Electronic Polarizability and Optical Basicity of BaO-B2O3-TeO2 Glass System

[(TeO 2 ) 0.7 (B 2 O 3 ) 0.3 ] 1-x (BaO) x , x = 0.00, 0.05, 0.10, 0.20, 0.25, 0.30 and 0.35 mol fraction glass series were successfully synthesized by conventional melt quenching method. Amorphous phase of all samples was confirmed through X-ray diffraction while optical properties were determined using UV-VIS spectrophotometer. Fourier Transform Infrared (FTIR) analysis showed that the glass structure consisted of TeO 3 , TeO 4 , TeO 6 , BO 3 and BO 4 structural units. The optical band gap energy, E opt which was calculated from Tauc’ plots decrease d as the amount of BaO increases, whereas, the Urbach energy value increased. The increase in Urbach energy value was attributed to the increase of defects in glass structure. The refractive indices of glass were found to increase along with the increased amount of BaO, due to the high polarization and high density of host material and glass modifier. The mol ar polarizability, α m, oxide ion polarizability , α o2- and optical basicity, Λ of the glasses are calculated by Lorentz-Lorenz equation. The glasses were found to possess α m values between 8.106 – 8.489 Å 3 , and α o2- values between 3.303 to 4.772. Meanwhile, optical basicity increases from 0.115 to 0.893.


Introduction
Developments of tellurite glasses began in the early 1950s especially in the optoelectronics field such as fiber optic and laser technology. Tellurite glasses are excellent in glass stability aspect among other oxide glass formers and possess high linear and nonlinear refractive index compared to fluoride and silicate glasses [1][2][3]. It is also reported that by comparing the density of glasses, the density of tellurite glass falls in the following order: tellurite > germinate > phosphate > silicate [2].
Borate is also one of the most popular glasses which possesses high chemical durability, good solubility of rare earth and easy for synthesisation. In addition, by combining with heavy metal oxide, it is possible to use as an electronic sensor [4], electronic device [5] and radiation shielding material [6][7]. Normally, borate glass consists of [BO4] and [BO3] structural units together with non-bridging oxygen when combined with alkali or heavy metal oxide. But it is interchangeable with other BxOy structural group [4; 8-9].
Investigation on borotellurite glass system has been growing in recent years. Combination of two glass formers, borate and tellurite is found to affect its physical properties which produces good quality glass. Structural units of TeO4 from tellurite and BO4 from borate have strong tendency to link with each other and produce a high connectivity in glass network. On top of that, suitable ratio of both glass formers will result to a reduction in its hygroscopic nature, while increasing the IR transmission and refractive index [10][11][12]. However, the number of publications for boro-tellurite glass are still small compared to other glass systems such as silicate, phosphate, boro-silicate, boro-phosphate etc.
In present work, a barium oxide is selected as glass modifier which mixed into the glass composition. The ability of barium composites in sustaining a strong electric field without conducting electricity has been useful as a component of high temperature conductors and electroceramics. Due to the non-carcinogenic property of barium, it is not poisonous and does not bioaccumulate. This is a useful perk in glass making industries as it also can increase the refractive index of glass [13,14,15]. Therefore, this work offers a new insight for boro-tellurite glass system that focuses on optical properties by adding barium oxide as its glass modifier. The density, structural and optical properties of this glass system are also explored to see their correlations with barium concentration.
B2O3-TeO2 based glass is selected as a glass former due to its low melting temperature, high transparency, and stability against devitrification. In addition, barium tellurite glass with borate is expected to possess high density, high refractive index and large polarizability which lead to the improvement of optical properties, thus making it suitable for photonic devices.

Sample preparation
A series of glass samples were prepared using melt quenching technique. Tellurium Next, the crucible along with the mixed chemical powder were placed inside a furnace with internal temperature of 400°C for 30 minutes at room temperature for a pre-heating process.
This process was to decompose the mixture and allow water vapor to evaporate. After 30 minutes of pre-heating, the crucible was transferred immediately to a second furnace for melting process at 900˚C for 50 min. When the mixture in the crucible was completely melted, the molten was then poured into a cylindrical stainless steel split mould which was pre-heated at 400°C. This temperature was achieved empirically through trial and error. If the temperature of the mould was too low, the formed glass will have too much internal stress that will lead to crack and fracture.
The mould was then placed inside the first furnace at 400˚C for 1 hour to eliminate any residual stress in the glass sample. Suitable temperature and time for annealing process were stressed upon to reduce any thermal stresses caused by the quenching process. Afterward, the furnace was turned off. The glass sample was left inside the furnace until the temperature cooled down and achieved room temperature.

Sample characterization
Glass density of all sample are obtained using densimeter with sensitivity of 10 -3 g/cm 3 that operated according to Archimedes' principle with distilled water as an immersion liquid. The relation used to calculate density is as follows; where is weight of sample in air, is weight of sample in distilled water and is density of distilled water.
Molar volume ( ) is the volume occupied by one mole of a substance from a chemical element or a chemical compound at a given temperature and pressure. The molar volume of a glass can be obtained by the relation of; where M is molecular weight of the substances and ρ is the density of glass sample. The band edge is measured using the theory of Davis and Mott in order to obtain values for the optical energy gaps. Davis and Mott found an empirical relation that provided an accurate depiction for the shape of the band edge [15]. 19] had stated that for glass and amorphous material, the best experimental data was at r = 2, and the band edge was dominated by indirect transitions. Using r = 2 in Equation (2), the following relation can be attained as: As implied by Equation 2, a plot of (αℏω) 1/2 versus ℏω will turn out as a straight line in the vicinity of the band edge. Furthermore, the x-intercept of this line will be the value of Eopt.
In the case of glasses and any amorphous materials, a band tail exists in the forbidden energy band gap. The extent of band tailing is a measurement of the material disorder and is estimated by Urbach rule [20]. For fundamental absorption edge in lower incident photon energy (ℏ ) between 10 2 -10 4 cm -1 , the absorption coefficient ( ) follows Urbach law given by; where B is a constant and ∆ is the Urbach's energy. The Urbach energy values are determined from the slopes of linear region of the plots in ln ( ) vs ℏ . The values of wavelength correspond to the absorption edge, where the intensity reaches the maximum value in optical absorption spectra is taken as cut-off wavelengths, λcut-off.
The refractive indices, noted as n, for the glass samples are appraised from the optical band gap values using the relation put forward by Dimitrov and Sakka (1996) and Eraiah (2006) [21][22].
The quoted refractive index values correspond to the respective Eopt values of the present glass samples. However, there are chances of small errors creeping into the process of estimating the n using the value of Eopt, owing to the extrapolation of (αℏ ) 1/2 vs (ℏ ) plots.
The average molar refraction, Rm of the glasses is taken from the Lorentz-Lorenz equation [23].
where the quantity of [( 2 − 1)/( 2 + 2)] refers to reflection loss, M is molecular weight, ρ is density and Vm is the molar volume. This parameter is able to give an insight on the contribution of ionic packing in controlling the general refractive index of the glass. Besides that, it is also directly proportional to the polarizability of the constituent ions used in a glass system.
There are two types of polarizabilities that can be calculated which are molar polarizability and oxide ion polarizability. Molar polarizability is defined as the total polarizability of a mole of a substance, which relies on temperature, pressure, and refractive index. According to the Clausius-Mosotti relation, molar polarizability of the materials ( ) is given by the relation: where N is the Avogadro's number. On the other hands, the polarizability of oxide ions 2− has been calculated using the equation proposed by Dimitrov and Sakka, (1996) [21].

Density and molar volume
All glasses prepared in this work were transparent and clear from air bubbles. The addition of BaO content in (TeO2)0.7(B2O3)0.3 glass system changed the colour tone from colourless to lightgreen.
Density is the key concept in analysing the structure of glasses, relying on the number and type of atoms as well as atom combination bonding [26]. It can be defined as the degree of compactness of a substance or glass structure. The densities values of all samples were listed in Table   1. It had been revealed that the density of glass system increased with increased in BaO concentration.
As expected, the small increment of this glass density was due to the replacement of lighter density  [28][29], respectively. Meanwhile, Desirena et al., (2009) [30] reported that the density of 20BaO-10Cs2CO3 -70TeO3 glass was 4.733 gcm -3 , which was higher than the density observed in this work. Furthermore, Ghada (2018) [31] stated that the increment of density in the glass system also was probably caused by an increment in the number of non-bridging oxygen (NBO) atoms. Besides, the introduction of modifier barium ions will attempt to occupy the interstices within the network and contribute to the creation of a more compact glass. This parameter also can be used as a numerical guideline to recognize an open structure in a glass structure. Therefore, the sample with the highest molar volume corresponds to the maximum open structure [32]. Typically, molar volume and density shows an opposite behaviour. Based on the data recorded in Table 1, the similar case also occurs in this study. In present work, the density of glass system increased while molar volume decreased as BaO content increased. The trend of this result followed the relationship between density and molar volume as shown in Equation (2), which is the inversely proportional relation between density and molar volume. The decreasing molar volume might be caused by the decreasing bond length and the space between atoms in a glass network. This process then affected the compactness of the glass network. This behaviour also can be observed in BaO borate glass explained by Obayes et al., (2016) [33]. It was stated that the addition of BaO (20 to 60 mol%) in MgO -Na2O -B2O3 glass system reduced average boron-boron separation (dB-B) from ~0.415 nm to ~0.333 nm, and thus, yielded a compact glass.
In order to confirm the increase in the compactness of the glass structure after addition of glass modifier, the average boron-boron separation, 〈dB-B〉 had been calculated and also listed in Table 1. The average boron-boron separation 〈dB-B〉 was calculated using the equation; where NA was Avogadro's number (6.0228 × 10 23 g/mol) and was the volume which corresponded to the volume that contained one mole of boron within the given structure. This volume, was determined using the following relation; where was the molar fraction of boron oxide, B2O3 and was the molar volume of glass.
The average boron-boron separation 〈dB-B〉 was calculated to give clearer understanding in the modification of glass network due to the addition of BaO. Boron atoms were the central atoms with negatively charged tetrahedral -BO4/2 units [34]. Since depended on cation species, the calculated values of average boron-boron separation 〈dB-B〉 decreased with the increase in the BaO contents in the glass system as shown in Table 1. Thus, the incorporation of on expense of B2O3 led to a significant contraction of the glass structural network which confirmed the obtained density and molar volume values [35].  [36][37]. Due to the absence of a sharp peak, it can be inferred that there was no long range order in atomic arrangements thus ensures the glass had an amorphous structure.

FTIR analysis
The FTIR spectra for present glass samples were plotted in Figure

Optical Properties
Optical absorption spectra of all glasses are measured for the wavelength in range of 220 -800 nm. The optical absorption for BTe-0.25BaO glass are illustrated in Figure 3. A cutoff wavelength was found to shift towards higher wavelength when BaO was inserted into the glass system. Addition of BaO content in (TeO2)0.7(B2O3)0.3 glass system was believed to increase the non-bridging oxygen atoms into glass structure, thus resulting in cutoff wavelength towards higher wavelength.

Fig. 3. Optical absorption for BTe-0.25BaO glass
The variation of the optical band gap Eopt for this glass series as listed in Table 2 determined from the data in Figure 3 and calculated by using Equation ( [43] for barium -fluorotellurite glass. The low value indicated there was minimum defects that present in the glass, which meant that the glass was stable and highly homogeneous.
where Rm is molar refraction and Vm is molar volume. The necessary and sufficient conditions for predicting nonmetallic or metallic character of solids were: Rm/Vm > 1, for metal and Rm/Vm < 1,  [47]. Besides, the range value of metallization criterion also indicated that this glass samples could be the best material for non-linear application compared to other glasses.
Hence, it can inferred that as barium oxide glass is known for its high refractive index, hardness and stability as well as low dispersion, barium oxide glass is known to have a good optical performance. It has a high potential to be used in the production of a desirable multifocal ophthalmic lens, which formed by fusing a glass segment of high refractive index onto a glass blank of lower refractive index. In the process, the glass used in the segment shall preferably have a relatively high refractive index in order to reduce the thickness of the lens [48].