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SN Applied Sciences

, 1:209 | Cite as

EPR, optical, physical and structural studies of strontium alumino-borate glasses containing Cu2+ ions

  • Mohamad Raheem AhmedEmail author
  • Md. Shareefuddin
Research Article
  • 183 Downloads
Part of the following topical collections:
  1. 5. Physics (general)

Abstract

Glass composition (30 − x) SrO–xAl2O3–69.5B2O3–0.5CuO (0 ≤ x ≤ 15 mol%) captioned as SABC was prepared by the conventional melt quenching technique. Peak-free X-ray diffractograms and homogeneous plain SEM image confirm the glassy nature of the prepared samples. EPR and optical absorption spectral studies were carried out to understand the effect of modifier oxide (SrO) and transition metal (TM) ion Cu2+ in the glass network. From the EPR spectra, spin-Hamiltonian parameters were evaluated. It was observed that \(g_{\parallel } > g_{ \bot } > g_{e} \,{\text{and}}\,A_{\parallel } > A_{ \bot }\) suggest that the ground state of Cu2+ is  \(d_{{x^{2} - y^{2} }}\) (2B1g state) and the site symmetry around Cu2+ is tetragonally distorted octahedral. The optical absorption spectra revealed a broad absorption for all the glass samples. This band is assigned to 2B1g → 2B2g transition. From optical absorption spectra, optical band gap and Urbach energy values were calculated. The FTIR and Raman spectral studies showed that the glass network consists of BO3 and BO4 structural units.

Keywords

Borate glasses Physical Optical properties EPR FTIR Raman 

1 Introduction

Borate is an excellent glass former, and its derived glasses are well known for their high thermal stability, low melting point and good solubility. They find wide application in optical glasses, gamma ray shielding and bioactive materials [1, 2]. Physical and optical properties of borate glasses can be enhanced by the addition of modifiers such as alkali and alkaline earth oxides. In continuation, alkaline earth oxides like SrO-, BaO-, CaO-, MgO-based borate glasses have been prepared and they are being used in various applications such as vacuum ultraviolet (VUV) optics which has the shortest wavelength in the EM spectrum, radiation dosimetry and solar energy converters [3]. Various studies have been conducted to explore the probable physical and optical properties of pure borate glasses added with different modifiers and dopants. Although strontium also plays the main role in biological systems and resembles the calcium element in its properties, like calcium, it is taken up and preferentially located in bones. The ionic size of strontium (Sr2+) is nearly equal to calcium ion (Ca2+) and easily replaceable, and its role in bone metabolism has been reported as well as both anti-resorptive and bone forming for different applications [4, 5]. In addition to this, SrO improves the rigidity of the glass samples and extends its applications in nonlinear optical devices and biological systems [6]. It has been reported that alumino-borate glasses (Al2O3–B2O3) are refractory compounds possessing excellent physical properties such as low density, high hardness, high Young’s modulus, high electrical resistivity and low coefficient of thermal expansion [7]. Abd El-Moneim [8] suggested that ultrasonic velocities and elastic moduli results are purely glass composition dependent and Al2O3 acts as network former with Al3+ ions by the formation of AlO4 tetrahedra. From literature survey, Al2O3 is a typical intermediate material for borate glasses; it can be changed into either glass network former or modifier controlled by the large or small amount of Al2O3, respectively [9]. Al2O3 is assumed to enter the structure of SrO–B2O3 glasses as AlO4 tetrahedra and/or AlO6 octahedra, depending on Al2O3/SrO ratio. If the ratio Al2O3/SrO < l, only four coordinated aluminum ions AlO4 are formed. For Al2O3/SrO > l, AlO6 groups are favored [10]. Some of the strontium aluminate glasses are used as photosensitive applications and thus are potential candidates for optical information storage devices. The research on the strontium alumino-borate glasses is essential as it finds application in many fields as described above. So to cater the need for probing, electron paramagnetic resonance (EPR) technique is a potential tool to study the lowest energy levels and hence the electronic state of the unpaired electrons of paramagnetic ions. FTIR and Raman studies are carried out to know the structural changes in the glass.

2 Experimental

The glass samples of the composition (30 − x) SrO–xAl2O3–69.5B2O3–0.5CuO, 0 ≤ x ≤ 15 mol% (Table 1) in the present investigation are prepared by melt quenching method. Appropriate amounts of reagent grade of SrCO3, Al2O3, H3BO3 and CuO powders were thoroughly mixed and taken in platinum crucible and then melted in an electrical furnace at the temperature ~ 1200 °C. The molten mixture is quenched to obtain blue-colored, transparent glass samples shown in Fig. 1. Phillips X-pert pro X-Ray diffractograms and Carl Zeiss model SEM were used to confirm the glassy nature. Density measurements were taken for these glasses. Optical absorption (UV–Vis) spectra were recorded using Shimadzu UV-1800 spectrophotometer in the region 200–1000 nm. ESR spectra are also recorded at room temperature on JOEL FE1X spectrometer operating X-band frequency with 100 kHz field modulation. JOBIN YVON HR800 (HORIBA) instrument is used to record Raman Spectra with (473 nm) a solid-state diode laser in the range 200–1600 cm−1. A fine glass powder is used to record FTIR spectra on Shimadzu 8400S instrument in the 400–1600 cm−1 wavelength range. Glass transition temperature (Tg) of prepared glasses was determined using DSC thermograms, recorded on NETZSCH DSC 404F3 spectrometer with a heating rate of 10 K/min.
Table 1

Composition of the (30 − x)SrO–xAl2O3–69.5B2O3–0.5 CuO glasses

Glass code

SrO mol%

Al2O3 mol%

B2O3 mol%

CuO mol%

Tg (oC)

SABC0

25

5

69.5

0.5

613

SABC1

22.5

7.5

69.5

0.5

587

SABC2

20

10

69.5

0.5

584

SABC3

17.5

12.5

69.5

0.5

573

SABC4

15

15

69.5

0.5

561

Fig. 1

Physical appearance of SABC glasses

3 Result and discussion

3.1 XRD and SEM

Figure 2 shows the typical X-ray diffraction pattern for SABC glasses. There are no sharp peaks in the spectra, indicating that glasses are of amorphous nature. The SEM image of SABC2 glass shown in Fig. 3 revealed the clear smooth homogenous surface with no origination of clusters which also supports the amorphous nature.
Fig. 2

X-ray diffraction pattern of SABC glasses

Fig. 3

SEM image of SABC 2 glass

3.2 DSC studies

Figure 4 shows the decreasing Tg (from SABC0 to SABC4) with the increase in the Al2O3 content; this is often as a result of increase in the number of non-bridging oxygen (NBO) and the formation of AlO4 tetrahedra and void space [11]. The increase in the concentration of Al2O3 in the SABC glass magnificently resulted in the blocking of network-modifying action of Sr2+ ions, thereby creating more BO3 units [12].
Fig. 4

DSC thermograms of SABC glasses

3.3 Physical properties

Density measurements pave the way for understanding and analyzing the structure of a glassy network. From the literature survey, it is found that the density of the glass depends on modifier quantities like SrO, MgO, Na2O, Al2O3, etc. [1, 3, 13]. Most of the research work on glasses includes density measurements employing Archimedes principle. Using density values, molar volume Vm is estimated [1, 14]. It is observed that the density values are decreasing from 2.705 to 2.451 g/cc with increasing Al2O3 from 5 to 15 mol% with decreasing equal amount of SrO content in the glass composition. The crystalline density of Al2O3 (3.95 g/cc) is much lower as compared to the density of SrO (4.7 g/cc) which might be one of the reasons behind the decrease in density when SrO is replaced by Al2O3. From the literature, it is understood that SrO acts as modifier and converts BO3 units to BO4 units. Al2O3 in the glass network generates AlO4 tetrahedra and BO3 units. The remaining aluminum reacts with CuO which leads to creation of the non-bridging oxygen (NBO) leading to more open structure [12, 15, 16]. This explains the increase in molar volume and decrease in density as shown in Fig. 5.
Fig. 5

Variation of density and molar volume with Al2O3 mol% of SABC glasses

The average boron–boron separation \(\left\langle {d_{{{\text{B}}{-}{\text{B}}}} } \right\rangle\) for SABC glasses is calculated using the relation [16] as
$$V_{\text{m}}^{\text{B}} = \frac{{V_{\text{m}} }}{{2\left( {1 - X_{\text{B}} } \right)}}$$
(1)
where \(X_{\text{B}}\) is the molar fraction of boron trioxide
$$\left\langle {d_{{{\text{B}}{-}{\text{B}}}} } \right\rangle = \left( {\frac{{V_{\text{m}}^{\text{B}} }}{{N_{\text{A}} }}} \right)^{{\frac{1}{3}}}$$
(2)
where \(N_{\text{A}}\) is 6.0221 × 1023 being the Avogadro number. The calculated values of \(\left\langle {d_{{{\text{B}}{-}{\text{B}}}} } \right\rangle\) are given in Table 2.
Table 2

Physical properties of SABC glasses

Physical property parameters

X = 5 mol%

X = 7.5 mol%

X = 10 mol%

X = 12.5 mol%

X = 15 mol%

Glass code

SABC0

SABC1

SABC2

SABC3

SABC4

Average molecular weight M (g)

79.784

79.742

79.701

79.659

79.618

Density ρ (g/cm3) (± 0.001)

 Experimental

2.705

2.703

2.612

2.550

2.451

 Theoretical

2.682

2.605

2.531

2.459

2.389

Molar volume (cm3/mole) (± 0.01)

29.48

29.49

30.50

31.23

32.48

Oxygen packing density (mol/cm3) (± 0.01)

84.449

86.117

84.900

84.520

82.810

Ionic packing density (Vt)

0.617

0.626

0.615

0.609

0.594

Refractive index nd

2.280

2.304

2.318

2.345

2.294

Molar refractivity Rm (cm−3)

17.193

17.384

18.090

18.736

19.358

Dielectric constant ɛ

5.198

5.308

5.373

5.499

5.262

Reflection loss R %

0.152

0.155

0.157

0.161

0.154

Transition metal ion concentration (Ni) 1021 (ions/cc) (± 0.01)

10.21

10.20

9.87

9.64

9.27

Polaron radius (rp) (A˚) (± 0.005)

2.20

2.21

2.23

2.25

2.28

Interionic distance (ri) (A˚)(± 0.005)

5.46

5.47

5.53

5.56

5.63

Molar polarizability αe (10−24 ions/cm3) (± 0.005)

6.82

6.89

7.17

7.43

7.68

Field strength (F) (1015 cm−2)

4.13

4.09

4.02

3.95

3.85

Average boron–boron distance (dB–B) (nm)

0.511

0.512

0.517

0.521

0.528

Optical band gap (Eg) (eV) (± 0.01)

3.47

3.37

3.31

3.20

3.41

Urbach energy (eV) (± 0.001)

0.536

0.618

0.639

0.681

0.825

Optical basicity Λth (± 0.001)

0.619

0.605

0.592

0.578

0.565

Average boron–boron separation increases from 0.511 to 0.528 nm with increasing Al2O3 favoring increase in molar volume.

The refractive index (\(n_{d}\)) of BABC glasses has been computed by utilizing the relation given below [17]
$$\frac{{(n_{d}^{2} - 1)}}{{\left( {n_{d}^{2} + 2} \right)}} = 1 - \sqrt {\frac{{E_{\text{g}} }}{20}}$$
(3)
where \(E_{\text{g }}\) is the energy band gap

Generally speaking, the increment is observed in a refractive index \((n_{d} )\) from 2.375 to 2.501 in the present system as a result of the rise in the count of NBO.

From the refractive index (\(n_{d}\)) values, the dielectric constant (ɛ) was calculated by utilizing formula [17]
$$\varepsilon = n_{d}^{2}$$
(4)
The molar refractivity \(R_{\text{M}}\) of the glass samples was evaluated using [16]
$$R_{\text{M}} = \left[ {\frac{{n_{d}^{2} - 1}}{{n_{d}^{2} + 2}}} \right]*V_{\text{m}}$$
(5)
where \(n_{d}\)—refractive index, \(V_{\text{m}}\)—molar volume
Measurement of TM ion concentration per cc for host glasses provides information about the observed changes in its different properties. Hence, the measurement of TM ion concentration (\(N_{\text{i}}\)) is of great importance in the present case. It is being calculated by the formula [18]
$$N_{\text{i}} = \frac{{N_{\text{A}} *X\left( {{\text{mol}}\% } \right)*d}}{M}$$
(6)
where \(X\left( {{\text{mol}}\% } \right)\) refers to a transition metal ion, d density, \(M\) average molecular weight.
The polaron radius (\(r_{\text{p}}\)) and interionic separation \(\left( {r_{\text{i}} } \right)\) are calculated through the following relation [18]
$$r_{\text{p}} = \frac{1}{2}\left( {\frac{\pi }{{6N_{\text{i}} }}} \right)^{{\frac{1}{3}}}$$
(7)
and
$$r_{\text{i}} = \left( {\frac{1}{{N_{\text{i}} }}} \right)^{{\frac{1}{3}}}$$
(8)
The electronic polarizability \(\alpha_{\text{M}}\) for all the glass samples was evaluated using [17]
$$\alpha_{\text{e}} = \left( {\frac{3}{4\pi N}} \right)*R_{\text{M}}$$
(9)
The field strength is calculated using the oxidation number (Z) from the following formula [18]
$$F = \left( {\frac{Z}{{r_{\text{p}}^{2} }}} \right)$$
(10)

In general, polaron radius (\(r_{\text{p}}\)) is increasing and field strength is decreasing, showing the opposite trend which is clearly observed in the present work (Table 2). The value of \(r_{\text{p}}\) increases from 2.20 to 2.28 Å. This increment is attributed to the open structure caused by Cu2+ addition. Field strength (F) decreases also support the open structure, which resulted in an increase in molar volume [11]. The polaron radius (\(r_{\text{p}}\)) and interionic distance \(\left( {r_{\text{i}} } \right)\) results are in tune with each other. The polaron radius in all the glasses (SABC series) is less than the corresponding interionic distance which is in accordance with the usual prediction of the polaron theory that the polaron radius should be smaller than the site separation.

The theoretical optical basicity (Λth) is calculated using the relation mentioned below [16]
$$\varLambda_{\text{th}} = X_{{ {\text{BaO }}}} \varLambda_{\text{BaO}} + X_{{{\text{Al}}_{2} {\text{O}}_{3} }} \varLambda_{{{\text{Al}}_{2} {\text{O}}_{3} }} + X_{{{\text{B}}_{2} {\text{O}}_{3} }} \varLambda_{{{\text{B}}_{2} {\text{O}}_{3} }} + X_{\text{CuO}} \varLambda_{\text{CuO}}$$
(11)
where \(X_{\text{BaO}}\), \(X_{{{\text{Al}}_{2} {\text{O}}_{3} }}\),\(X_{{{\text{B}}_{2} {\text{O}}_{3} }}\),\(X_{{{\text{CuO}} }}\) are fractions of present oxides and \(\varLambda_{\text{BaO}}\), \(\varLambda_{{{\text{Al}}_{2} {\text{O}}_{3} }}\),\(\varLambda_{{{\text{B}}_{2} {\text{O}}_{3} }}\), \(\varLambda_{\text{CuO}}\) are optical basicity values. The calculated values of optical basicity are given in Table 2. These values are decreasing from 0.619 to 0.565 with an increase in Al2O3 content. This trend shows the decreasing electron donar capability of oxide ions to the coordinate cations. The increase in the refractive index (nd) and molar refractivity (Rm) indicates the presence of more polarizable electrons surrounding the oxygen. The variation of nd and Rm with Al2O3 mol% is shown in Fig. 6.
Fig. 6

Refractive index versus molar refractivity with Al2O3 mol% of SABC glasses

3.4 EPR studies

Figure 7 shows EPR spectra of (30 − x) SrO–xAl2O3–69.5B2O3–0.5CuO glass systems. The general behavior of an ideal Cu2+ ion is to provide four parallel and four perpendicular hyperfine components satisfying the condition (2I + 1), where I = 3/2 and S = 1/2 for the Cu2+ ion. However, Cu2+ ion in glassy network has shown three feebly resolved parallel components while fourth being mixed with perpendicular component. Perpendicular hyperfine components are not well resolved. EPR signal intensity is modulated by the addition of Al2O3. This is clear from Fig. 7 that signal intensity is decreasing as moving from SABC0 to SABC4 glass sample. This shows that ligand field around Cu2+ ion is being greatly influenced by the addition of Al2O3. The presence of Al2O3 greatly hinders the hyperfine splitting.
Fig. 7

EPR spectra of SABC glasses

Spin-Hamiltonian parameters are calculated using the expressions [19, 20]. \(g_{\parallel }\) values varying between 2.335 and 2.340 are much greater than \(g_{ \bot }\) values (2.059–2.076) which in turn are greater when compared to free spin g value ge = 2.0023. The hyperfine splitting tensor is found be \(A_{\parallel } > A_{{ \bot }}\). From the above-mentioned data, it is clear that Cu2+ ions are located in tetragonally distorted octahedral site and the ground state of Cu2+ ions is \(d_{{x^{2} - y^{2} }}\) orbital 2B1g state [20]. To fetch some more information regarding the concentration of Cu2+ ions that have actually participated in the resonance process, we calculated (N) values using the area under the EPR spectra and its values are given in Table 3. The following equation is used to obtain the N value [21]
Table 3

Spin-Hamiltonian parameters and molecular orbital bonding coefficient values of the SABC glasses

Glass code

\(g_{\parallel }\)

\(g_{ \bot }\)

\(\alpha^{2}\)

\(\beta^{2}\)

\(\beta_{1}^{2}\)

ΔExy (cm−1)

\(N \times 10^{21}\) (per kg)

\(\chi \times 10^{3}\) m3/kg

SABC0

2.334

2.061

0.7890

0.9747

0.8053

12674

1.57

1.90

SABC1

2.340

2.059

0.7972

0.9647

0.8041

12578

1.60

1.94

SABC2

2.340

2.061

0.7703

0.9983

0.8290

12531

1.79

2.06

SABC3

2.335

2.076

0.7965

0.9656

0.7873

12500

1.56

1.91

SABC4

2.338

2.065

0.7950

0.7917

0.9673

12547

1.57

2.51

$$N = \frac{{A_{x} \left( {{\text{Scan}}_{x} } \right)^{2} G_{\text{std}} \left( {H_{\text{m}} } \right)_{\text{std }} \left( {g_{\text{std}} } \right)^{2} \left[ {S\left( {s + 1} \right)} \right]_{\text{std}} \left( {P_{\text{std}} } \right)^{{\frac{1}{2}}} }}{{A_{\text{std}} \left( {Scan_{\text{std}} } \right)^{2} G_{x } \left( {H_{\text{m}} } \right)_{x} \left( {g_{x} } \right)^{2} \left[ {S\left( {s + 1} \right)} \right]_{x} \left( {P_{x} } \right)^{{\frac{1}{2}}} }}\left[ {\text{Std}} \right]$$
(12)
The subscripts x and std represent corresponding quantities for SABC glasses and the reference (CuSo4∙5H2O). The magnetic susceptibility (χ) of Cu2+ ions has been calculated using the equation [21]
$$\chi = \frac{{Ng^{2} \beta^{2} J\left( {J + 1} \right)}}{{3K_{\text{B}} T}}$$
(13)
where N is the number of spins per kg, J = 5/2. Figure 8 shows the variation of N and χ with the Al2O3 (mol%) which is nonlinear.
Fig. 8

Variation of spin concentration (N) and magnetic susceptibility (χ) with Al2O3 mol%

3.5 Optical absorption spectra and energy band gap

The optical absorption spectra resulted due to the presence of Cu2+ ions in the glass system of SABC which has exhibited one broad absorption band (Fig. 9). This band is attributed to 2B1g → 2B2g transition. Using these spectra, the absorption coefficient ‘α’ can be measured as a function of frequency using the given formula [22]
$$\alpha \left( \nu \right) = \frac{A}{d} \times 2.303;$$
(14)
in the above equation, absorbance is represented by ‘A’ at frequency ν and ‘d’ represents the thickness of the sample.
Fig. 9

Optical absorption spectra of SABC glasses

The indirect transitions are calculated using the relation [14]
$$\alpha \left( \nu \right) = b\left( {h\nu - E_{\text{opt}} } \right)^{n} /h\nu$$
(15)
where ‘b’ is the energy-independent constant and in the above equation index n has different values, n = 2 and 1/2 for direct and indirect allowed transitions, respectively. The graph plotted for indirect transition is shown in Fig. 10. The optical band gap energy (Eopt) for indirect transitions is shown in Table 2 which is found to decrease with increasing Al2O3 due to an increment of NBO number in the glass sample. From the general definition, Urbach energy (ΔE) gives significant data about the density of energy states present in the optical band gap which is obtained from the plots drawn between hv versus ln(α) shown in Fig. 11.
Fig. 10

Tauc plots of SABC glasses

Fig. 11

Urbach plots of SABC glasses

Table 3 contains the peak position (ΔExy) obtained from optical absorption spectra (Fig. 9). This can be identified as the dd transition band due to Cu2+ ion. The peak position is shifted toward higher wavelengths due to the ligand field around Cu2+ ion. Bonding parameters (α2, β2 and β 1 2 ) describing the bonding nature of bonding between the Cu2+ ion and its ligands are effectively evaluated by the data obtained from both EPR and optical absorption studies. The in-plane σ bonding represented by α2 bonding between ligands (surrounding oxygen) and copper \(d_{{x^{2} - y^{2} }}\) orbital is moderately ionic. The out-of-plane Π-bonding between ligands and copper dxz,yz orbital is represented by β2 is found to be ionic, while β 1 2 , the measure of in-plane Π-bonding with dxy orbital, is mostly ionic in nature [19, 23]. Bonding parameters are given in Table 3. From these values, it may be concluded that Cu2+ ion is mostly in an ionic environment in the present SABC glass system [24].

3.6 FTIR studies

The FTIR spectra of SABC glasses revealed the presence of different vibrational units shown in Fig. 12. The FTIR transmission bands falling around ~ 509, ~ 650, ~ 775, ~ 1017, ~ 1214, ~ 1380, ~ 1608 cm−1 lying between wavenumbers 500–2500 cm−1 with their assignments are listed in Table 4. The present study shows that the quantitative evolution of these glass structures is enormously affected by the Al2O3 concentration. The band at ~ 509 cm−1 represents the metal cation, Sr2+ vibrations and also due to the transition metal ion Cu2+ [25, 26]. The FTIR band ~ 650 cm−1 is resulted due to the bending vibration of B-O-B in BO3 triangles [27, 28]. The transmittance peak intensity is increasing for 775 cm−1 with Al2O3 mol percentage due to the BO3–O–BO4 bond bending vibrations [29, 30]. B–O symmetric stretching of BO4 units is seen ~ 1017 cm−1, while the wide band ~ 1214 cm−1 belongs to B–O stretching vibration BO3 units from mixed borate groups like phyro- and orthoborates [31, 32]. The intensity of the sharp peak ~ 1380 cm−1 increases with Al2O3 increment due to B–O asymmetric vibrations in BO3 and BO2O units and indicates BO3 increment with the increase in Al2O3 mol percentage [29, 31]. Another sharp band at ~ 1607 cm−1 is due to bending vibrations of O–H groups [31, 33].
Fig. 12

FTIR spectra of SABC glasses

Table 4

FTIR band assignments of SABC glasses

Glass code

Band assignment

References

SABC0

SABC1

SABC2

SABC3

SABC4

520

506

485

510

509

Vibration of metal cations (Sr2+) and CuO bond vibrations.

[25, 26]

701

663

672

694

650

Bending vibration of B–O–B in BO3 triangles

[27, 28]

781

775

775

775

775

BO3–O–BO4 bond bending vibration

[29, 30]

1013

1016

1010

1024

1017

B–O symmetric stretching of BO4 units

[30, 31]

1210

1224

1225

1214

1208

B–O stretching vibration BO3 units from phyro and orthoborate groups

[30, 32]

1395

1371

1364

1374

1380

B–O asymmetric vibrations in BO3 and BO2O units

[29, 31]

1608

1605

1602

1605

1607

Bending of O–H

[31, 33]

3.7 Raman studies

Raman spectroscopy is extensively used to analyze the information about the structure and to explore the probable functional groups present in the glass. Raman spectra of SABC glasses are shown in Fig. 13, and deconvoluted Raman spectra of SABC4 glass are shown in Fig. 14. Raman peaks are explored from the deconvoluted graphs, and their assignments are listed in Table 5. The band seen ~ 468 cm−1 is due to the vibration of BO4 isolated tetrahedra or isolated diborate groups [34, 35]. Raman band ~ 674 cm−1 is assigned as B-O-B stretching of metaborate rings [35, 36]. The sharp peak centered ~ 777 cm−1 is attributed to symmetric breathing vibrations of boroxol rings and also due to AlO4 units [34, 36]. The band intensity is increasing with the addition of Al2O3 mol percentage, and shifting of the peak to higher wavelength is also observed. The band ~ 858 cm−1 represents [25, 34] the pyroborate groups, while the band ~ 942 cm−1 is due to the B–O bond stretching of orthoborate groups [25, 35]. A weak intensity peak at ~ 1028 cm−1 is assigned as diborate groups [34, 36]. The Raman peak at ~ 1261 cm−1 is due to B–O stretching in pyroborate units. The band at ~ 1362 cm−1 is generally due to B–O stretch in BO4 units from varied borate groups [34, 38]. Raman peak at ~ 1437 cm−1 is attributed to stretching vibrations of BO3 triangles with large segment of borate network [39, 40]. From Raman spectra analysis, it is clearly observed that the absence of the peak at 805 cm−1 concluded the absence of boroxol rings and existence of AlO4 and BO3 units observed at ~ 791 cm−1 and ~ 1533 cm−1, respectively. The presence of pyroborate and orthoborate groups accepted the existence of non-bridging oxygen.
Fig. 13

Raman spectra of SABC glasses

Fig. 14

Deconvoluted Raman spectra of SABC4 glass

Table 5

Raman assignment for SABC glasses

Glass code

Band assignment

References

SABC0

SABC1

SABC2

SABC3

SABC4

468

476

472

463

467

Vibration of BO4 isolated tetrahedra/isolated diborate groups

[34, 35]

674

680

693

690

665

B–O–B stretching of metaborate ring

[35, 36]

777

784

790

796

791

Symmetric breathing vibrations of boroxol rings and AlO4 Units

[34, 36]

858

884

880

889

800

Pyroborate groups

[25, 34]

942

985

977

958

923

B–O bond stretching of orthoborate group.

[25, 35]

1028

1039

1035

1012

1023

Diborate groups

[34, 36]

1128

1139

1126

1134

1123

Vibration of trigonal boron

[35, 37]

1261

1277

1223

1281

B–O stretching in pyroborate units

[36, 38]

1362

1374

1379

1358

1369

B–O stretch in BO4 unit from varied borate group

[34, 38]

1437

1494

1514

1533

Stretching vibrations of BO3 triangles with large segment of borate network

[39, 40]

4 Conclusions

From the above investigations, the following conclusions are drawn:
  1. 1.

    XRD and SEM morphology confirms the glassy nature of the SABC glasses

     
  2. 2.

    Decreasing Tg with the increase in the Al2O3 content is an evidence for the increasing of NBO.

     
  3. 3.

    The decreasing density value for present glasses indicates the structural changes, due to Al2O3 increment. Al2O3 reacts with glass composition and from AlO4 and BO3 units with NBO. Using density and molar volume, OPD and ionic packing densities are calculated.

     
  4. 4.

    The Cu2+ ions in all the glass systems studied are in tetragonally distorted octahedral sites with \(d_{{x^{2} - y^{2} }}\) orbital (2B1g) ground state. The spin-Hamiltonian parameters are influenced by the glass composition which may be attributed to the change in the ligand field strength around Cu2+. From covalency parameters, it is concluded that \(\alpha^{2}\) is moderately ionic,\(\beta_{1 }^{2}\) is mostly ionic, and \(\beta^{2}\) is ionic.

     
  5. 5.

    The optical absorption spectra of present glasses have shown a single broad peak assigned to 2B1g → 2B2g transition. Optical band gap and Urbach energies are calculated, and using this, data refractive index, polaron radius and molar refractivity are calculated. Decreasing values of optical band gap with increasing Al2O3 indicate the creation of NBO.

     
  6. 6.

    The FTIR and Raman studies confirmed the presence of BO3 and BO4 units in the glass network. The metal cation Sr2+ and Cu2+ vibrations are observed ~ 520 cm−1 in FTIR spectra. The sharp Raman peak ~ 777 cm−1 is attributed to symmetric breathing vibrations of boroxol rings and AlO4 units.

     

Notes

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Muffakham Jah College of Engineering and TechnologyOsmania UniversityHyderabadIndia
  2. 2.Department of PhysicsOsmania UniversityHyderabadIndia

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