

Concerning the evolution of the boson peak (band A), the position of its maximum intensity insignificantly red-shifts accompanied with an intensity decrease with addition of TiO 2 or WO 3 contents (cf. Figs. 4 and6). According to previous works [START_REF] Sokolov | Medium-range order in glasses: Comparison of Raman and diffraction measurements[END_REF][START_REF] Kalampounias | A temperature dependence Raman study of the 0.1 Nb 2 O 5 -0.9 TeO 2 glass-forming system[END_REF], the evolution of boson peak's position and intensity in TTxWy glasses could suggest a slight crosslinking density increase in the glass network at the medium-range scale. This behavior is in agreement with the continuous increase of the glass transition temperature upon adding TiO 2 or WO 3 .

Since the same compositional evolutions of structural features as revealed by micro-Raman spectroscopy are observed upon increasing WO 3 content at different TiO 2 concentrations with no further emerging bands (Fig. 5), it seems unlikely that Ti-O-W bridges participate in the crosslinking within the glass network.

From the above discussion, we argue that unlike the peculiar structural effect of TiO 2 promoting a phase separation within the amorphous glass network, the incorporation of WO 3 results in uniformly dispersed (throughout the glass network) WO 6 octahedra which tend to form amorphous WO 3 -like regions before ending with crystallized γ-WO 3 at 30 mol.%. This might explain why the addition of TiO 2 and WO 3 results in relatively moderate thermal stabilities of TTxWy glasses.

Linear optical properties

Refractive index, optical band gap and Urbach energies by UV-Vis-NIR optical transmission

The optical transmission spectra of 14 out of 15 TTxWy glass samples show a transparency over 75% in the 800-2600 nm range (cf. Fig. 8). Only the sample TT15W25 showed a markedly lower transparency of ~68% in this region. This can be related to its peculiar composition (richest in both TiO 2 and WO 3 oxides) at the boundary of the glass-forming domain. The large absorption band onsetting at ~2800 nm is recorded in all of the samples. It corresponds to the stretching mode of bound hydroxyl groups (R-OH), and both symmetric and asymmetric stretching modes of H 2 O molecules [START_REF] Efimov | Water-related bands in the IR absorption spectra of silicate glasses[END_REF]. Similar evolutions are generally observed upon adding TiO 2 (or WO 3 ) at constant WO 3 (or TiO 2 ) content. Therefore, in the following, we only consider TTxW5 and TT5Wy sets when discussing the effects of TiO 2 and WO 3 on the optical properties.

The UV absorption edge red-shifts with increasing WO 3 content (cf. Fig. 8 (a) for TT5Wy glasses). Likewise, a minor red-shift is observed within a narrower wavelength range (400-475 nm) upon adding TiO 2 . The color evolution of glass samples (described in section 3.1) is in agreement with the detected red-shifts of the absorption edge.

The refractive indices were extracted from the optical transmission data in the transparent region using the following equation: T = 2n/(n 2 +1). We provide in Table 1 the We have extracted the optical band gap energy E g for all TTxWy glasses from their respective UV absorption edges. Based on the wavelength-dependence of the absorption coefficient α (cf. Fig. 9 (a) for TT5Wy glasses), we plotted (αhν) ½ versus the incident photon energy hν (commonly known as Tauc plot) to estimate E g (Fig. 9 (b) and Table 1). Decreasing E g upon adding WO 3 in different TeO 2 -based systems has been previously reported [START_REF] Yousef | Optical and acoustic properties of TeO 2 -WO 3 glasses with small amount of additive ZrO 2[END_REF][START_REF] Umair | Effects of increasing tungsten on structural, elastic and optical properties of xWO 3 -(40-x)Ag 2 O-60TeO 2 Glass System[END_REF]. In agreement with literature, a slight decrease of E g is recorded upon increasing WO 3 content from 5 mol.% (2.88 eV in TT5W5) to 25 mol.% (2.74 eV in TT5W25). Upon adding TiO 2 , E g tends towards a marginal decrease in WO 3 -poor compositions (TTxW5) and seems to remain constant in WO 3 -rich ones (TTxW25).

Moreover, we have extracted the Urbach energy E U values. E U is characteristic of the width of the band-tails at the top of the valence band and the bottom of the conduction band, thus estimating the density of localized states [START_REF] Urbach | The long-wavelength edge of photographic sensitivity and of the electronic absorption of solids[END_REF]. A highly disordered glass network features a higher density of localized states in the band gap and thus having a high E U value. The E U value is obtained from the inverse of the slope of the straight line by plotting ln(α) against hν.

For TTxWy glasses, the E U values are gathered in Table 1 and lie in the range 0.096-0.108 eV. E U is found to remain globally stable with increasing WO 3 content (~0.106 eV in TT5Wy glasses). However, a slight decrease is observed upon adding TiO 2 at constant WO 3 content (for instance from 0.105 to 0.096 eV in TTxW5 glasses). Ghribi et al. [START_REF] Ghribi | Synthèse, caractérisations structurale et élastique de nouveaux matériaux tellurites pour des applications en optique non linéaire[END_REF] recently reported the same behavior in (100-x-y)TeO 2 -xTiO 2 -yZnO glasses, namely decreasing E U from 0.105 eV (at x = 5 and y = 15) to 0.097 eV (at x = 10 and y = 15). It is also worth mentioning that our estimated E U values are very close to those reported in literature for WO 3containing TeO 2 -based glasses [START_REF] Munoz-Martín | TeO 2 -based film glasses for photonic applications: structural and optical properties[END_REF]. Therefore, it can be argued that adding TiO 2 to the TeO 2rich network does not induce any significant structural modification of the glass network but instead, considering the compositional dependence of E U , its incorporation might "lessen" the average structural disorder. In this connection, it is important to recall that upon adding TiO 2 , we found the glass forming ability to be substantially reduced due to the crystallization of rutile TiO 2 in TT20Wy samples.

Refractive index dispersion by ellipsometry

From the dispersion curves of TTxWy glasses measured by spectroscopic ellipsometry (Fig. 10 for TT5Wy glasses), we derived the refractive indices n ∞ (cf. Table 1) extrapolated to infinite wavelength using the following Sellmeier's dispersion formula [START_REF] Sellmeier | Zur Erklärung der abnormen Farbenfolge im Spectrum einiger Substanzen[END_REF][START_REF] Ghosh | Sellmeier coefficients and dispersion of thermo-optic coefficients for some optical glasses[END_REF]: n = (A + B/(1-C/λ 2 ) + D/(1-E/λ 2 )) 1/2 where A-E are fitting constants specific to each sample, allowing to determine n ∞ = (A + B + D) 1/2 . An excellent fitting quality was achieved as demonstrated by the R 2 coefficients ranging between 0.9984 and 0.9999. The dependence of n ∞ on TiO 2 content suggests a steady evolution from an average value of ~2.187 in TT5Wy to 2.199 in TT15Wy glasses with the highest value (n ∞ ≈ 2.206) recorded from the TT15W15 composition. Adding WO 3 results in a steady behavior with an average value of ~2.193 in TTxW5 and TTxW25. A similar steady evolution of refractive indices is observed in binary TeO 2 -TiO 2 and TeO 2 -WO 3 glasses [START_REF] Dai | TeO 2 -based glasses containing Nb 2 O 5 , TiO 2 , and WO 3 for discrete Raman fiber amplification[END_REF] with n = 2.211-2.226 from 5 to 15 mol.% upon adding TiO 2 and n = 2.211-2.219 from 10 to 30 mol.% upon adding WO 3 respectively.

From optical transmission and ellipsometric measurements, the refractive index remains practically constant upon adding TiO 2 and WO 3 . This behavior is coherent with the results of the structural analysis (elaborated in section 3.3.) that incorporating TiO 2 or WO 3 in the TeO 2rich network does not markedly alter the inherent structural features of Te-O-Te bridges. In contrast, when the structural depolymerization takes place (e.g., in presence of MgO, ZnO, BaO [START_REF] Manning | Ternary tellurite glasses for the fabrication of nonlinear optical fibres[END_REF]), the refractive index progressively decreases upon adding the network modifiers.

Bond network nature from the dispersion energy and electronic polarizability

By applying the single-effective-oscillator model of Wemple and DiDomenico [START_REF] Wemple | Refractive-index behavior of amorphous semiconductors and glasses[END_REF] to the refractive index dispersion data, we extracted optical constants that are closely connected with the bond network nature. According to this model, the refractive index dispersion can be interpreted using the following equation:

. By plotting 1/(n 2 -1) as a function of (hν) 2 and fitting a straight line, oscillator parameters E d (dispersion energy) and E 0 (single oscillator energy) can be determined. In fact, E d was found to obey the following empirical relationship in more than 100 ionic and covalent crystals [START_REF] Wemple | Refractive-index behavior of amorphous semiconductors and glasses[END_REF]: E d = β.N c .Z a .N e , where β parameter approaches 0.37 ± 0.05 eV and 0.26 ± 0.04 eV in covalent and ionic crystals respectively, N c is the coordination number of the cation, Z a the absolute charge number of the anion and N e the total number of valence electrons per anion. Since the overall glass network structure (especially that of TeO 2 -rich network) is only slightly altered upon adding TiO 2 or WO 3 (section 3.3.), consider the short-range N c parameter in TTxWy glasses to be slightly increasing upon substituting TeO 2 with TiO 2 or WO 3 since globally, Te 4+ cations are four-fold coordinated while Ti 4+ and W 6+ cations are six-fold coordinated.

E d and E 0 values lie in the range 26.11-30.37 eV and 6.93-7.99 eV respectively (Table 1).

They are found to show an overall decrease with increasing TiO 2 and WO 3 contents. From the above equation, it is rational to assign this trend to a decreasing  parameter. Hence, we suggest that this evolution is associated with an ionic character amplification of the glass network, which can simply be supported by the expected ionicity increase upon substituting Te with Ti and W transition metals.

Moreover, we have calculated the electronic polarizability of oxide ions α O2-based on optical band gap energies. The used method is based on the Lorentz-Lorenz equation [START_REF] Lorentz | Ueber die Beziehung zwischen der Fortpflanzungsgeschwindigkeit des Lichtes und der Körperdichte[END_REF][START_REF] Lorenz | Über die Refractionsconstante[END_REF]: R M = [(n 2 -1)/(n 2 +2)] V M = (4/3) π N A α M = 2.52 α M , where R M is the molar refraction, n the refractive index, V M the molar volume, α M the molar polarizability and N A the Avogadro's number. This equation allows to calculate α O2-in oxide materials by subtracting the cation polarizability (considered as constant) from α M . Following the polarizability approach developed by Dimitrov and Komatsu [START_REF] Dimitrov | An interpretation of optical properties of oxides and oxide glasses in terms of the electronic ion polarizability and average single bond strength[END_REF], we calculated for TTxWy glasses the α O2-based on E g using the following equation: α O2-= {(V M /2.52) [1 -(E g ½ -0.98)/1.23] -∑pα i } q -1 , where p and q are the numbers of cations and oxide ions in the chemical formula of the oxide A p O q .

The free-cation polarizability α i values of Te 4+ (1.595 Å 3 ), Ti 4+ (0.184 Å 3 ) and W 6+ (0.147 Å 3 ) ions were collected from ref. [START_REF] Dimitrov | An interpretation of optical properties of oxides and oxide glasses in terms of the electronic ion polarizability and average single bond strength[END_REF]. The obtained results are listed in Table 1.

The calculated α O2-values are comprised between α O2-(min) = 1.56 Å 3 and α O2-(max) = 1.82 Å 3 . It is found that upon adding WO 3 from 5 to 25 mol.%, α O2-slightly increases from ~1.6 to ~1.8 Å 3 while remaining practically constant with addition of TiO 2 . This slight increase of α O2- values indicates that the overall glass network becomes slightly more "polarized", and thus a little more ionic in nature.

Conclusions.

A new TeO 2 -based glass-forming domain has been explored and investigated within the ternary TeO 2 -TiO 2 -WO 3 system. Density, thermal, structural and linear optical properties have been measured and analyzed as a function of TiO 2 and WO 3 contents.

The structural properties of TTxWy glasses were examined using micro-Raman spectroscopy to highlight the structural roles and effects of adding TiO 2 and WO 3 on the short-to mediumrange structure. We report the following structural trends: Fig. 3. Total-area averaged Raman spectra of TTx glasses (x = 0, 2.5, 5, 10 and 15) along with the spectrum of TiTe 3 O 8 (from [START_REF] Soulis | The role of modifier's cation valence in structural properties of TeO 2 -based glasses[END_REF]). The inset is adapted from Fig. 2 in ref. [START_REF] Swamy | Size-dependent modifications of the Raman spectrum of rutile TiO 2[END_REF] and shows spectra recorded from rutile TiO 2 samples with different average crystallite sizes ranging from 5 nm up to the bulk. (eV.cm -1 )



  are detected in TTxW30 samples. The corresponding peaks become sharper and more intense upon adding TiO 2 , suggesting an increase of the number and/or average size of WO 3 crystallites. At 20 mol.% of TiO 2 , crystals of rutile TiO 2 phase are detected in TT20Wy samples without any trace of crystallizing TiTe 3 O 8 or anatase TiO 2 phases.

  , the features shown by the set of vibrational modes of Te-O bonds in asymmetric and nearly symmetric Te-O-Te bridges (bands E-J), compared to those of pure TeO 2 glass, have only been slightly modified in terms of their bands' profiles, positions and intensity ratios upon increasing WO 3 content. The key information with respect to the crosslinking of Te-O-Te bridges within the glass network is contained within the 400-550 cm -1 range (bands E and F). In this region, the intensity barely decreases suggesting that the network of Te-O-Te bridges has been only slightly modified. Thus, it seems that adding WO 3 does not considerably alter the original structural features within the network of Te-O-Te bridges. In order to exclude the compositional effect, we have calculated the normalized intensities of the decomposed bands E-H by dividing their areas by TeO 2 molar content (attributed to vibrations of Te-O bonds) and those of the bands D and K-M by WO 3 molar content (attributed to vibrations of W-O bonds). It should be noted that this approach of normalization to the composition yields dissimilar band evolutions compared to the total-area averaged spectra (cf. bands D, L and M for example). The compositional-dependence of the normalized intensities of bands D, E-H and K-M in TT5Wy glasses is plotted in Fig. 7. Regarding the TeO 2 -rich network, the evolution of the normalized intensities of bands E-G suggests a minor structural modification of the glass network since the bands F and G seem to be unaffected upon adding WO 3 . The band E shows a moderate increase of its normalized intensity that might be due to a slightly increasing number of nearly symmetric Te-O-Te bridges. Given the relatively insignificant modification of the crosslinked network of Te-O-Te bridges, and in agreement with the forthcoming discussion on WO 3 effect, we assign the increasing intensity of bands I and J (attributed to vibrations of Te-O and W-O bonds) to an increase of W-O-W bridges as in γ-WO 3 . Since these two bands are highly interactive via overlapping, we intentionally omitted their compositional evolutions from Fig. 7. Let us know consider the Raman bands D, K, L and M associated with distorted WO 6 octahedra. The normalized intensity decrease of the bands D, L and M upon increasing WO 3

  refractive indices n 1.5 at λ = 1.5 µm (typical wavelength used for optical communication). The n 1.5 indices of TT5Wy glasses are plotted in Fig. 8 (b) against WO 3 content. In general, the results suggest a practically constant behavior of the refractive index upon adding TiO 2 or WO 3 with high values ranging between ~2.10 and ~2.15. It should be stressed that the above equation slightly underestimates the refractive index value since the multiple reflections of light are neglected and most of the reflection is considered to occur at the two air/glass interfaces (Fresnel reflection). The actual refractive indices will then be provided from the ellipsometric measurements.



  Globally, only minor structural modifications are taking place upon adding TiO 2 or WO 3 , (i) indicating that TTxWy glasses collectively share common features of the pure TeO 2 glass network and (ii) suggesting the absence of Te-O-Te bond network depolymerization (induced transformation of TeO 4 into TeO 3 units). Upon adding TiO 2 , we argue that a phase separation occurs between amorphous TeO 2network and amorphous "rutile-like" TiO 2 -rich regions (assumed to be of a nanometric size) in binary TTx and ternary TTxWy glasses. Therefore, it seems unlikely that hybrid Te-O-Ti bridges build the bond network in these glasses, thus explaining the limited glass-forming domain to only 15 mol.%. Upon adding WO 3 , we argue that (i) only a minor change affects the crosslinked network of Te-O-Te bridges; (ii) at low WO 3 contents, uniformly dispersed WO 6 octahedra exist in the glass network; and (iii) at higher WO 3 contents, amorphous WO 3 -like regions continuously grow before the crystallization of γ-WO 3 at 30 mol.% of WO 3 . Let us recall that Sekiya et al.[START_REF] Sekiya | Structural study of WO 3 -TeO 2 glasses[END_REF] have suggested the same scenario for TeO 2 -WO 3 glasses.It is important to emphasize that even though these Raman spectroscopy-based findings support the presence of distinct regions as a result of adding TiO 2 and WO 3 , they still do not provide a direct proof of their actual existence. In this regard, we aim in the future to probe the existence of such regions using transmission electron microscopy.The linear optical properties of TTxWy glasses were measured by the UV-Vis-NIR optical transmission and spectroscopic ellipsometry. In general, the obtained results suggest a steady evolution of the optical properties upon adding TiO 2 or WO 3 which is consistent with the only minor structural modifications within the TTxWy glasses. The latter possess high refractive index values (n ∞(min) = 2.173 and n ∞(max) = 2.206) that remain practically constant upon adding TiO 2 or WO 3 . The evolution of the dispersion energy suggests a slight increase of the network's ionicity upon substituting TeO 2 with TiO 2 and WO 3 , which is in agreement with the observed slight increase of the electronic oxide ion polarizability.As a future work, this evolving bond network nature could be verified by deriving the splitting of the longitudinal optical and transverse optical (LO-TO) modes by means of infrared reflectance spectroscopy.

  Figure captions.

Fig. 1 .

 1 Fig. 1. (a) Glass-forming domain obtained within the (100-x-y)TeO 2 -xTiO 2 -yWO 3 system (labeled TTxWy) surrounded by the yellow frame. Blue and green colored squares and spots correspond to partially crystallized samples in TiO 2 and WO 3 respectively. (b) Photograph of the obtained glass pellets in their final polished state. The relative positions of pellets are in accord with the black spots in the glass-forming domain.

Fig. 2 .

 2 Fig. 2. DSC curves of TT5Wy glasses. The glass transition and onset crystallization temperatures are given in bold and non-bold typefaces respectively.

Fig. 4 .

 4 Fig. 4. Evolution of total-area averaged Raman spectra of TTxWy glasses as a function of TiO 2 content. The low-wavenumber (boson peak) and mid-to high-wavenumber regions are supplied apart for more clarity.

Fig. 5 .Fig. 6 .

 56 Fig. 5. Evolution of total-area averaged Raman spectra of TTxWy glasses as a function of WO 3 content. The low-wavenumber (boson peak) and mid-to high-wavenumber regions are supplied apart for more clarity. The Raman spectrum of γ-WO 3 is reproduced from Fig. 2 in ref. [71]. Fig. 6. Total-area averaged Raman spectra of TT5Wy glasses. The labels A-M correspond to the inserted bands for spectral decomposition. Inset: measured Raman spectrum of pure TeO 2 glass featuring the three major broad bands (E+F, G+H and I+J).

Fig. 7 .

 7 Fig. 7. Compositional dependence of the normalized intensities of inserted bands for the Raman spectral decomposition of TT5Wy glasses. The areas of bands E-H were divided by the TeO 2 content; those of bands D and K-M by the WO 3 content (see text).

Fig. 8 .

 8 Fig. 8. UV-Vis-NIR light transmission spectra of TTxWy glasses. Insets for TT5Wy: (a) Zoom-in plot of the UV absorption edge; (b) Compositional dependence of the refractive index calculated at 1.5 µm.

Fig. 9 .

 9 Fig. 9. (a) Evolution of the absorption coefficient as a function of the incident wavelength in TT5Wy. (b) Tauc's plot for the optical band gap energy derived from the absorption coefficient in TT5Wy.

Fig. 10 .

 10 Fig.10. Refractive index dispersion of TT5Wy glasses. Each dispersion curve corresponds to the average of three measurements from different spots on the same glass pellet. The given refractive index values are obtained by fitting the curves using Sellmeier's equation[START_REF] Sellmeier | Zur Erklärung der abnormen Farbenfolge im Spectrum einiger Substanzen[END_REF]. Inset: compositional dependence of the refractive index.

 8  1 

 81 Symmetric stretching vibrations in nearly symmetric Te-O-Te bridges (as in γ-TeO 2 ) [4,51]. Motions of oxide ions in O-Ti-O bridges along the c axis (E g mode of rutile TiO 2 ) [46]. Symmetric stretching vibrations in Te-O-Te bridges [4,51]. G 607 (fixed)  Asymmetric stretching vibrations in nearly symmetric Te-O-Te bridges (as in γ-TeO 2 ) [4,51]. Motions of oxide ions in O-Ti-O bridges perpendicular to the c axis (A 1g mode of rutile TiO 2 ) [46]. Symmetric stretching vibrations in W-O-W bridges (as in -WO 3 ) [71]. J 772 (fixed)  Asymmetric stretching of essentially covalent Teeq O bonds [4,51]. K 820 (fixed)  Asymmetric stretching vibrations in W-O-W bridges (as in -WO 3 ) [71]. L 852.3 -863.Stretching vibrations of W-O bonds in W-O-W bridges [29].  Antisymmetric stretching vibrations of Ti-O bonds (B 2g mode of rutile TiO 2 ) [44,45]. M 925 -934  Asymmetric stretching vibrations of W-O bonds [71].

  Figure 1

  Figure 3

  Figure 7

Table captions .Table 1 .

 captions1 Thermal properties, densities and linear optical properties of TTxWy glasses. * Refractive indices obtained at λ = 1.5 µm from transmission spectra. § Refractive indices derived from ellipsometric dispersion curves using Sellmeier's equation.

Table 2 .

 2 Wavenumbers and vibrational assignments of the inserted bands. Raman bands arising from TeO 2 network (Te-O-Te bridges) and their assignments are highlighted in bold typeface. In the spectral decomposition process, the wavenumbers of F, H, L and M bands were unconstrained and are given herein as wavenumber ranges in TT5Wy glasses.

	Sample	T g (°C) ±1	ΔT (°C) ±1	Density (g/cm 3 )	E g (eV) ±0.020	E U (eV) ±0.002	n 1.5 ±0.010 *	n ∞	§	E 0 (eV)	E d (eV)	 O2-(Å 3 )
	TT5W5	328	31	5.586 ±0.003	2.880	0.105	2.132	2.195 ±0.015	7.992 ±0.126	30.371 ±1.116	1.593 ±0.048
	TT5W10	341	36	5.677 ±0.008	2.842	0.105	2.127	2.188 ±0.011	7.556 ±0.169	28.523 ±1.355	1.653 ±0.053
	TT5W15	355	38	5.777 ±0.007	2.789	0.106	2.122	2.186 ±0.027	7.667 ±0.233	28.792 ±3.164	1.727 ±0.047
	TT5W20	365	38	5.764 ±0.006	2.764	0.105	2.141	2.173 ±0.019	7.360 ±0.156	27.230 ±2.289	1.807 ±0.052
	TT5W25	376	41	5.925 ±0.008	2.744	0.107	2.118	2.191 ±0.020	7.599 ±0.174	28.669 ±2.066	1.810 ±0.055
	TT10W5	353	45	5.529 ±0.009	2.843	0.100	2.112	2.185 ±0.030	7.354 ±0.190	27.583 ±2.798	1.564 ±0.047
	TT10W10 364	45	5.643 ±0.010	2.812	0.103	2.123	2.197 ±0.014	7.763 ±0.132	29.545 ±1.137	1.682 ±0.060
	TT10W15 376	52	5.748 ±0.008	2.776	0.103	2.128	2.182 ±0.024	7.317 ±0.189	27.333 ±2.545	1.731 ±0.055
	TT10W20 389	54	5.799 ±0.006	2.758	0.103	2.133	2.191 ±0.013	7.022 ±0.095	26.519 ±0.851	1.777 ±0.066
	TT10W25 399	57	5.860 ±0.004	2.736	0.108	2.134	2.190 ±0.018	6.927 ±0.090	26.106 ±1.138	1.823 ±0.046
	TT15W5	380	38	5.454 ±0.004	2.826	0.096	2.120	2.198 ±0.024	7.067 ±0.119	26.851 ±1.673	1.669 ±0.045
	TT15W10 391	39	5.535 ±0.004	2.800	0.102	2.140	2.198 ±0.012	7.025 ±0.115	26.659 ±1.169	1.717 ±0.040
	TT15W15 405	40	5.623 ±0.008	2.782	0.100	2.113	2.206 ±0.015	7.048 ±0.081	27.027 ±0.834	1.750 ±0.046
	TT15W20 414	52	5.682 ±0.007	2.761	0.103	2.116	2.196 ±0.023	7.143 ±0.162	27.132 ±2.321	1.797 ±0.035
	TT15W25 426	48	5.757 ±0.005	2.758	0.100	-	2.199 ±0.011	6.987 ±0.080	26.604 ±0.809	1.813 ±0.032

Table 1

 1

	Band	Wavenumber (cm -1 )	Raman bands assignments
	A	44.0 -45.6	 The boson peak, ascribed to an excess density of vibrational states [56].
	B	146.9 (fixed)	 Intra-chain vibrations of Te-Te bonds (as in amorphous metallic trigonal Te) [52].
	C	230 (fixed)	 Bending vibrations of WO 6 octahedra (as in -WO 3 ) [71].
	D	351 (fixed)	 Bending vibrations of distorted WO 6 octahedra [71].

Table 2

 2

Acknowledgements.

We thank Dr. J. Cornette and Dr. R. Mayet for valued assistance with Raman and XRD experiments respectively. M.R. Zaki gratefully acknowledges the financial support from Conseil Régional du Limousin.

Synthesis, thermal, structural and linear optical properties of new glasses within the TeO2-TiO2-WO3 system