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

Abstract

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.

KEYWORDS:

• Raman and infrared spectroscopy
• Mixed alkali effect
• Borogermanate glass

1. Introduction

Germanium dioxide has been used for many years as a candidate for the formation of glass. Glasses made of GeO₂ 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 [5–7]. 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. Many factors influence the MAE's strength, including temperature, concentration of total alkali, radius and mass difference of mixed alkali ions, and so on [8–10]. MAE can be observed in phosphates, silicates, tellurite, tungstate, and borate glasses, among other glass systems [11–13]. Moulton et al. discussed MAE of Both alkali and alkaline-earth silicate glasses and their numerous industrial applications in window glass, fibre optics and biomaterials [14]. Glasses with a mixed alkali effect can be employed to develop materials with specific qualities and modify their features to meet specific industrial requirements. Addition of alkali oxide in GeO₂ glass increases density, but only to a maximum before declining. This is called Germanate anomaly in the glass. This pattern may be seen in a variety of physical characteristics like refractive index, density, hardness, elastic moduli etc in the presence of alkali oxides [10]. When the alkali oxide content of the GeO₂ network rises, the transformation of GeO₄ tetrahedra to GeO₆ octahedra 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 GeO₂–B₂O₃ 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]. Borate glasses have been shown to be good hosts for studying mixed alkali phenomena. Glasses containing B₂O₃ have been the subject of a number of investigations on combined alkali effects. To explore the influence of changes in the alkali fractions on the EPR parameters, EPR investigations were executed in the xK₂O:(30–x)Li₂O–10CdO–59B₂O₃–1MnO₂ amorphous system [20]. S Rahman et al. investigated the mass density, ionic conductivity, and glass transition temperature of (40-x)Na₂O:xLi₂O–50B₂O₃–10Bi₂O₃ and (40-x)Na₂O:xK₂O–50B₂O₃–10Bi₂O₃ borate glasses with Na₂O concentrations ranging from 0 to 40% [21]. Edukondalu et al. used the melt quench method to create xLi₂O–(30-x)Na₂O–10WO₃–60B₂O₃ alkali borotungstate glasses and then investigated FT-IR and Raman spectroscopy [22]. Although there is research on alkali germanate glasses, and there are few publications on the mixed alkali effect in borate glasses, including GeO₂. Brian M. Walsh et al. published a spectroscopic investigation of Tm-doped germanate glass for luminescence decay, absorption, and emission [23]. Density of states, DC conductivity and physical properties of Ag2S-Ge–Te–Se chalcogenide glassy system has been discussed by Chamuah et al. in the light of DC conductivity model and variable range hopping model [24]. Franco et al. have investigated thermal, structural properties of Germanium–Aluminum–Borate glass (85-x)GeO₂–(x)B₂O₃–10Na₂O–4Al₂O₃–1PbO using DSC, Raman scattering and NMR spectroscopies [25]. Liu et al. studied ultra broadband near-infrared emission in RO–Al₂O₃–B₂O₃–GeO₂ with 0.005 mol% Bi₂O₃ doping under 460 nm excitation, principally consisting of 3 bands [26]. As per our literature review, the structural characteristics of (30-x)Li₂O–xNa₂O–40B₂O₃–30GeO₂ 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)Li₂O–xNa₂O–40B₂O₃–30GeO₂ (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.

2. Experiment

Melt quenching was utilized to make a sequence of mixed alkali glasses (30-x)Li₂O–xNa₂O–40B₂O₃–30GeO₂ (x = 0, 5, 10, 20, and 30 mol%). As starting ingredients, highly pure Na₂CO₃, Li₂CO₃, B₂O₃, and GeO₂ (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.

3. Results and discussion

The most suited approach for glass studies is infrared transmittance spectroscopy. Figure 1 depicts the IR spectra of (30-x)Li₂O–xNa₂O–40B₂O₃–30GeO₂ measured in the region of 400–2000cm⁻¹. The apparent IR spectra of the present glass framework arises 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 [27–29]. The de-convoluted IR of (30-x)Li₂O–xNa₂O–40B₂O₃–30GeO₂ glass network for various x are shown in Figure 2. The deconvoluted specifications of the FTIR spectra of (30-x)Li₂O–xNa₂O–40B₂O₃–30GeO₂ are presented in Table 1.

Figure 1. Infrared spectra of present glasses (x = 0, 5, 10, 20 and 30).

Figure 2. The de-convoluted IR spectra of (30−x)Li₂O–xNa₂O–40B₂O₃–30GeO₂ glass.

Table 1. Deconvoluted parameters of the FT-IR spectra of the xNa₂O–(30−x)Li₂O–40B₂O₃–30GeO₂ 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.

X = 0			X = 5			X = 10			X = 20			X = 30
C	A	W	C	A	W	C	A	W	C	A	W	C	A	W
483	1.7	99.9	505	4.1	117	505	3.8	110	483	4.4	112	513	3.2	114
779	8.8	156	760	23.1	249	771	8.7	191	743	12.1	249	783	28	267
949	26.8	205	971	21.4	211	964	31.6	212	–	–		984	15.9	188
–	–		–	–	–	–	–	–	805	14.9	295	–	–	–
1098	12.6	139	1094	5.6	108	1108	19.1	109	1052	22.8	261	1102	5.3	106
1380	29.2	262	1363	22.1	274	1322	22.1	277	1344	21.1	281	1368	26.3	303
1555	20.7	364	1515	22.1	428	1461	14.5	333	1572	23.5	317	1531	21.3	499

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 BO₃ triangles with a specified fraction of 6 member rings [31]. In general, between 600 and 1200 cm⁻¹, the infrared spectra of vitreous B₂O₃ exhibits no active modes, while IR examination of modified borate glasses indicates discrete frequency areas in the mid-infrared range, where the vibrations of the B–O configuration dominate. IR absorption bands of borogermanate glasses are ascribed to bending vibrations of different borate units and stretching vibrations of germanate units. B–O–B stretching vibrations of polymerized BO₃ are attributed to the infrared band approximately 1380 cm⁻¹ of present glass, in which the boron atom is associated with 3 oxygen atoms in the BO₃ unit [32]. The B–O asymmetric stretching of tetrahedral BO₄ units is allocated to the next band at 1098 cm⁻¹ [33]. In the Li₂O–B₂O_3–GeO₂ glass system, H. Doweider et al. detected IR bands at 1080 and 1390 cm⁻¹, which were attributed to boron–oxygen bond vibrations and B–O–B stretching state of chemically bonded BO₃ groups respectively [33]. The symmetrical stretching oscillations of Ge–O–Ge bonds related to the three members of GeO₄ rings were assigned to the IR band at 483 cm⁻¹ in the current glasses. The intensity of 483 cm⁻¹ decreases as Na₂O concentration rises, and the IR band's core shifts to longer wavelengths. Sahar et al. discovered an IR band of range 448–493 cm⁻¹ in Na₂O–P₂O₅–GeO₂ glasses, which they assigned to symmetric stretching states of Ge–O–Ge bonds [34]. A moderate shoulder in the current glass system's infrared spectra was detected at 779 cm⁻¹ and it was allotted to the stretching vibration of Ge–O in GeO₄ [35]. In Alumina-bearing lanthanide borogermanate glasses, Sigaev et al. detected an IR band at 800 cm⁻¹, which they linked to stretching Ge–O vibration in GeO₄ units [35]. Stretching vibrations of B–O bonds in BO₄ units from diborate groups are assigned to the 948 cm⁻¹ IR band, while stretching vibrations of B–O bonds in BO₄ units from pentaborate groups is attributed to the 1098 cm⁻¹ band in the current glasses [36]. In the present glass system, a weak IR band at 1555 cm⁻¹ is coupled to asymmetric stretching vibrations with 3 non-bridging oxygen (NBO) of B–O–B groups [37]. The IR band of about 1570 cm⁻¹ in AgI–Ag₂O–GeO₂–B₂O₃ glasses is ascribed to the B–O–B stretching vibrational network for different “xˮ modes of polymerized BO₃ units [37]. Table 2 assigns the Fourier transform of IR spectra of the (30-x)Li₂O–xNa₂O–40B₂O₃–30GeO₂ glass scheme.

Table 2. Fourier Transform Infrared allocations in (30-x)Li₂O–xNa₂O–40B₂O₃–30GeO₂ glass network.

x = 0	x = 5	x = 10	x = 20	x = 30	IR allocations
483	505	505	483	513	3 members GeO4 rings are related with symmetrical stretching vibrations of Ge–O–Ge bonds.
769	760	771	743	783	Stretching vibration of Ge–O polyhedral (GeO4).
949	971	964	805	984	Stretching vibrations of B–O bonds in BO4 units from diborate groups.
1098	1094	1108	1052	1102	Stretching vibrations of B–O bonds in BO4 units from pentaborate groups
1380	1363	1422	1344	1368	B–O bond stretching vibrations of polymerized BO3.
1555	1515	1461	1572	1531	Asymmetric stretching vibration with 3 NBO of B–O–B groups.

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 BO₄ groups in favour of their BO₂O^– isomeric 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 well-defined 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 Na₂O was added. Figure 4 shows Raman spectra of (30-x)Li₂O–xNa₂O–40B₂O₃–30GeO₂ 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)Li₂O–xNa₂O–40B₂O₃–30GeO₂ glass network for various x. Table 3 shows the de-convoluted characteristics (band centre and relative percentage area of band) of the Raman spectra of the (30-x)Li₂O–xNa₂O–40B₂O₃–30GeO₂ glass system.

Figure 3. The Compositional dependency of several (1110 and 1500 cm⁻¹) IR band peak locations.

Figure 4. Raman spectra of current glass (30−x)Li₂O–xNa₂O–40B₂O₃–30GeO₂ with x = 0, 5, 10, 20, 30.

Figure 5. De-convoluted Raman spectra of present glasses using Gaussian functions.

Table 3. Deconvoluted parameters of the Raman spectra of the xNa₂O–(30−x)Li₂O–40B₂O₃–30GeO₂ glasses under present investigation.

X = 0			X = 5			X = 10			X = 20			X = 30
C	A	W	C	A	W	C	A	W	C	A	W	C	A	W
538	16.8	189	546	8.1	143	558	6.7	124	577	5.2	103	565	1	137
797	14.3	154	845	30.4	250	838	41.5	348	884	60.3	484	796	24.2	231
1040	23	270	1090	25.9	182	1095	14.4	152	1094	10.7	122	1071	35.2	225
1395	19.1	292	1294	2.5	182	1292	3.8	147	1285	4.1	143	1324	3.1	152
1514	26.8	312	1436	33.1	357	1501	33.6	453	1509	19.7	303	1501	37.5	495

Different Raman bands of glass samples are approximated as 538, 797, 1040, 1396, and 1514 cm⁻¹. The Raman band centered at 538 cm⁻¹ should have originated from the bending vibration of Ge–O–Ge [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 GeO₄ having one non-bridging oxygen and three bridging oxygen at 797 cm⁻¹ (Q³ 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 O^– is 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 BO₄ unit [44–46]. Raman band 1285–1396 cm⁻¹ was assigned to the vibrating mode of B–O terminal bonds of BO₃ triangles in this study [45,46]. In the current glass network, the B–O bond stretching vibration of polymerized BO₃ unit was assigned to the Raman band at 1437–1514 cm⁻¹ [45]. Table 4 shows the Raman band allocations of (30-x)Li₂O–xNa₂O–40B₂O₃–30GeO₂ glass network. Subhashini et al. also detected band about 1350 cm⁻¹ with shoulder around 1120–1250 cm⁻¹ in Na₂O–Li₂O–K₂O–ZnO–B₂O₃ glass system, which were attributed to the BO₂O⁻ triangles connected with BO units [47].

Table 4. Wave numbers and allocations of Raman spectra of (30-x)Li₂O–xNa₂O–40B₂O_3–30GeO₂ present glasses.

Position of Raman band (cm^−1)	Raman allocations
538.0 ∼ 577	Ge–O–Ge bonds’ symmetrical stretching oscillations linked to 3 members’ rings of GeO4 tetrahedra.
796 ∼884	Ge–O symmetric stretching vibrations in GeO4 with one NBO and three bridging oxygens.
1040 ∼1095	B–O bonds’ stretched oscillations in unit of BO4.
1285∼1396	Vibrations of B–O^– terminal bonds of triangle BO3.
1436∼1514	B–O bond stretching vibration of polymerized BO3.

Figure 6 illustrates a normalized change of ratio of intensity between the Raman bands situated at 797 and 538 cm⁻¹, indicated as I₇₉₇/I₅₃₈, as well as a shifting of the peak of band 538 cm⁻¹ with Na₂O content. In the Raman spectra of glasses (Figure 6), I₇₉₇/I₅₃₈ initially increases with x up to x = 0.20, then begins to drop. Table 5 shows change of relative intensity I₇₉₇/I₅₃₈ with Na₂O content. Raman spectroscopy investigations of alkali borate glasses for various Na₂O contents suggest the possibility of two chemical processes distributed throughout the glasses. The first step, which occurs at lower Na₂O 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 N₄ dependency.

Figure 6. Normalized change of the intensity ratio (I₇₉₇/I₅₃₈) 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 Na₂O content.

Table 5. Change of relative intensity I₇₉₇/I₅₃₈ with Na₂O content.

Composition  x	I797/I538	Peak position of band 538cm^−1
0	0.92	538
5	2.14	554
10	2.25	565
20	2.47	582
30	1.35	588

4. Conclusions

The FTIR and Raman spectra of (30-x)Li₂O–xNa₂O–40B₂O₃–30GeO₂ 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 GeO₄ 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).

Additional information

Funding

This research was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2022R1), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia. The authors express their gratitude to the Deanship of Scientific Research at King Khalid University for funding this work through the Research Group Program under grant number RGP 2/195/43.