The XRD results (Figure 1) show that the bismuth tellurite glass is in an amorphous phase. Sample TB 5 (refer to Table 1) is a transparent glass of a dark orange color, while TB 8, TB10, TB12 and TB15 are transparent glasses of a yellow color.

The density of the glass samples increases as the bismuth content increases. The densities of the bismuth tellurite glasses range from 5.43 g/cm³ to 6.26 g/cm³, as shown in Table 1. Figure 2 shows the molar volume and the density increase proportional to the bismuth content. The molar mass of the bismuth (iii) oxide (465.96 g/mol) is heavier than the molar mass of tellurite oxide (159.60 g/mol) and hence, the glass matrix becomes more dense when Bi³⁺ ions are added into the glass network [19]. In addition, the increase in molar volume is due to the atomic radius of Bi³⁺ (1.70 Å ) which is higher than that of tellurite (1.60 Å ). Usually, the density of the glass changes in the inverse direction of the molar volume, but in this study, the density and molar volume increase with the bismuth contents, this anomaly was also found by Saddek et al. (2007) and Halimah et al. (2010) [20,21].

As shown in Figure 3, the FTIR analysis shows that the TeO₂ is the framework former while Bi₂O₃ is the glass modifier in the bismuth tellurite glasses system. Table 2 shows the ranges of the wave numbers for difference mode. The wave number for the bismuth tellurite glass system is slightly shifted to the lower wave numbers when the ion Bi³⁺ concentration is increased. There is a diffuse band in the range of 500–600 cm⁻¹, due to the disordered structure; this is considered as the vibration modes of both the TeO₃ and the TeO₄ entities [22]. The vibration modes of the bonds for TeO₃₊₁ polyhedra is also found at bands around 580 cm⁻¹ where the TeO₃₊₁ polyhedra consists of an intermediate coordination of the tellurium atoms between 3 and 4 [17], this is found in the all glass samples. On the other hand, the shoulder form in the range of 800–900 cm⁻¹ in TB8, TB10, TB12 and TB15 is due to the symmetrical stretching vibration of the Bi–O bonds in BiO₃ units. This band is not observed in the TB5 because the Bi³⁺ ions present in this glass sample contain non-bridging oxygen in the form of [BiO]⁻ defects. As the ion Bi³⁺ content increases, bismuth oxide as the network modifier ion modifies the glass structure and enhances the breaking of axial Te–O–Te linkages in the trigonal bipyramids [TeO₄] (tbp) and creates the formation of trigonal pyramid [TeO₃] (tp) units and non-bridging oxygen.

When the bismuth content increases, the refractive index measured at the wavelength of 632.8 nm increases from 1.97 to 2.12. The refractive index increase is due to the ion Bi³⁺ which has a high polarity that can break the bridging oxygen [BO] to non-bridging oxygen [NBO]. In addition, non-bridging oxygen also has an effect on the refractive index, because of the polarity of non-bridging oxygen (NBO) is higher than that of bridging oxygen [29]. As a result, Bi₂O₃ will affect the TeO₂ based glasses and forms highly polarized non-bridging oxygen ions at the terminals of Te–O bonds.

The optical absorption spectra were taken in the ranges of 350 to 500 nm. For the optical band gap in this bismuth tellurite glass system, the optical band gap energy, E_opt decreases when the ion bismuth content increases as shown in Table 3.

The optical band gap energy is determined by using the following equation [30]

where α is the absorption coefficient, hv is the incident photon energy, A is a constant and E_opt is the optical band gap. Values of n are 2 and 1/2 for direct and indirect transitions, respectively. The indirect band gap as a function of photon energy for (Bi₂O₃)_x(TeO₂)_100−x glass system is plotted and shown in Figure 4.

According to Sayed (2005), amorphous or glassy materials consist of a band tailing in the forbidden energy band gap. The band tailing might arise from random fluctuations of the internal disorder in the amorphous materials. So it can be estimated using the Urbach equation shown as follows [31]

where B is a constant and ΔE is the width of the band tail of the electron states. From Figure 5, Urbach energy, E_g is determined from the slope of plot ln (α) versus hv.

The range of the indirect optical band gap decreases from 2.40 eV to 2.60 eV and 3.00 eV to 3.02 eV for the direct band gap as the concentration of ion Bi³⁺ increases in the bismuth tellurite glass system. However, sample TB5 has a low bismuth content and a low energy band gap because of the formation of TeO₂ linkages in Te–O where bridging oxygen is more frequent than non-bridging oxygen. Shifting of the absorption band to a lower energy can be related to the formation of non-bridging oxygen (NBO) which binds exited electrons of non-bridging oxygen less tightly than bridging oxygen [10], and this cause the glass network to become less rigid (Rajendan, 2003) [27]. Another possibility is due to the high polarizability of the Bi³⁺ ions which can be attributed to the empty d orbitals of the corresponding cations and also their high coordination number towards oxide ions. The coordination number of TeO₂ changes from 4 to 3 while the coordination number of Bi₂O₃ changes from 3 to 6, which then forms a BiO₆ octaherdal structural unit with the addition of Bi₂O₃ into the glass system. According to Yanfei (2008) [16], less tight oxygen anions were allowed when the amount of non-bridging oxygen increased, This was because of an increase in Lewis basicity of oxide ions, which form stronger covalent Te–O bonds in TeO₃ units, and so allow less tight oxygen anions in the glass network. Hence the more the Bi₂O₃ content increases, the fewer tightly bound oxygen anions (valence electrons) are found and the greater the decrease of the optical band gap energies.

The optical band gap energy is opposite to the Urbach energy, which can be attributed to the formation of non-bridging oxygen (Figure 6). The optical band gap energy and Urbach energy values for glass systems are found to lie between 3.10–3.00 eV and 0.25–0.44 eV, respectively. Rajendran (2003) [27] reported that the addition of Bi₂O₃ glass modifier would make the glass network more loosely packed (breaking the O–Te–O chains to form TeO₃ units and non-bridging oxygen) and therefore produce unstable glass. When the concentration of ion Bi³⁺ is increased, the appearance of TeO₃ units and non-bridging oxygen in the glass networks increases. As the concentration of the ion modifier Bi³⁺ increases, non-bridging oxygen and TeO₃ units in the glass structure increase and as a result, the Urbach energy for this glass system also increases.