In the present investigation, glass samples of the desired composition xCaF₂–(20−x)ZnF₂–20Bi₂O₃–59.8B₂O₃–0.2Cr₂O₃ (0 ≤ x ≤ 20 mole%) were prepared by the classical melt-quenching technique. Analar grade chemicals CaF₂ (Himedia), ZnF₂ (Himedia), Bi₂O₃ (Sigma-Aldrich), H₃BO₃ (Sigma-Aldrich), and Cr₂O₃ (Sigma-Aldrich) were used in preparation. These starting materials were taken in proportions and mixed homogeneously in an agate mortar with a pestle. These mixtures were melted in porcelain crucibles at 1173 K in an electrical furnace for 45 min. To attain better homogeneity, the mixture was stirred periodically. The bubble-free transparent melt was poured onto a pre-heated stainless steel (SS) plate and subsequently pressed by the other SS plate to obtain glass samples. The glass compositions are given in Table 1.

Table 1.

Table 1.

Table 1. Compositions of xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0≤ x ≤ 20 mole%) glass samples in mole%. . Glass code CaF2 ZnF2 Bi2O3 B2O3 Cr2O3 CZBBCR1 0 20 20 59.8 0.2 CZBBCR2 5 15 20 59.8 0.2 CZBBCR3 10 10 20 59.8 0.2 CZBBCR4 15 5 20 59.8 0.2 CZBBCR5 20 0 20 59.8 0.2

Table 1. Compositions of xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0≤ x ≤ 20 mole%) glass samples in mole%. .

A Philips Xpert PRO XRD (Pan Analytical) model was used for recording XRD spectra. The 2θ values were recorded over the range from 10° to 80°. Density (ρ) measurements were carried on CZBBCR glasses using Archimedes principle in which xylene (ρ_b = 0.865 gm/cc) is used as the immersion liquid. For each glass composition, a minimum of three samples were used to estimate the density. A random error in the measurement of density values was about ± 1%. The JASCO V-760 UV–visible spectrophotometer, built with a double beam monochromator for obtaining high resolution, is used for recording optical absorption spectra of the polished samples in absorption mode at room temperature (RT). Electron paramagnetic resonance (EPR) spectra of the samples were recorded at RT using (BRUKER) EPR spectrometer at frequency (9.7 GHz) with a modulating frequency of 100 kHz. The errors in ‘g’ and ‘A’ values are about ± 0.002 and ± 2 × 10⁻⁴ cm⁻¹, respectively. The FTIR spectrum of the samples is scanned in the range 400–1600 cm⁻¹ on JASCO-FTIR-4600 spectrophotometer with an accuracy of ± 0.01 cm⁻¹ and resolution of 0.7 cm⁻¹ at RT. A KBr pellet technique was used, 1:100 ratios of powdered glass sample and KBr are mixed and obtained in the form of a pellet under an applied load. The sample in pellet form is used for recording the FTIR spectrum of the glasses. The Raman measurements were performed on CZBBCR glasses in the range 200–1600 cm⁻¹ on a Jobin–Yvon Horiba (LABRAM HR-800) system inbuilt with high stable confocal microscope for micro Raman 10x, 50x, 100x objective lens. A suitable objective lens can be taken to focus the laser onto the spot from ∼ 1–5 micron.

X-ray diffraction studies of the samples confirm the crystalline structure with sharp Braggs characteristics peaks. The sharp peaks are due to long range orderliness in the materials. The absence of such sharp peaks indicates the short range order. Particularly in glassy materials only short range order is prevailing and shows no sharp peaks. X-ray diffraction patterns of CZBBCR glasses have shown broad humps over the range from 10° to 80° of 2θ values as indicated in Fig. 1. The absence of Bragg peaks in the spectra did not show any crystalline nature.

Fig. 1.

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	Fig. 1. XRD spectra of xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0 ≤ x ≤ 20 mole%) glass samples.

Density (ρ) is the primary physical property of any solid material, which prospects the strength and structural changes in any material. Its variation depends on many physical quantities like coordination number, compactness, etc. Density plays an important role in understanding the modification of BO₃ into four fold BO₄ units, and vice versa.^[1,34–36] In the present study, it was found that the density decreases from 5.236 gm/cc (CZBBCR1) to 4.819 gm/cc (CZBBCR5) with increasing CaF₂ concentration. The molar volume (V_m) was estimated from density and the molecular weight (M) of the glass sample. Figure 2 shows the response of ρ and V_m with CaF₂.

Fig. 2.

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	Fig. 2. Variation of density and molar volume with CaF2 in xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0 ≤ x ≤ 20 mole%) glass samples.

From previous literature, the variation in density is explained using the physical parameters like crystal density, molecular weight and/or ionic radius.^{[1,34,37–39]} The gradual enhancement in the mole percentage of CaF₂ in the CZBBCR glass system caused a nonlinear decrease in the density, which is suitably explained using the crystal density of calcium fluoride (3.18 gm/cc) which is lesser than zinc fluoride (4.95 gm/cc). It can also be assigned to fractional replacement of low molecular weight CaF₂ (78.07 g/mol) by the high molecular weight ZnF₂ (103.406 g/mol). The decrease in density may be due to the conversion of [BO₄] into [BO₃] units. It was reported that the [BO₄] structural units are denser than [BO₃] structural units, hence decrease in strong connectivity in the glass network results in an increase in the inter-atomic spacing and forms less compact glass.

Figure 3 shows optical absorption spectra of CZBBCR glass samples. All the spectra have shown characteristic peaks at around 440 nm, 610 nm, and 680 nm, and are ascribed to ⁴A_2g → ⁴T_1g, ⁴A_2g → ⁴T_2g, ⁴A_2g → ²E transitions, respectively. Crystal field and Rachah parameters (D_q, B, and C) have been calculated using these transitions and the following relations and presented in Table 3.^[34,40–43]

Fig. 3.

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	Fig. 3. Optical absorption spectra of xCaF2–(20−x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0 ≤ x ≤ 20 mole%) glass samples. Inset: absorption spectra of CZBBCR5 sample with peak positions.

In the present investigation of CZBBCR glass samples, the B value is 515 cm⁻¹ for CZBBCR1, 625 cm⁻¹ for CZBBCR2, 575 cm⁻¹ for CZBBCR3, 565 cm⁻¹ for CZBBCR4, and 694 cm⁻¹ for CZBBCR5 glass sample. It can be seen that the obtained B values for all CZBBCR glass samples are far less than the free-ion B value 1030 cm⁻¹. The lesser value of B suggests that the d-shell electrons are shielded by boron network in CZBBCR glasses. D_q/B value estimates the strength of the crystal field. It is well known that from earlier work, for weak crystal field D_q/B ≪ 2.3, for strong fields D_q/B ≫ 2.3, and for intermediate crystal fields D_q/B is 2.3.^[44] In the present glass system, the D_q/B values for all the samples are greater than 2.3, which suggests that the Cr³⁺ ions are in the strong crystal field.

Figure 4 shows the Tauc plots of CZBBCR glass samples. Optical band gap values were evaluated using Davis and Mott^[45] and Tauc and Menth^[46] relation for indirect optical transitions.

Fig. 4.

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	Fig. 4. Tauc plots of xCaF2–(20 − x) ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0 ≤ x ≤ 20 mole%) glass samples.

The optical band gap value for CaF₂ free sample (CZBBCR1) is 2.143 eV, whereas for CZBBCR2 the band gap is 2.691 eV and seems to decrease with the increase in composition of CaF₂. Only for the 5 mol% addition of CaF₂ to the glass network, the band gap is enhanced. Further, the gradual increase of CaF₂ (replacement of ZnF₂ with CaF₂, i.e., Zn²⁺ with Ca²⁺) decreases the band gap value. As electronegativity of Zn²⁺ (1.65) is greater than Ca²⁺ (1.00), ionicity of fluorine ions increases with the increase of CaF₂ and hence band gap values decrease from CZBBCR2 to CZBBCR4.

Figure 5 shows the variation of Urbach energy (ΔE) with CaF₂ mole%. The ΔE values vary from 0.882 eV (CZBBCR1) to 0.4276 eV (CZBBCR5). The materials with larger ΔE values are believed to have a greater tendency to convert weak bond into defects.^[47,48] The obtained values of ΔE for the present glass system decrease nonlinearly with the increase of CaF₂, suggesting an increase of more orderliness.

Fig. 5.

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	Fig. 5. Variation of Urbach energy with CaF2 in xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0 ≤ x ≤ 20 mole%) glass samples.

The refractive index (n), molar refraction (R_m), and molar polarizibility (α_m) values of the CZBBCR samples are evaluated from the following relations:^[49,50]

Table 2.

Table 2.

Table 2. Molecular weight (M), density (ρ, gm/cc), molar volume (Vm, cc/mole), Urbach energy (ΔE, eV), optical band gap (Eopt, eV), refractive indexes (n), molar refraction (Rm, cc/mole), and molar electronic polarizability (αm, cc/mole) of xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (5 ≤ x ≤ 20 mole%) glass samples. . Glass code M ρ Vm ΔE Eg n Rm αm/10−24 CZBBCR1 188.12 5.236 35.93 0.882 2.143 2.677 24.15 9.26 CZBBCR2 186.86 4.944 37.79 0.7359 2.691 2.486 23.93 9.18 CZBBCR3 185.59 4.843 38.32 0.8624 2.651 2.498 24.37 9.34 CZBBCR4 184.32 4.833 38.14 0.5867 2.615 2.509 24.35 9.33 CZBBCR5 183.06 4.819 37.98 0.4276 2.544 2.532 24.43 9.36

Table 2. Molecular weight (M), density (ρ, gm/cc), molar volume (Vm, cc/mole), Urbach energy (ΔE, eV), optical band gap (Eopt, eV), refractive indexes (n), molar refraction (Rm, cc/mole), and molar electronic polarizability (αm, cc/mole) of xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (5 ≤ x ≤ 20 mole%) glass samples. .

The theory of EPR spectra related to chromium ions was reported by many researchers.^{[30,34,37,51,52]} Figure 6 shows the EPR spectra of xCaF₂–(20 − x)ZnF₂–20Bi₂O₃–59.8B₂O₃–0.2Cr₂O₃ (x = 0, 5, 10, 15, and 20 mole%) glass samples. EPR spectra exhibited an intense resonance signal centered at g ≈ 4.3 and g ≈ 1.97. The g values varied from 4.265 to 4.316 and 1.988 to 1.993 for low field and high field portion, respectively. According to the Landry theory, the spectrum on the lower field side (g ≈ 4.3) is from the isolated ions of chromium, whereas the high field side (g ≈ 1.99) is due to exchange coupled pairs Cr³⁺–Cr³⁺.^[53–55]

Fig. 6.

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	Fig. 6. EPR spectra of xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0 ≤ x ≤ 20 mole%) glass samples.

Noticeable change in g values is not observed, but nonlinear reduction in the intensity of the EPR spectra at g ≈ 2.0, i.e., in the high field region, has been noticed as we move from CZBBCR1 to CZBBCR5 glass sample. Hence, on increasing CaF₂ concentration, interaction of the exchange coupled pairs of chromium ions decreases.

The number of spins (N) participating in the resonance and the magnetic susceptibility (χ) were calculated and are given in Table 4.^[56,57] The N value for CZZBBCR glasses decreases nonlinearly from CZBBCR1 to CZBBCR5, which is attributed to the decrease in the intensity of EPR signal. The χ values vary proportionally with N.

Table 3.

Table 3.

Table 3. Absorption peak positions, corresponding transitions and Racah parameters (Dq/B, B, and C) for xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0 ≤ x ≤ 20 mole%) glass samples. . Glass code 4A2g → 4T1g 4A2g → 4T2g 4A2g → 2E Dq B Dq/B C λ/nm cm−1 λ/nm cm−1 λ/nm cm−1 CZBBCR1 456 21929 606 16501 720 13888 1650.1 515 3.200 3314 CZBBCR2 439 22779 608 16447 725 13793 1644.7 625 2.629 3043 CZBBCR3 442 22624 600 16666 728 13736 1666.6 575 2.895 3131 CZBBCR4 449 22271 609 16420 714 14005 1642.0 565 2.905 3243 CZBBCR5 436 22935 620 16129 718 13927 1612.9 694 2.325 2944

Table 3. Absorption peak positions, corresponding transitions and Racah parameters (Dq/B, B, and C) for xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0 ≤ x ≤ 20 mole%) glass samples. .

Table 4.

Table 4.

Table 4. g-value, number of spins (N), and magnetic susceptibility (χ) of xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0 ≤ x ≤ 20 mole%) glass samples. . Glass code g (low field side) g (high field side) N(1022) per Kg χ/10−3 m3/Kg CZBBCR1 4.265 1.992 5.22 12.54 CZBBCR2 4.316 1.988 5.16 12.34 CZBBCR3 4.301 1.993 4.54 10.92 CZBBCR4 4.310 1.990 3.61 8.65 CZBBCR5 4.274 1.992 2.08 4.99

Table 4. g-value, number of spins (N), and magnetic susceptibility (χ) of xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0 ≤ x ≤ 20 mole%) glass samples. .

Structural details of glasses can be gathered from the FT–IR spectroscopy. Figure 7 shows the FT–IR spectra of CZBBCR glass samples. Generally, IR spectra of borate glasses are mainly classified into three regions. The bands in the region i) 1600–1200 cm⁻¹ are due to BO₃ units, ii) 1200 and 800 cm⁻¹ belongs to BO₄ units; and iii) ∼ 700 cm⁻¹ corresponds to the B–O–B bending vibrations.^[58–60] The band positions and assignments are listed in Table 5. The band positioned at 450 cm⁻¹ is due to the metal cation (Zn²⁺ and Ca²⁺) vibrations. The bands around 540 cm⁻¹ are attributed to the vibrations of Bi–O bonds in BiO₆ units. The wave numbers from 672–720 cm⁻¹ are attributed to bending vibration of B–O–B in [BO₃] triangles and the symmetric stretching vibrations of Bi–O bonds in BiO₃ pyramidal units and stretching vibrations of B–O–B bonds in BO₃ unit groups.^[18,61,62] The second one, 867–1052 cm⁻¹, is attributed to B–O stretching vibration of BO₄ units which were shared by various borate groups, like penta-borate, tri-borate, and di-borate groups. Band at 880 cm⁻¹ may be attributed to the B–O stretch in BO borate groups and/or ascribed to the symmetrical stretching vibrations of Bi–O bonds of [BiO₃] units.^[18,62,63] Another band ∼ 960 cm⁻¹ is attributed to the B–O stretch in BO₄ units from diborate groups. The stretching vibrations of B–O–B in the [BO₄] tetrahedron are found in the wavenumber range from 1052 to 1038 cm⁻¹. The deep band at 1250 cm⁻¹ is caused due to the B–O stretching vibration of trigonal BO₃ units from meta and ortho-borate group. Stretching vibrations of B–O bond from ortho-borate groups are due to the band at 1213–1246 cm⁻¹. The band at 1370 cm⁻¹ is the stretching vibration of B–O–B in [BO₃] triangles. The bands between 1174–1266 cm⁻¹ arise due to stretching of BO₃ units from pyro- and ortho-borate groups and 1320–1435 cm⁻¹ arise due to the asymmetric stretching of the B–O bonds in BO₃ units.^{[1,18,64–67]}

Fig. 7.

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	Fig. 7. FTIR spectra of xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0 ≤ x ≤ 20 mole%) glass samples.

Table 5.

Table 5.

Table 5. FTIR band assignments of xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0 ≤ x ≤ 20 mole%) glass samples. . Wavenumber/cm−1 Band assignments ∼ 450 Metal cation (Zn2+ and Ca2+) vibrations ∼ 545 Vibrations Bi–O bonds in BiO6 units ∼ 720 Bending vibration of B–O–B in [BO3] triangles and stretching vibrations of B–O–B bonds in BO3 units groups ∼ 874 B–O stretch in BO borate groups ∼ 962 B–O stretch in BO4 units from diborate groups ∼ 1038 Stretching vibrations of B–O–B in [BO4] tetrahedron ∼ 1256 Stretching vibration of trigonal BO3 units from meta and ortho-borate group 1356–1452 Asymmetric stretching of B–O bonds in BO3 units

Table 5. FTIR band assignments of xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0 ≤ x ≤ 20 mole%) glass samples. .

Raman spectra of CZBBCR glass samples are shown in Fig. 8 and deconvoluted spectrum of CZBBCR2 is shown in Fig. 9. Raman spectra are spread over in the region from 200 cm⁻¹ to 1600 cm⁻¹. The Raman bands found at ∼ 133, ∼ 278, ∼ 368, ∼ 451, ∼ 580, ∼ 683, ∼ 863, ∼ 1258, ∼ 1326, and ∼ 1400 cm⁻¹ (Table 6). It has been observed that the vibrations arising from Bi₂O₃ groups fall at lower numbers as compared to the vibrations of B₂O₃. The peak centered at 133 cm⁻¹ indicates the presence of the Bi³⁺ in [BiO₆] units, which supports that the bismuth has network forming nature, while the bands between 270–580 cm⁻¹ are due to the symmetric stretching anion motion in angular-regulated cation–anion–cation configurations in [BiO₆] polyhedral.^[67,68] The band at 680 cm⁻¹ is due to the stretching of B–O- bonds connected to borate groups. The bands at ∼ 860 and ∼ 1250 cm⁻¹ are due to symmetric stretching vibrations of B–O–B bridges and pyroborate groups, respectively. The band at ∼ 1330 cm⁻¹ is due to BØ₂O⁻ triangles linked to units. The band at ∼ 1400 cm⁻¹ is assigned to the stretching of B–O bands attached to the borate groups.^[69–71]

Fig. 8.

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	Fig. 8. Raman spectra of xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0 ≤ x ≤ 20 mole%) glass samples.

Fig. 9.

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	Fig. 9. Deconvoluted Raman spectra of 5CaF2–15ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 glass sample.

Table 6.

Table 6.

Table 6. Raman band assignments of xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0 ≤ x ≤ 20 mole%) glass samples. . Wavenumber/cm−1 Band assignments ∼ 133 Presence of the Bi3+ cations as [BiO6] 270–580 Bi–O–Bi angular vibrations belong to [BiO6] polyhedra ∼ 683 Stretching vibrations of B–O-bonds ∼ 863 Symmetric stretching vibrations of B–O–B bridges ∼ 1258 Vibrations of pyroborate groups ∼ 1326 BØ2O− triangles linked to units ∼ 1400 Stretching of B–O bands attached to the borate groups

Table 6. Raman band assignments of xCaF2–(20 − x)ZnF2–20Bi2O3–59.8B2O3–0.2Cr2O3 (0 ≤ x ≤ 20 mole%) glass samples. .