Raman and Infrared Spectroscopy of Barium-Gallo Germanate Glasses Containing B2O3/TiO2

Previous reports showed that many types of germanate glass had been studied using vibrational spectroscopy [46,47,48,49]. Initially, the precise determination of the network units characterized by multicomponent germanate matrices was a complicated task due to the character of composed network-forming or network-modifier oxides. The network of germanate glass is formed by tetrahedral GeO₄ structural units, which share their corners, and the Ge atom is covalently bonded to four bridging oxygens. The thermodynamic instability of the GeO₆ octahedral units produces a large concentration of nonbridging oxygen ions. This evolution clearly indicates the conversion of [GeO₄] → [GeO₆] structural units [50]. According to the literature [51], the vibrational spectrum of the germanate glasses is rather remarkable. The germanate matrix is characterized by the structural units’ dominant contributions in low- and high-frequency regions. The low-frequency region is mainly characterized by a peak around 560 cm⁻¹ and is associated with the bending vibrations of Ge-O-Ge. The high-frequency region is reported to contain a band at approx. 915 cm⁻¹, and low inflections at approx. 1000 cm⁻¹ are attributed to the asymmetric vibrations of the Ge-O-(Ge) bridges. These bands occur in a typical infrared spectrum when the glassy germanium oxide is composed of germanium–oxygen tetrahedra with nonbridging oxygens. It was repeatedly demonstrated that changes in the local structure of the germanate network as the alkali concentration [52,53] increased resulted in a systematic shift in the band components associated with the vibrations of the GeO₄ units. This is evidenced by the broken Ge-O^– bridges at about 750 and 870 cm⁻¹ for the Q2 and Q3 units. Comprehensive studies of germanate glasses have also provided strong evidence regarding the replacement of GeO₄ by other units, including germanate–oxygen octahedra (GeO₆). This strong modification is demonstrated by the appearance of a band at about 715 cm⁻¹ in the midinfrared spectrum, evidently involving a change in the coordination number of the germanium ions from four (LK = 4) to six (LK = 6) [54,55]. According to the paper published by McKeaon and Marzbacher [56], when the GeO₂ content decreases, the midfrequency envelope shifts to higher frequencies while the high-frequency features shift to lower frequencies. It was interpreted as a reduction in the average ring size, as well as an average lengthening of the T-O (where T is Ge or Ga) band.

Figure 8 shows the measured FT-IR and Raman spectra in the wavelength range of 400–1600 cm⁻¹ of barium gallo-germanate glasses with various GeO₂:TiO₂ molar ratios. The spectra exhibited two groups of bands (i) in the low-frequency region located from 400 to 600 cm⁻¹ and (ii) in the high-frequency region from 700 to 900 cm⁻¹. A single band dominates the first region of 400–600 cm⁻¹ due to the GeO₄ structural units, which share their corners, where the germanium atom bending is covalently bonded to four biding oxygens. The second high-frequency region, between 620–900 cm⁻¹, is attributed to the GeO₆ structural units, where the central atom is germanium and is surrounded by six oxygen atoms [57,58]. As expected, the molar ratio of TiO₂ strongly affected these structural properties of glasses. As one can see from Figure 8, the intensities of the IR and Raman bands related to the GeO₄ and GeO₆ structural units underwent significant changes by incorporating the TiO₂ content into the germanate glass host. With the introduction of TiO₂ up to 30mol% in the GeO₂-BaO-Ga₂O₃ glass network, the intensity band centered at about 450 cm⁻¹ and 800 cm⁻¹ was observed to significantly decrease with the shifting towards the lower frequency region. However, when the TiO₂ concentration was greater than 30mol%, it was observed that the band due to the GeO₆ structural units was shifted to higher frequencies. The explanation for these results lies in the dual role of titanium dioxide in the glass network. Titanium dioxide acted both as the network modifier and network former, which participated during the formation of the glass network in the form of TiO₄ or existed in the gap outside the network in the form of TiO₆ units [57,58,59,60]. In this work, the effect observed with the increased TiO₂ content very well confirms that the doping titanium ions of the germanate matrix generated a strong destruction of germanate tetrahedra and octahedra units caused by the formation of more Ti-O structural units. We considered the Raman spectrum for the GeO₂:TiO₂ = 1:5 (GT3) glass sample when titanium dioxide acted as a network former. The main problem with this material is the interpretation of the local structure due to the overlapping bands. The most interesting observation concerning all registered spectra was the presence of a band in the frequency range of 600–700 cm⁻¹. The mentioned band was very detectable with the increase in the amount of titanium ions introduced at the expense of germanium ions, which resulted in a systematic decrease in the amount of GeO₄ tetrahedrons and GeO₆ octahedrons. According to the data, this Raman peak is considered strong evidence of Ti-O stretching vibration connected with the TiO₆ unit. Earlier studies of other glasses containing TiO₂ exhibited a well-resolved band at about ~720 cm⁻¹ identified due to the vibration of TiO₄ structural units [61,62].

As a final point of our investigations, we characterized the structural properties of the barium gallo-germanate glass host containing B₂O₃ (Figure 9). As expected, independently, the chemical composition germanate glass structure resulted in the appearance of the spectra, revealing signals from the bending and stretching modes of the GeO₄ and GeO₆ structural units. In contrast to TiO₂, the addition of boron oxide was found to be a weak scatterer in low-frequency and high-frequency ranges between 400–1600 cm⁻¹. Adding B₂O₃ to glass causes progressive changes in the low and higher frequency range. These changes are accompanied by the decreasing of a strong band at 450 cm⁻¹ (almost vanishing at 50 mol% B₂O₃ in the FT-IR spectrum) and the one at 800 cm⁻¹ with increasing borate content and shifts to lower frequencies. The obtained results indicated that the Raman shift decreased from 1411 cm⁻¹ (glass GeO₂-rich composition) to 1340 cm⁻¹ (glass B₂O₃-rich composition). A clear correspondence was observed between the bands in the Raman spectra and the measurements carried out of the infrared spectra for the obtained glasses. In general, in the boron-based glass host, boron is three-coordinated. Moreover, previous studies indicated the presence of trigonal and tetrahedral boron with different ratios and the partial conversion of BO₃ into BO₄ units [63,64].