Figure 1 shows the FT-IR spectrum of the x = 9 glass sample, for which absorption bands are clearly observed around 500, 720, 840, 980 - 1080 and 1200 - 1500 cm⁻¹. The corresponding assignments are listed in Table 1.

FT-IR spectra of glasses are generally interpreted using the method proposed by Tarte [12] and Condrate [13] , by comparing the experimental data with those of related crystalline compounds. In this study, the characteristic absorption bands for vitreous Bi₂O₃ [14] [15] and B₂O₃ [16] [17] were used. The absorption band centered at ~ 500 cm ⁻¹ is characteristic of the stretching vibrations of Bi-O bonds in strongly distorted BiO₆ octahedral units [14] [15] . These can overlap with B-O-B bond-bending vibrations [16] - [18] . The absorption band at ~ 715 cm ⁻¹ has been assigned to the symmetric stretching vibrations of Bi-O bonds in pyramidal BiO₃ units [20] , and this can overlap with O₃B-O-BO₃-bending vibrations [17] [19] . The absorption band at ~ 880 cm ⁻¹ arises from symmetric stretching vibrations of Bi-O bonds in pyramidal BiO₃ units [14] , possibly superposed with stretching vibrations of B-O bonds in the BO₄ units of diborate groups [17] . The band at ~ 990 cm ⁻¹ can be attributed to B-Q stretching vibrations in BQ₄ units from tri- (B₃O₅⁻), tetra- (B₈O₁₃²⁻), and penta-borate (B₅O₈⁻) groups [18] . In the Q_n nomenclature, n indicates the number of bridging oxygen for a given network-forming unit, in this case boron. The strong band at ~ 1200 cm ⁻¹ is assigned to stretching vibrations of B-O bonds in BO₃ units from

meta- and ortho-borate groups [19] [21] . The broad shoulder present in the spectrum at ~ 1295 cm ⁻¹ is assigned to the asymmetrical stretching vibrations of B-O bonds in BO₃ and BQ₂O^– units [19] [22] .

For all samples, FT-IR absorption bands for vitreous B₂O₃ were identified at ~720, ~1260, and ~ 1420 cm ⁻¹, which are attributed to B-O bond vibrations in trigonal BO₃ units [16] - [18] . The vibrational modes of the borate glass network appear in three infrared spectral regions [19] . The first group of bands, between 1200 and 1600 cm ⁻¹, arises from the asymmetric stretching relaxation of B-O bonds in BO₃ units. The second group, between 800 and 1200 cm ⁻¹, is due to B-O bond stretching in tetrahedral BO₄ units, and the third group, around 700 cm ⁻¹, is due to the bending of B-O-B connections in the borate network [20] . Characteristic absorption bands for crystalline ZnO were identified at ~430 and ~ 520 cm ⁻¹ and are attributed to Zn-O bond vibrations in tetrahedral ZnO₄ units [23] .

The assignments in Table 1 suggest that [BiO₆], [BiO₃], [BO₃], and [BO₄] units may be present in the Bi₂O₃-ZnO-B₂O₃-BaO quaternary glasses.

Further information on the structure of the glass can be gathered from the absorption peaks around 990 - 1080 cm⁻¹, 1200 cm ⁻¹, and 1300 - 1350 cm ⁻¹, that are characteristic of [BO₃] and [BO₄] units. The FT-IR absorption peaks around 500 cm ⁻¹, 720 cm ⁻¹, and 840 cm ⁻¹ can be attributed to the Bi-O bond in [BiO₆] units and to the B-O bond in [BO₃] units, or to the Bi-O bond in [BiO₃] units and the B-O bond in [BO₄] units. These attributions are however uncertain and can only be confirmed using Raman spectroscopy.

Figure 2 shows a typical Raman spectrum for these glasses, consisting of broad peaks in the 100 - 1600 cm⁻¹ range. For the x = 9 glass sample, peaks are observed around 130 cm ⁻¹, 390 cm ⁻¹, 575 cm ⁻¹, 920 cm ⁻¹, and 1250 cm ⁻¹.

The vibrational Raman and infrared spectra of the investigated glasses with 45 - 65 mol% bismuth oxide concentrations are dominated by bands associated with structural units built around the heaviest cation, Bi³⁺. The Raman bands arising from heavy metal oxides, such as Bi₂O₃, can be classified into four groups: 1) low wavenumber Raman modes (< 100 cm ⁻¹), 2) heavy metal ion vibrations in the range 70 - 160 cm⁻¹, 3) bridged anion modes in the intermediate 300 - 600 cm⁻¹ region, and 4) non-bridging anion modes at higher wavenumbers [24] .

The presence of a band around 135 cm ^{− 1 in Raman spectra is evidence for the presence of [BiO}₃] and [BiO₆]

polyhedra [25] . In the spectrum shown in Figure 2, this band is observed at 130 cm ⁻¹, showing that the Bi³⁺ cations are incorporated into [BiO₃] and [BiO₆] groups. Considering that the vibrations from bismuth groups appear at significantly lower wavenumbers than those from boron units, the peaks in the 120 - 650 cm⁻¹ range are assigned to bismuth units with a bridging anion [24] [26] . Thus, the broad and strong band at ~ 370 cm ^{− 1 in the present Raman spectrum can be attributed to Bi}-O-Bi vibrations, while the shoulder at 591 cm ⁻¹ is assigned to Bi-O^– stretching vibrations in [BiO₆] octahedral units. Similar observations have been made in lithium bismuthate [27] and other heavy metal oxide glasses [26] . The band around 921 cm ⁻¹ can be ascribed to isolated orthoborate groups [29] while the weak band from 1285 to 1309 cm ⁻¹, observed over the entire composition range, is due to Bi-O^– non-bridging oxygens (NBOs) in BiO₃ and pyroborate groups. Indeed, in the infrared spectra from these glasses, the band observed at 708 cm ⁻¹ is assigned to B-O-B bending vibrations in trigonal [BO₃] groups [21] , the band around 1218 cm ⁻¹ arises from Bi-O^– NBO stretching vibrations of [BiO₃] units, and the band at 1256 cm ⁻¹ is due to B-O^– vibrations. To facilitate the interpretation of the spectra, the different Raman peaks and their related functional groups are summarized in Table 2.

Figure 2 and Table 2 show that [BO₃], [BiO₆] and [BiO₃] units may be present in these boro-bismuthate glasses, but the characteristic peak for [ZnO₄] units around 254 cm⁻¹ is not clearly visible in Figure 2, suggesting that ZnO acts as a network modifier in this glass system.

Based on the FT-IR and Raman spectra obtained for this glass, the main network forming units are trigonal [BiO₃] and [BO₃] groups with a smaller number of [BO₄] tetrahedrons. However, [BiO₆] units are not incorporated in the glass network. Bi-O⁻ NBOs may be present as well as some Bi-O-Bi and bonds between [BiO₃] and [BiO₃] or between [BiO₃] and [BiO₆] groups. The connections between [BO₃] groups, and between [BO₃] and [BO₄] groups, may occur via B-O-B bonds or B-O⁻ links. ZnO acts as a network modifier and is not integrated in the glass network.

Bi₂O₃ plays a unique role in the structure of these glasses. The presence in large proportions of the two network formers, Bi₂O₃ and B₂O₃, leads to the formation of a vitreous structure with exceptional properties. Figure 3 shows the FT-IR spectra obtained for glasses in this system with different Bi₂O₃ contents, with the corresponding Raman spectra shown in Figure 4. Both the FT-IR and Raman curves shift slightly with increasing Bi₂O₃ contents.

The IR absorption spectra obtained from glasses in the Bi₂O₃-ZnO-B₂O₃-BaO system with different Bi₂O₃ contents are shown in Figure 3. The peak around 500 cm ⁻¹ clearly increases in intensity with increasing Bi₂O₃ content, indicating an increase in the number of [BiO₆] units. The peak around 720 cm⁻¹ also tends to increase in intensity, possibly due to increasing numbers of [BO₃] and [BiO₃] units. Similarly, the strengthening of the peak around may be attributed to increasing numbers of [BiO₃] and [BO₄] groups. Moreover, while the peak around 990 - 1080 cm⁻¹ does not change significantly in intensity, it shifts towards lower wave numbers, highlighting the tendency of [BO₄] units to become looser in the glass structure. Finally, the increase in intensity of the peaks around 1200 cm ⁻¹ and 1300 - 1350 cm⁻¹ is ascribed to increasing numbers of [BO₃] units in the glasses.

Figure 4 shows the Raman spectra obtained for glasses with different Bi₂O₃ contents. A clear increase in intensity is observed around 130 cm ⁻¹, which indicates increasing numbers of [BiO₃] and [BiO₆] units in the glass. Furthermore, from sample A₁ to A₅, the peaks shift to higher wavenumbers, from 124 cm ⁻¹ to 136 cm ⁻¹, highlighting an increasing proportion of [BiO₃] units. In these glasses, the network is formed around [BO₄] tetrahedrons and trigonal [BO₃] and [BiO₃] units but does not involve [BiO₆] units. Bi-O-Bi bonds might be formed between [BiO₃] units, or between [BiO₃] and [BiO₆] units; however, the vibration frequency of Bi-O-Bi bonds varies little between different structural units, since the vibration is stronger inside the network than outside.

The scattering peak around 390 cm ^{− 1 in the Raman spectra increases in intensity with increasing Bi}₂O₃ contents, indicating that the numbers of [BiO₃] and [BiO₆] units increase in the glass. The peak around 575 cm ⁻¹ also becomes stronger, highlighting an increase in the number of [BiO₆] units. Finally, the increased intensity of the peak around 1250 cm ⁻¹ may be ascribed to greater numbers of [BiO₃] and [BO₃] units.

To summarize, the FT-IR and Raman spectra of these glasses suggest that bismuth is present in the form of [BiO₆] and [BiO₃] units, and boron as [BO₃] and [BO₄] units. ZnO is not incorporated in the network and provides free oxygen atoms. In the glasses with higher Bi₂O₃ contents, a clear increase in the numbers of [BiO₆] and [BiO₃] units is evidenced. The FT-IR absorption peaks at 990 - 1080 cm⁻¹ shift to lower wavenumbers from samples A₁ to A₅, indicating that the [BO₄] units tend to become loose in the glass network, the latter being built mainly around trigonal [BO₃] groups.

Figure 5 shows the infrared spectra obtained for the glasses in group B. For all compositions, bands around 500 cm ⁻¹, 720 cm ⁻¹, 840 cm ⁻¹, 980 - 1080 cm ⁻¹, and 1200 - 1500 cm⁻¹ are observed. The assignments are similar to those described for group A, showing that the same four structural units—[BiO₃], [BiO₆], [BO₃], and [BO₄]— are present in these glasses.

The absorption band at 980 - 1080 cm⁻¹, attributed to stretching vibration in [BO₄] groups, increases in intensity from sample B₁ to B₅. Higher B₂O₃ contents result in stronger B-O-B bond vibrations from trigonal [BO₃] units, which appear around 1100 - 1400 cm^{− 1 in the FT-IR spectra. A similar increase is observed for the absorption peak at 720 cm −1}, arising from B-O-B bond stretching vibrations in trigonal [BO₃] groups and from symmetric Bi-O stretching vibrations in trigonal [BiO₃] units, indicating and increase in the number of these two groups. In contrast, the absorption peak at 470 cm ⁻¹, arising from Bi-O bond bending in [BiO₆] octahedrons, weakens.

Figure 6 shows the Raman spectra obtained for different glasses in group B, covering the spectral range of 100 - 1600 cm⁻¹. For the B₁ sample, scattering bands are clearly observed at 130 cm ⁻¹, 390 cm ⁻¹, 575 cm ⁻¹, 920 cm ⁻¹, and 1250 - 1500 cm⁻¹.

As they belong to the same glass system, the samples from groups A and B produce similar Raman peaks, as evidenced by comparing Figure 4 and Figure 6. However, as the B₂O₃ content increases, the scattering intensity at 130 cm ^{− 1 in the Raman spectra, arising from symmetric Bi}-O-Bi stretching vibrations in trigonal [BiO₃] groups and [BiO₆] octahedron, decreases, highlighting a corresponding decrease in the relative numbers of [BiO₃] and [BiO₆] units.

As the B₂O₃ content increases from sample B₁ to B₅, the band around 390 cm ⁻¹, due to symmetric Bi-O-Bi stretching vibrations in trigonal [BiO₃] units and [BiO₆] octahedrons, becomes stronger, reflecting enhanced crosslinking of the [BiO₃] and [BiO₆] groups in the glass network. The peak around 575 cm ⁻¹, associated with Bi-O bond stretching vibrations in [BiO₆] octahedrons, decreases in intensity with increasing B₂O₃ contents, indicating a decrease in the relative number of [BiO₆] units. However, no clear variation is observed for the Raman scattering peak at 920 cm⁻¹, attributed to B-O bond stretching vibrations in the orthoborate formed by BO₃ pyramidal units. For all compositions, the Raman scattering peak at 1250 - 1500 cm⁻¹ arises mainly from the

Bi-O⁻ NBO of [BiO₃] units and the B-O-B stretching vibration of [BO₃] units. As the B₂O₃ content is increased in going from sample B₁ to B₅, this peak weakens, indicating a relative decrease and increase in the number of [BiO₃] and [BO₃] units, respectively. Indeed, the decrease in the Raman peak around 1250 - 1500 cm⁻¹ suggests that the decrease in the number of [BiO₃] units is more substantial than the increase in the number of [BO₃] groups.

The above analysis of the B samples suggests that [BiO₆], [BiO₃], [BO₃], and [BO₄] units are present in the glasses. With increasing B₂O₃ content, the relative numbers of [BO₃] and [BO₄] units increase with a corresponding decrease in the number of [BiO₃] and [BiO₆] units. It is notable however that no Raman peak assignable to [ZnO₄] tetrahedrons appears in the group B glasses.