## 3 Results and Discussion
### Optical Properties  
UV-Vis absorption spectra of the PS system in the 200-800 nm spectral region are depicted in Figure 7. As may be observed in the figure, no sharp absorption edge has been found, which indicates the disorder in the matrix and corresponds to the characteristic of the glassy state of the samples [32]. The UV-Vis spectrum of the PS1 sample exhibits a broad absorption band with a maxima center at 353 nm corresponding to the O\({}^{2-}\)\(\rightarrow\) Pb\({}^{2+}\) charge transfer band [33]. The shoulders located at 219 and 263 nm are suggested to be correlated with trace Fe\({}^{3+}\) impurity ions [34]. The introduction of amounts of SiO\({}_{2}\) gradually shifts the maximum absorption band to lower wavelengths, at 298 nm, while simultaneous decreasing the absorbance intensity. This band ascribed to the electronic transitions of Pb\({}^{2+}\) ions with the 6s\({}^{2}\) electron configuration became evident beginning with the PS5 sample. The shift of the absorption band to a lower wavelength indicated that the fraction of BO content increases with increasing SiO\({}_{2}\) concentration [35]. Analysis of UV-Vis spectra suggested that Pb\({}^{2+}\) occupies different local sites in the network, and the evolution of the distribution of Pb\({}^{2+}\) ions depends on the PbO\({}_{2}\) concentration. Figure 6: Variation in MFP as a function of incident photon energy for (**a**) PS, (**b**) PSC, (**c**) PSE, and (**d**) PSY glass samples. The higher and lower energy parts of the absorption edge, known as the Tauc and the Urbach region, were analyzed to evaluate the optical band gap energy, E\({}_{\text{g}}\), and the Urbach energy, E\({}_{\text{U}}\), values of the studied glass system. The band gap represents the minimum amount of energy required for an electron to jump from the valence to the conduction band. The size of the E\({}_{\text{g}}\) is very important for technological applications, and it affects the properties of the materials. The value of the E\({}_{\text{g}}\) gives the materials distinct properties. On the basis of the band structure, a semiconductor's optimal E\({}_{\text{g}}\) must be below 3.0 eV in order to allow for light absorption for production of electron-hole pairs [36], and, for dielectric materials, E\({}_{\text{g}}\) is greater than 3 eV. The optical band gap energy for indirect allowed transition was estimated from Tauc's relation by plotting \((\alpha\hbar\alpha)^{2}\) versus photon energy, hv, and extrapolating the linear fitted regions of \((\alpha\hbar\alpha)^{2}\) to 0 [37], as shown in Figure 8a. The Urbach energy is a measure of material disorder. High E\({}_{\text{U}}\) values are due to the disorder and the wide distribution of local electric field intensities. The values of E\({}_{\text{U}}\) are obtained by plotting natural ln\(\alpha\) versus hv and taking the reciprocals of the straight line's slope [38], as shown in Figure 8b. Figure 8: Plot of (a) \((\alpha\hbar\
u)^{2}\) versus hv; (b) ln\(\alpha\) versus hv. Figure 7: UV-Vis spectra of PS glass system. Figure 9 presents the variation in \(\rm E_{g}\) and \(\rm E_{U}\) with glass composition. The progressive decrease in the amount of non-bridging oxygen content shifts the absorption edge to a lower wavelength, leading to an increase in the \(\rm E_{g}\) values from 2.76 to 3.55 eV as the SiO\({}_{2}\) content increases. The PS0-PS3 samples exhibit semiconductor behavior, \(\rm E_{g}\leq\) 3 eV, and PS4-PS7 present insulating properties. The \(\rm E_{U}\) values show small variations with composition; for the PS0-PS4 samples, the \(\rm E_{U}\) decreases progressively from 0.189 at 0.150 eV as the SiO\({}_{2}\) content increases. Meanwhile, for the PS5 sample, the \(\rm E_{U}\) value increases at 0.285 eV and then slowly decreases with high SiO\({}_{2}\) content. Variations in \(\rm E_{g}\) and \(\rm E_{u}\) values are a consequence of short-range order inversion from the PS5 sample as the transition from a lead-oxygen network to a silicate network occurs.