Utilization of infrared, Raman spectroscopy for structural analysis of alkali boro-germanate glasses

Melt quenching technique was used to prepare (30-x)Li2O–xNa2O–40B2O3–30GeO2 glasses (x = 0, 5, 10, 20 & 30 mol %). The room temperature infrared spectra of the powdered glass samples were recorded in the range of 400–2000cm−1. Raman measurements were performed in the range of 200–2000cm−1 at room temperature. Raman and Infrared spectroscopy was utilized for structural study of this glass system. Specific modes of vibration and peak position were identified from de-convolution of infrared and Raman spectra using Gaussian fitting. Ge–O–Ge bonds’ symmetrical stretching vibrations related to 3 members GeO4 rings were allotted to the IR band of 483 cm−1. The strongest Raman band was allotted to symmetrical stretching vibrations of Ge–O in GeO4 having one non-bridging oxygen with 3 bridging oxygens at 797 cm−1. The observed Raman and IR spectra of various specimens are utilized here to explain the structural characteristics of current glass.


Introduction
Germanium dioxide has been used for many years as a candidate for the formation of glass. Glasses made of GeO 2 have excellent thermal stability, high phonon energy, and excellent infrared transmission. Germanium-based glasses are promising candidates for use in waveguides, laser amplifiers, sensors, light and other optical devices due to the relatively low energy losses in the mid-infrared region [1,2]. These glasses are extensively used in modern optical fibres because of their chemical resilience, mechanical strength, and high refractive index. Multi-components glass shows nonlinear changes in characteristics when one of the alkali component is replaced by some other species [3,4]. Nonlinear characteristics are also seen in mixed alkali-alkaline earth oxide glasses [5][6][7]. This is called the mixed-alkali effect (MAE), and this phenomenon may be used to make low-loss electrical glass while also increasing the chemical robustness of the material. is predicted, resulting in a Germanate anomaly. When alkali oxide is added to GeO2, the density of the resultant glass increases with the amount of alkali oxide applied, rising at 10-20 mol% alkali oxide and subsequently declining. The type of the alkali oxide applied determines the anomaly maximum. Henderson [15,16] has addressed the nature of this behaviour as well as the possible structural mechanisms behind it. According to Raman investigations [17], there is no alteration in the germanium/boron coordination in GeO 2 -B 2 O 3 glasses.
The structural development of the glass network was investigated using Raman spectra and FT-IR absorption spectra because NIR emission sort of behaviour is shown to be responsive to glass network local structure [18,19] [26]. As per our literature review, the structural characteristics of (30-x)Li 2 O-xNa 2 O-40B 2 O 3 -30GeO 2 alkali borogermanate glasses are inadequately documented in the literature. Borate glasses with a wide range of composition and characteristics have an excellent scope in nonlinear as well as in linear optics. The goal of this article is to investigate the structure of (30-x)Li 2 O-xNa 2 O-40B 2 O 3 -30GeO 2 (x = 0, 5, 10, 20, and 30) on glasses using two important spectroscopic methods: Raman and IR. This research is essential since the recent results will give knowledge about the fundamental units that make up this multi-component glass structure.

Experiment
Melt quenching was utilized to make a sequence of mixed alkali glasses (30-x)Li 2 O-xNa 2 O-40B 2 O 3 -30GeO 2 (x = 0, 5, 10, 20, and 30 mol%). As starting ingredients, highly pure Na 2 CO 3 , Li 2 CO 3 , B 2 O 3 , and GeO 2 (99.99% pure, Sigma-Aldrich) were used. A stoichiometric ratio of 20 g of these powders blended well in an agate mortar before being liquefied in a ceramic vessel placed in an electric oven at a high temperature range of 1100-1200°C for about 30 min. The melt was periodically spun until a clear, bubble-free liquid was created. The resulting melt was warmed to 200°C and pressed with a steel disc that had been preheated to the same temperature. Different glass compositions were annealed for around 12 h at 200°C. Lack of any Bragg peaks in the x-ray diffraction patterns revealed that the glass samples were amorphous. The current materials were crushed and mixed with KBr to make pellets that were about 0.3 mm thick for FTIR examination. The IR absorption spectra at room temperature were obtained utilizing a Bruker Optics Spectrometer. The IR observations were taken between 400 and 2000 cm −1 . A Renishaw (UK) spectrometer was utilized for investigation of micro Raman spectra at 514 nm excitation wavelength. An external CCD sensor was used in combination with an NIR-adjusted Olympus LMPl microscope objective, and 1800 lines/mm grating.

Results and discussion
The most suited approach for glass studies is infrared transmittance spectroscopy. Figure 1 depicts the IR spectra of (30-x)Li 2 O-xNa 2 O-40B 2 O 3 -30GeO 2 measured in the region of 400-2000cm −1 . The apparent IR spectra of the present glass framework arises   Table 1. Deconvoluted parameters of the FT-IR spectra of the xNa 2 O-(30−x)Li 2 O-40B 2 O 3 -30GeO 2 glasses under study. C is the component band center (cm −1 ), A is the component band's relative area (percentage) and W is full width at half maxima. mainly from altered borate networks in active spectral range from 900 to 1600 cm −1 and from the altered germanate groups in the active spectral range of 450-770 cm −1 . The IR spectra with overlapping broad spikes are de-convoluted using 6 peaks and Gaussian functions [27][28][29]. The de-convoluted IR of (30-x)Li 2 O-xNa 2 O-40B 2 O 3 -30GeO 2 glass network for various x are shown in Figure 2. The deconvoluted specifications of the FTIR spectra of (30-x)Li 2 O-xNa 2 O-40B 2 O 3 -30GeO 2 are presented in Table 1.
The boron ion is a cation for the formation of glass network that may be found at the center of oxygen triangles or in tetrahedral structures [30]. The structure of boron oxide glass, as per Krogh Moe's model, is a randomized network of planar BO 3 triangles with a specified fraction of 6 member rings [31]. In general, between 600 and 1200 cm −1 , the infrared spectra of  The compositional dependency of IR band peak in the current investigation is depicted in Figure 3. The MAE is demonstrated by a nonlinear change in peak location for the IR bands located at approximately 1098 and 1555 cm −1 . It is evident from these graphs that the IR peak positions with composition deviate  from linearity. The IR reflectivity of mixed-alkali borate glasses was investigated by Kamitsos et al. who discovered that alkali mixing induced partial annihilation of BO 4 groups in favour of their BO 2 Oisomeric triangles. They also discovered that fraction of NBO exhibited a positive deflection from linearity [38]. By using IR spectroscopy along with nuclear magnetic resonance, Kamitsos et al. discovered that the alkali ion has a welldefined effect on the network structure: the proportion of tetrahedrally connected boron atoms reduces as the alkali ion size increases, or identically the ion's field strength increases [39]. The Raman spectra of varios specimens were examined in order to better understand how the glass characteristics changed when Na 2 O was added. Figure 4 shows Raman spectra of (30x)Li 2 O-xNa 2 O-40B 2 O 3 -30GeO 2 with various x. Using Gaussian functions, a Raman spectra with overlapped wide peak is de-convoluted into 5 peaks. Figure 5 depicts the de-convolution of Raman spectra using a Gaussian fit of (30-x)Li 2 O-xNa 2 O-40B 2 O 3 -30GeO 2 glass network for various x. Table 3 shows the  [40,41]. G.S. Henderson has also mentioned that the Ge-O-Ge bending modes are observed in the broad region between 500 and 620 cm −1 [42]. The strongest Raman band was assigned with asymmetrical stretching vibrations of Ge-O in GeO 4 having one non-bridging oxygen and three bridging oxygen at 797 cm −1 (Q 3 species) of current glass [43]. According to a literature study, the asymmetrical stretching vibration of four-fold coordinated Ge (i.e. stretching of Ge-O-Ge and O-Ge-O -, where Ois the non-bridging) is associated to the Raman signal between 700 and 900 cm −1 [43]. As a consequence, the Ge-O stretching vibration was attributed to the Raman peak about 797 cm −1 . For glasses, we detected a wide peak at 797 cm −1 and 1292 cm −1 , indicating that there are several vibrational modes linked together. The peak's breadth might be related to structural heterogeneity, which varies depending on ring diameters and the number of NBOs. The detected Raman band with an interval of 1045-1096 cm −1 in the current glass network was ascribed to the stretching vibration associated with B-O bonds in BO 4 unit [44][45][46]. Raman band 1285-1396 cm −1 was assigned to the vibrating mode of B-O terminal bonds of BO 3 triangles in this study [45,46]. In the current glass network, the B-O bond stretching vibration of polymerized BO 3 unit was assigned to the Raman band at 1437-1514 cm −1 [45]. Table 4 Figure 6. Normalized change of the intensity ratio (I 797 /I 538 ) between the Raman bands situated at 797 and 538 cm −1 , as well as a shifting of the peak of band 538 cm −1 with Na 2 O content. Figure 6 illustrates a normalized change of ratio of intensity between the Raman bands situated at 797 and 538 cm −1 , indicated as I 797 /I 538 , as well as a shifting of the peak of band 538 cm −1 with Na 2 O content. In the Raman spectra of glasses ( Figure 6), I 797 /I 538 initially increases with x up to x = 0.20, then begins to drop. Table 5 shows change of relative intensity I 797 /I 538 with Na 2 O content. Raman spectroscopy investigations of alkali borate glasses for various Na 2 O contents suggest the possibility of two chemical processes distributed throughout the glasses. The first step, which occurs at lower Na 2 O content, produces boron in four fold coordination with Na + ion providing local charge neutrality. The 2nd phase is the creation of NBO (O -) close to the Na + ion. The two chemical processes mentioned above can also be used to explain N 4 dependency.

Conclusions
The FTIR and Raman spectra of (30-x)Li 2 O-xNa 2 O-40B 2 O 3 -30GeO 2 glass systems were analyzed to identify local structural variations. In the glass network, we allocated vibrational modes to various sorts of atomic motions. The examined glasses' de-convoluted Raman and IR spectra demonstrated the existence of five Raman bands and six IR bands, respectively. IR absorption bands of the borogermanate glasses were allotted to bending oscillation of different borate sections and stretching oscillations of germanate unit. Symmetrical stretching vibrations of Ge-O-Ge bonds related to the IR band at 483 cm −1 . The intensity of this band diminished and IR band moved to longer wavelengths with x. The MAE was observed by a nonlinear change in peak location for the IR bands at 1098 and 1555 cm −1 . Different Raman bands of glass samples are approximated as 538, 797, 1040, 1396, and 1514 cm −1 . The symmetric stretching vibrations of Ge-O-Ge bonds connected to 3 section rings of GeO 4 tetrahedra were assigned to the Raman band centered at 538 cm −1 .

Disclosure statement
No potential conflict of interest was reported by the author(s).