Figure 1 shows FTIR absorption spectra of GeO₂-PbO-Bi₂O₃ glass where two envelops at around 500–600 and 700–800 cm⁻¹ wavenumbers are present. Table 1 provides the wavenumbers of the deconvoluted bands. Absorption at 550 cm⁻¹ indicates bending and symmetric stretching vibration of the Ge–O–Ge of GeO₄ tetrahedral unit [9], Bi–O⁽⁻⁾ vibration bond of BiO₆ unit [10] and antisymmetric bending vibration of Pb-O-Pb bond [11]. The band at 700–800 cm⁻¹ wavenumbers was deconvoluted to peaks at 715 cm⁻¹ and 785 cm⁻¹. The first peak is related to stretching vibration mode of the Ge–O–Ge bond in GeO₆ octahedral units of rutile germanium oxide [12]; the second is related to antisymmetric stretching vibration mode of the Ge–O–Ge bond in GeO₄ tetrahedral units [9].

FTIR absorption at 830 cm⁻¹ is characteristic of the pyramidal BiO₃ group [13]. Absence of this peak indicates that only BiO₆ units participate in the glass structure. The peak at 470 cm⁻¹ is related to symmetric bending vibration of Pb–O in PbO₄ tetragonal pyramid (PbO covalent bond) and Bi–O bond bending of strongly distorted BiO₆ octahedral units formed when both Pb and Bi act as network former in a glass matrix. Absence of this peak clearly indicates that the modification role of both lead and bismuth is predominant. Pb²⁺ ions in the germanate-lead oxide system act as a modifier in low PbO content with an increase of non-bridging oxygen, yet, participate in the glass network in high composition as PbO chains [6]. Based on Figure 2, the wavenumber at 550 cm⁻¹ decreases gradually with increasing x. This is attributed to replacement of lead with bismuth ions where Bi³⁺ is surrounded by more oxygen than lead. Figure 2 shows two linear decreases of wavenumber in both Pb-rich and Bi-rich regions, but a deviation from the linear interpolation is observed when x changes from 0.2 to 0.3. This can be related to the appearance of Bi²⁺ and/or Bi¹⁺ which have less oxygen in their surroundings in comparison with Bi³⁺ ions. Bi³⁺ is shared with three oxygens but Bi²⁺ and/or Bi¹⁺ ions are surrounded with less oxygens. So Bi–O bonding should be stronger for Bi²⁺ or Bi⁺ ions in contrast to weaker bonding for the Bi³⁺ ion and appears in the deviation shown in Figure 2 to greater wavenumbers for GPB631 and GPB640.

The 700–800 cm⁻¹ band was deconvoluted into two peaks at 715 and 785 cm⁻¹ which are related to GeO₆ and GeO₄ units, respectively, and show the existence of both four and six coordination. Coordination number (CN), proposed by the Dachille and Roy study [14], relates the coordination number of cation “T” to the wavelength of the strongest infrared peak of the T–X stretching bond in TX₂ compounds like SiO₂, GeO₂ and BeF₂:

where A_T and A_X are the atomic number of atoms T and X, respectively; Z_T and Z_X are valance of atoms T and X, respectively; μ is reduced mass and K is a constant. With μ in atomic mass units and λ in μm, Dachille and Roy [14] proposed the average value of K = 0.168. In this study, crystalline GeO₂ precursor was used to determine the K value, supposing that all germanium atoms are 4-fold coordinated. The FTIR peak in 853 cm⁻¹, substituted in the Dachille and Roy relation, yielded K = 0.163 which is in accordance with previous results [10,11].

Figure 3 shows the average coordination number of Ge atoms in 6-fold coordination that was evaluated with Equation 1. The average coordination number decreases from 5.38 to 5.14 by increasing bismuth content, with a step-up between GPB631 and GPB622 showing the existence of CN values of both six and four, and a gradual change in germanium coordination from six to four with the sudden coordination change in the intermediate region.