Physical and optical investigations of Bi2O3-TeO2-B2O3-GeO2 glasses

A new series of quaternary glasses with chemical composition xBi2O3-(80-x)TeO2-10B2O3-10GeO2 where 40 ≤ x ≤ 65 have been prepared by melt quenching technique. X- ray diffraction measurements have been achieved to check the amorphous nature of the glasses. The effect of Bi2O3 content on the physical, thermal and optical properties of the prepared glasses was studied. It is observed that oxygen packing density decreased with the increase of molar volume with increasing Bi2O3 content implying the formation of non-bridging oxygen’s and expands the glass. The glass transition temperature (Tg) increased whereas the glass stability decreased with Bi2O3. In addition to that, indirect optical band gap and Urbach energy values of the titled glasses have been calculated from Tauc plots using absorption spectra. The indirect optical band gap (Eopt) decreased with the increase of Bi2O3 concentration in the present glass system. The Raman spectroscopy at room temperature was employed to study the influence of bismuth oxide on the boron-oxygen network structure. The analysis of Raman spectra shows the presence of fundamental vibrations of TeO3, GeO6, BO3, BO4, BiO6 structural units.

oxygens, refractive index, density, and oxygen packing density increased while the optical band gap and molar volume showed a decreasing trend.
Since B 2 O 3 and TeO 2 are glass formers, the addition of heavy metal oxides such as Bi 2 O 3 , WO 3 or PbO to boro-tellurite glasses can change the structure of the glass. Bismuth is one of the heavy metals considered to be harmless, non-toxic and non-carcinogenic material has electronic applications, ceramic production and also used in high temperature superconductors because of the high polarizability of Bi 3+ cations [8][9][10][11][12]. Addition of heavy metal oxides like Bi 2 O 3 to tellurium glasses modifies physical as well as optical properties of the system. TeO 2 -Bi 2 O 3 glasses exhibit high second and third order non-linear optical susceptibility due to which they are used in up-conversion lasers and non-linear optical materials. There are many reports on the structural studies of the binary glasses such as B 2 O 3 -Bi 2 O 3, B 2 O 3 -GeO 2 [13,14]. Hasegawa [15] and Saddeek et al [16] developed Bi 2 O 3 -TeO 2 -B 2 O 3 ternary glass system and investigated their physical, linear and non-linear optical properties. Munoz-Martin et al [17] prepared ternary tellurite-tungstate glass system with alkaline oxide, ZnO, Bi 2 O 3 or Li 2 O as third component and demonstrated that these ternary glasses are promising materials for developing broad band integrated optical amplifiers. Zhou et al [18] prepared and characterized new tellurium quaternary TeO 2 -PbO-Bi 2 O 3 -B 2 O 3 glass system and explained the variations in thermal stability with the glass composition using FTIR measurements. The present glasses can be used for photonic devices and low melting point sealing glasses.
GeO 2 is a typical glass former that consists of tetrahedral and octahedral germanium coordinated by oxygen. Bismuth germanate glasses have been widely used various applications such as infrared transmitting windows, as active media for Raman optical fiber amplifiers, and thermal and mechanical sensors [19][20][21]. Mansour [22] prepared Li 2 O-B 2 O 3 -GeO 2 glasses with high Li 2 O content and measured Molar volume, density, and Raman spectra of the prepared glasses. Glasses containing Bi 2 O 3 in combination of GeO 2 have been investigated as IR transmitting windows and optical fibers with low loss [23]. Fan et al [24] performed thermal, infrared, Raman and XPS spectroscopic studies on prepared Bi 2 O 3 -B 2 O 3 -GeO 2 ternary glass system. Xia and Yang [25] carried out thermal, emission and absorption measurements on Bi 2 O 3 -GeO 2 -WO 3 and Bi 2 O 3 -GeO 2 -BaO glasses and concluded that Bi 5+ ions are responsible for broad band emission. There are many reports on GeO 2 ternary glasses such as GeO 2 -PbO-Nb 2 O 5 , GeO 2 -PbO-CaCO 3 and Li 2 O-GeO 2 -P 2 O 5 [26][27][28].
In order to exploit the technological potential of glasses, it is crucial to investigate the structural studies by infrared and Raman spectroscopy. These two methods play an important role in the identification of the basic structural units like BO 4 , BO 3 , TeO 3 , TeO 4 , GeO 6 . Structural investigations of B 2 O 3 , Bi 2 O 3 , TeO 2 and GeO 2 based glasses were published [29,30]. Osaka et al [31] and Mattarelli et al [32] examined the structure of GeO 2 -TeO 2 by EXAFS and Raman studies. Osipov et al [33] employed Raman and FTIR spectroscopy to study the influence of zinc oxide on BaO-B 2 O 3 glass matrix. Vani et al [34] presented the Raman data on fluorotellurite glasses based on zinc and barium fluorides and observed the formation of more non-bridging fluorine instead of non-bridging oxygens. Quaternary glasses Bi 2 O 3 -TeO 2 -B 2 O 3 -TiO 2 were investigated by Raman studies [1] and found that the coordination number of tellurium ions with oxygen remains constant with varying TiO 2 content. Recently Seema Thakur et al [35]

Characterization
Structural characterization of the glass samples was done by using x-ray diffraction technique. Differential scanning calorimetry was used for the thermal characterization of the samples. Physical properties of the glasses like, density, density related parameters such as molar volume, oxygen packing density, oxygen molar volume, bismuth ion concentration, polaron radius, and inter-nuclear distance were systematically examined. Optical properties of the glasses were investigated using UV-Visible spectroscopy. The investigated optical parameters are cut-off wavelength, indirect optical band gap energy, Urbach energy ΔE, Possion's ratio (Makishima and Mackenzie) μ cal , Possion's ratio (Bridge) m¢ , cal bond density and average coordination number. The working conditions of the characterization tools along with the procedures followed for the investigation of structural, thermal, physical, and optical properties have been given in ESM.

Results and discussions
3.1. Structural analysis Figure 1 shows the XRD patterns obtained for xBi 2 O 3 -(80-x)TeO 2 -10B 2 O 3 -10GeO 2 glass system. As observed from the figure, the broad low intense peak appeared at the angles 20°-30°confirms the amorphous nature of the sample without any strong and high intense peaks.

Physical properties
The experimental density of the present glasses with composition xBi 2 O 3 -(80-x)TeO 2 -10B 2 O 3 -10GeO 2 is presented in figure 2 with varying Bi 2 O 3 content. It is revealed that the density of the present glasses varied nonlinearly with the increase of Bi 2 O 3 content, the maximum value obtained for x=50 mole% of glass.
Theoretically, glass density from its chemical composition is given by the following equation [36].
where M i is the molecular weight (Kg mole −1 ), x i is the mole fraction (mole%), and V i is the packing density parameter (m 3 mole −1 ). The packing density parameter V i for M x O y can be calculated using the following formula where r M and r O are the ionic radii of metal and oxygen respectively. Shannon's ionic radii were used [37,38] for the present study. The packing density parameter V i (m 3 [39]. Other physical parameters such as Bi 3+ ion concentration (N), polaron radius (r p ) and inter-nuclear distance (r i ) were also calculated and can be seen in table 1. Decreasing trend of r p and r i , and increase of Bi 3+ concentration with increasing Bi 2 O 3 content were observed. The r p of each prepared glass is observed less than the corresponding Bi-Bi spacing. Figure 4 presents the DSC thermograms for the prepared glasses with different Bi 2 O 3 content. A typical DSC thermogram for 40Bi 2 O 3 -40TeO 2 -10B 2 O 3 -10GeO 2 glass is shown in figure 4 (inset). A single endothermic peak at 455°C corresponds to the glass transition temperature (T g ), followed by the onset of crystallization temperature (T x ). The exothermic peak at 569 is assigned to the full crystallization temperature (T c ). Furthermore, the good homogeneity of the glasses also revealed from the single peak in T g . The thermodynamical parameters such as T g and T x were determined and represented in table 2. The T g values increase from 455°C to 540°C with increase of Bi 2 O 3 content. Generally, the thermal stability of the glass can be estimated from the difference between the T x and T g (i.e., S=T x −T g ). The thermal stability for the prepared glasses was calculated and tabulated in table 2. The wide thermal stability range is highly favorable for the process of glass formation [40]. Figure 5 plots the variation of T g as a function of Bi 2 O 3 content. It was found that the glasses with S100 are suitable for drawing optical fibers [41]. The observed S values of the prepared glasses decrease with the increase of Bi 2 O 3 content. Structural changes presented in the glass   [43] reported that the T g is a function of the stretching force constant and the average crosslink density. In the present glass system, the values of the average coordination number (m) increase from 4.8 to 5.3 (table 2). The increasing average coordination number of the glass network increases the rigidity and leads to an increase in the T g . The cross link density (n c ) of the glasses was calculated by the following equation [44] å å

( ) ( )
The total packing density (V t ) was calculated using the equation where V i is the packing density factor of the oxides was defined in equation (10) where m is the coordination number of the glass. In the present glass system the bond density decreases from 5.77×10 22 to 4.84×10 22 /cc as the bismuth content is increased from 40 to 65 mole% (table 2).

Optical properties
The information related to the optically induced transitions, energy gap, and band structure of amorphous glassy materials can be obtained from the optical absorption studies (in particular, absorption edge). A systematic analysis of absorption spectra at lower and higher energies will give the accurate information about the atomic vibrations and electronic states in the atom respectively [48]. The interaction between an electromagnetic wave and a valence electron results in both direct and indirect optical transitions across the energy gap. Figure 6  Here hν is the energy of incident photon, B is a constant related to the amount of band tailing, E opt is the optical energy gap and n is a digit which symbolizes the transition progression (for n=1/2, 2, 3/2, and 3 the transitions are direct allowed, indirect allowed, direct forbidden, and indirect forbidden respectively).  in the prepared glasses. The cation polarizability of the Bi 3+ ion (1.508 Å 3 ) is weaker as compared to the cation polarizability of Te 2+ ion (1.595 Å 3 ) which leads to the weakening of bond strengths between metal and oxygen and finally forms the non-bridging oxygen atoms. Defect concentration in the glass networks was calculated using Urbach energy ΔE. The relation between α (ν) and ΔE is given by well-known Urbach law [51].
where B is constant and ΔE is Urbach energy. The relation can also be written as Urbach plots (a graph between ln(α) versus hυ) for all the prepared glasses are shown in figure 8. The values of Urbach energy (ΔE) were determined from the inverse slope of linear regions of Urbach plots and are presented in table 2. Earlier reports [49] suggested that the ΔE values for amorphous materials should lie between 0.045 and 0.66 eV. Generally, large values of ΔE convert the weak bond into defects and reduce the long   The Raman spectra of α-TeO 2 consists of several peaks in the low frequency range. In all the present glasses the Raman strong peaks observed at lower wavenumbers in the range 128-145 cm −1 are due to the librational   [55] observed this band at 418 cm −1 in Rb 2 O-GeO 2 glass system. A well distinguished Raman peak between 529-547 cm −1 belongs to Bi-O stretching vibrations (vibrations of bismuth associated with nonbridging oxygens) of BiO 6 octahedral units. This peak is observed only in x=40, 55 and 65 glasses. The Raman intensity and the area of this peak increased with Bi 2 O 3 content, indicating that the BiO 6 group containing NBO, s increased with Bi 2 O 3 content. [51,56,57]. Raman peak centered in the range 627-654 cm −1 is ascribed to a symmetric stretching vibration of Ge-O-Ge linkages in GeO 6 octahedral [58] present in the glass structure and these peaks are observed only in x=40 and 45 glass.
In the present Raman spectra the peak around 701-740 cm −1 is related to the stretching vibrations of Te-O − bonds (O − stands for non-bridging oxygens) in TeO 3 structural units. It is observed that the TeO 3 structural groups remains constant throughout the glass system [59]. This Raman peak was observed at 750-760 cm −1 in zinc bismuth boro-tellurite glasses [60]. The sharp Raman bands in the present study occurring in the spectral range 1349-1354 cm −1 may be assigned to B-O − vibrations of the units attached to large segments of the borate networks [61]. This band intensity and area remains constant.
The Raman peak intensity observed in the range 1201-1215 cm −1 is found to increase with Bi 2 O 3 content. Prominent Raman bands were observed from 1465-1794 cm −1 range. These band intensity and area remains constant. The Raman bands observed in the range 1201-1215 cm −1 and 1465-1794 cm −1 can be assigned to the B-O stretching vibrations in BO 3 units from boroxol rings [62,63]. The Raman assignments for all the peaks in the Raman spectra are given in table 4.

Conclusions
Synthesis of six glass samples with the composition xBi 2 O 3 -(80-x)TeO 2 -10B 2 O 3 -10GeO 2 where 40x65 mol% has been done successfully using melt quenching technique. XRD, density, and UV-Visible characterization of the glass samples were studied. The amorphous nature of the present bismuth tellurium boro Peak Center Center (C) Center (C) Center (C) Center (C) Center (C)  145  135  130  129  128  128  319  335  333  324  332  335  392  411  411  -410  408  529  --547  -537  627  654  --687  694  740  739  707  728  701  701  1201  1205  1202  1201  1215  1210  1349  1351  1352  1353  1353  1354  1471  1465  1417  1477  1479  1481  1676  1678  1681  1682  1681  1684  1787  1774  1786  1794 1768 1778 Table 4. Raman peaks Assignments of present glasses. germanate glasses has been confirmed by XRD spectra. The density of the glasses was measured and density related parameters were calculated. It is observed that the molar volume and oxygen molar volume increase with increase in Bi 2 O 3 content, whereas oxygen packing density decreases. Theoretically, Poisson's ratio of the present glasses calculated by Makhishima & Mekhanzie theory and Bridge model was found to vary from 0.095 to 0.155 and 0.205 to 0.211 respectively. The glass transition temperature is found to increase with increase in Bi 2 O 3 content and the reverse trend is followed by the glass stability. The indirect optical band gap energy for all the glass samples was determined from the Tauc plots, the values were in the range 3.07-2.81 eV. One can conclude that the studied Bi 2 O 3 -TeO 2 -B 2 O 3 -GeO 2 glasses in the present work could be considered as candidates suitable for drawing optical fibers. The structure of the present glasses was studied by Raman spectroscopy. The Raman spectra were deconvoluted. The structural result analysis indicates the existence of TeO 4 , BiO 6 , BO 3 , GeO 4 units stretching vibrations.