1 Introduction

The study of the structures of glasses is necessary to obtain a good understanding of structure–property relations. Boron atoms in lithium borate glasses are either three- or four-coordinated and are generally designed as BO3 and BO4 units. The presence of BO4 units leads to the tetrahedral network of the glass [1].

The effect of copper addition on lithium borate glass samples was studied: The copper ions can act as network modifier and some ions as network former by increasing the copper content. The magnetic, electrical and optical properties show a variable behavior due to the presence of copper ions in two valances, cuprous (Cu+) and cupric (Cu2+), in glass samples [2]. In our study, lithium borate glass containing lead was studied. The results showed the presence of lead oxide lowering the average coordination number of oxygen [3]. This causes the increase in refractive index and density.

The structure of PbO–B2O3 glasses containing 10–80 mol% PbO was investigated. The structure studies revealed that the PbO content varied between 10 and 20 mol% and PbO acts as a network modifier. With the increase in PbO content, the borate group changed from [BO3] to [BO4] units. When the content is over 60 mol%, PbO plays the role of glass former. Four possible structure models have been suggested to explain the effects of PbO on glass network [4]:

  1. 1.

    Three-coordinated boroxol rings modified by Pb2+;

  2. 2.

    Formation of Pb–O–B covalent bonds;

  3. 3.

    Bridge networks between [BO3] and [BO4] units;

  4. 4.

    Complex structures of Pb2+-modified boron oxygen rings and chains.

In this work, we study the effect of substitution of Pb3O4 with CuO in lithium lead borate glass system on structure, electrical, magnetic and optical properties.

2 Experimental work

A series of lithium lead borate glass samples containing CuO were prepared. The details of the compositions chosen for the present study are given in Table 1. The homogenous mixture was melted in a platinum crucible in an electrically programmable heated furnace, (type UAF 15/10 Lenton Thermal Designs), equipped with an automatic temperature controller. The samples were melted at 1200 ± 20 °C for 2 h with a heating rate of 30 °C/min. The molten materials were stirred several times to ensure complete homogeneity. Then, they were quenched in air and poured at room temperature between two Cu plates.

Table 1 Glass samples’ composition (mol%)
Full size table

The samples were examined using a Philips Analytical X-ray diffraction system, type PW 3710 with Cu tube anode. It can be seen that all samples are in a glassy state. FTIR spectra were recorded with a FTIR spectrometer (type JASCO FT/IR-430, Japan) in the wave number range 220–2000 cm−1 at room temperature by using the KBr disc technique. Differential scanning calorimetric measurements (DSC) were taken using a SHIMADZU DTA-50 analyzer. The measurements were taken between 25 and 650 °C (in air using Al2O3 powder as a reference material) at the rate of 35 °C/min.

The density of glasses was determined by Archimedes method in which glass sample was weighed three times in air and when immersed in toluene at 25 °C. The density was calculated from the formula

$$\rho = \, \left[ {W_{\text{a}} /\left( {W_{\text{a}} - W_{\text{b}} } \right)} \right] \times 0.8635$$

where ρ is the density of the glass sample, W a is the weight of the glass sample in air, W b is the weight of the glass sample in toluene and 0.8635 is the density of toluene.

For good electrical contact during measurements, the sample surfaces were highly polished and coated with an air-drying silver paste. The electrical conductivity measurements at room temperature were obtained with a programmable Keithley electrometer (type 617). Magnetic susceptibility was measured by applying Gouy method using a Faraday electromagnet. Optical absorption measurements were carried out using V 570 JASCO spectrophotometer.

3 Results and discussion

The X-ray diffraction patterns of the [65 mol% Li2B4O7-(35-x) mol% Pb3O4-x mol% CuO] glass samples (x = 0, 5, 10, 15 or 20 mol%) are shown in Fig. 1. All the samples show broad halo around a diffraction angle 2θ = 32°, characteristic of an amorphous structure and confirming the absence of crystal in the samples.

Fig. 1
figure 1

X-ray diffraction patterns of the glass samples

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The density is related to how the modifier and former ions groups are packed together in the structure. Figure 2 shows the relation between the density and copper content. From Fig. 2, it can be observed that the density decreases with the increase in Cu content. Substitution of Pb (having high molecular weight) with Cu (having low molecular weight) decreases the density. Density is directly proportional to the molecular weight, and the increase in Cu content decreases the formation of tetrahedral groups (BO4), and increasing the triangle groups (BO3) led to the increase in borate non-bridging oxygen (NBO) [5], increasing volume which is inversely proportional to the density.

Fig. 2
figure 2

Relation between the density and copper oxide content

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Density plays an essential role in calculating the concentration of ions added, polaron radius and the time between additions of ions. In the glass samples under study, the concentration of copper ions per unit volume can be calculated using the equation [6]:

$$N \, = \, \rho {\text{ Na}}/100\;W\;{\text{cm}}^{ - 1}$$

where ρ is the density of glass sample, Na is the Avogadro’s number and W is the atomic weight of copper in the glass sample. From the calculated values of N, the polaron radius (r p) can be determined using the relation:

$$r_{\text{p}} = \, 0.5 \, \left( {\pi /6N} \right)^{1/3} {\text{cm}}$$

Also, the space between Cu–Cu atoms in the glass system can be calculated from the values of N from the equation:

$$R \, = \, \left( {1/N} \right)^{1/3} {\text{cm}}$$

All the calculated parameters of N, r p and R are shown in Table 2.

Table 2 Concentration of copper ions per unit volume N, the polaron radius (r p) and the space between Cu–Cu atoms R
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From these results, it can be concluded that the separation between Cu–Cu (R) is greater than polaron radius (r p) for all samples. And the values of r p are smaller than R and greater than the ionic radius of copper (ionic radius of Cu ion is 0.74E-12 cm), which indicates that electrons are localized [6].

In the thermal studies using DTA, the appearance of single peak due to the glass transition temperature of all glass samples indicates the high homogeneity of the glass prepared [5, 7]. Figure 3 indicates the relation between the glass transition temperature and the Cu content. From Fig. 3, it can be observed that the value of Tg increases with Cu content up to 15 mol% and then decreases.

Fig. 3
figure 3

Relation between the glass transition temperature and the Cu content

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The glass transition temperature depends on the glass structure, and glass transition is related to the density of covalent cross-linking, the oxygen density of the network and the number and strength of the cross-links between oxygen and the cation [8, 9].

In the glass samples, the substitution of Pb by Cu decrease the oxygen density which led to the compactness of the structure and increasing the glass transition temperature up to 15 mol% and then decrease, the decrease in the value of Tg as between 10 < PbO < 20 mol%, shows that the Pb ions act as a network modifier [4].

The temperature difference between the crystallization temperature Tc and transition temperature Tg reflects the glass-forming ability (GFA) and is expressed by ΔTc = Tc − Tg, which is called the width of the supercooled liquid region (SCL). ΔTc is a good indication of thermal stability because the higher the value of this difference, the more is the delay in the nucleation process [10].

Figure 4 shows the glass-forming ability as a function of copper oxide content. From Fig. 4, it can be observed that ΔTc = Tc − Tg increases with the addition of copper oxide content up to 10 mol% and then decreases. A large value of ΔTc at 10 mol% CuO implies that the supercooled liquid can exist in a wide temperature range without crystallization and has a resistance to the nucleation and growth of crystallization phases, leading to good GFA and then decreasing with increasing CuO content.

Fig. 4
figure 4

Glass-forming ability as a function of copper oxide content

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Figure 5 shows the FTIR spectra of the glass samples free from copper and samples containing 20 mol% CuO.

Fig. 5
figure 5

FTIR spectra of the glass samples free from copper and samples containing 20 mol% CuO

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From Fig. 5 (the spectrum of the sample free from CuO), it can be observed that the fundamental absorption bands are at 1450, 1300, 1165, 1015, 900, 850, 700 and 430 cm−1. The band at 1450 cm−1 attributed to triangle boron oxygen triangle groups [11] while the band at 1356 cm−1 is due to asymmetric stretching modes of borate triangles BØ3 and BØ2O (Ø representing an oxygen atom bridging two boron atoms and O-NBO) [12] various borate rings. The band at 1165 cm−1 is due to the B–O stretching modes of borate triangles BO3 [13]. The band at 1038 cm−1 is due to the vibration of BO4 tetrahedra, which is present as pentaborate, tetraborate, diborate and triborate groups [4, 14]. The band at 950 cm−1 is attributed to Pb–O bonds [15].

The band at 840 cm−1 is due to the pentaborate, diborate and triborate vibrations. The band at 700 cm−1 results from oxygen bridges forming between one tetrahedral and one trigonal boron atom [2]. Moreover, the band at 620 cm−1 is the characteristic of the vibration frequency of lead [16].

The replacement of PbO by CuO causes a decrease in the intensity of bands at 620, 840, 1180, 1250 and 1356 cm−1, which then disappear in sample containing 20 mol% CuO. This may occur due to the change in the network. No bands appear for Cu ions, and this indicates that the Cu ions act as network modifiers in this region of concentration.

From the relative peak areas of {B´Ø3 and BØ2O–} (A3) and {BØ4–} (A4), which were separated by a Gaussian deconvolution, the value of N4 is calculated as A 4/(A 4 +A 3). The quantities A 4 and A 3 reflect the relative content of tetrahedral (BØ4–) and triangular (BØ3 and BØ2O–) borate species, respectively (Ø representing an oxygen atom bridging two boron atoms).

The following method is used in the calculation of the fraction N 4 of the four-coordinated boron atoms in the glass, where [17]

$$\begin{aligned} N_{4} & = ({\text{concentration}}\,{\text{of}}\,{\text{BO}}_{ 4} \,{\text{tetrahedral}})/({\text{concentration}}\,{\text{of}}\,{\text{BO}}_{ 4} \,{\text{tetrahedral}} \\ & \quad + {\text{ concentration}}\,{\text{of}}\,{\text{BO}}_{ 3} \,{\text{triangle}}) \, = A_{4} /\left( {A_{4} + A_{3} } \right) \\ \end{aligned}$$

The obtained values are shown in Fig. 6 as a function of composition. N 4 decreases firstly with the addition of 5 mol% CuO. This decrease due to the decrease in the BO4 groups with the replacement of lead oxide by copper oxide depends on the number of oxygen atoms, converts BO4 to BO3 and forms non-bridging oxygen. In addition, Pb ions act as former structure and Cu ions as network modifiers.

Fig. 6
figure 6

Fraction N4 of the four-coordinated boron atoms in the glass samples

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Figure 7 shows the relation between electrical conductivity and copper oxide content.

Fig. 7
figure 7

Relation between the electrical conductivity and copper oxide content

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From Fig. 7, it can be observed that the (Ln σ) decreases with the increase in the copper content. (Ln σ) decreases with the gradual increase in copper oxide content. The number of electron hopping processes in the glass network increases where the sample free from copper has the ionic conduction, although the conduction decreases with the increase in the copper content. In addition, as the number of Cu ions increases, they enter the interstitial vacancies to occupy the modifier positions replacing lead and at high concentration of copper occupy the network former and modifier positions.

Copper ions exist in two valance states, Cu+ and Cu+2 [18]. Electronic conduction occurs by polarons hopping between these two sites. Introduction of high ionic strength Cu in the form of CuO6 instead of PbO3 or PbO4 decreases the conduction, and this reflects the change from ionic to mixed conduction [19].

Figure 8 shows the relation between the magnetic susceptibility and the concentration of CuO. It can be observed that the magnetic susceptibility increases with increasing Cu content up to 10 mol% and then decreases. The figure also shows a sudden decrease for sample containing CuO between 10 and 15 mol% and very slight decrease between 15 and 20 mol%. It is clear that the magnetic susceptibility χ depends on the change of the role of copper ions in the sample’s structure due to the change in the copper glass-forming oxide [20]. In various glasses, Cu ions exist as Cu+, Cu2+ and Cuo, although most glasses melting under ordinary atmospheric conditions are usually assumed not to contain Cuo [21–23]. The increase in the magnetic susceptibility χ by increasing the copper content up to 10 mol% CuO is attributed to the formation of Cu2+ ions, which have high magnetic moment (1.7 μB). By increasing the copper oxide content over 10 mol% CuO, the magnetic susceptibility χ decreases due to the conversion of Cu2+ ions to Cu+ ions, which have zero magnetic moment. The magnetic susceptibility χ showed stability in the glass samples having 15 mol% CuO and above. This may be because no more Cu2+ ions change to Cu+ ions, and this result is confirmed by electrical conduction results.

Fig. 8
figure 8

Relation between the magnetic susceptibility and the concentration of CuO

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The visible absorption bands observed from the Cu-doped glass can be understood and related to the presence of Cu2+ ions. For the Cu2+ ion (d9), the d–d transition can be considered [17].

Figure 9 shows the optical transmission of the glass samples. The cutoff for the UV and infrared is determined and tabulated in Table 3. From Fig. 9 can be observed that the glass samples has two absorption bands, one in the UV and the other in infrared. But the sample free from copper has one absorption bands, in the UV. In addition, as the copper oxide increases, the infrared bands shifts and the optical transmission of UV band decreases (Table 3).

Fig. 9
figure 9

Optical transmission of the glass samples

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Table 3 UV and IR cutoff, bandstop, bandpass, band gap and band tail
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The effect of copper oxide contents of the cutting bands in UV and IR due to the copper has two absorption bands, one in the UV and the other in infrared. Moreover, as copper oxide increases, the band shifts and the absorption UV band increases, until threshold contents of copper oxide begin to have a cutoff in infrared region.

Figure 10 shows the cutoff and bandstop filter of UV. These filters (glass sample) prevent the spectral lines’ bandstop (UV). This filter, the UV bandstop, begins with 190 nm and increases with increasing the CuO doping; the UV cutoff, the end of the bandstop filter which starts from 350 to 460 nm, depends on the CuO concentration from 5 to 20 %. The IR cutoff and IR bandstop filter are shown in Fig. 10. The bandstop in the IR band begins with 800 nm and ends to 1060 nm.

Fig. 10
figure 10

Cutoff and bandstop filter of UV and IR

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The absorption broadband around 740 nm for all glass samples was observed which is due to Cu+2 in distorted octahedral group [23–25]. By the addition of copper content, the absorption peak shifts to high wavelength and increases the intensity up to 25 mol%. The observed shift to near-infrared region is due to the formation of copper tetrahedral group [24–26].

These observations indicate a gradual increase in the concentration of Cu2+ ions that take modifying positions in the glass network. This caused the higher content of NBOs in the glass matrix [5]. This leads to increase in the degree of localization of electrons, thereby increasing the donor centers in the glass matrix. This decreases the optical band gap.

In the present glass network, Cu2+ ion is considered to be in octahedral, tetragonal and square planar coordination. As the concentration of CuO is increased, an increase in the intensity of the broadband is observed. Such an increase indicates the conversion of a part of Cu+ ions to Cu2+ ions. The highest intensity of these two kinds observed in the spectrum of glass containing 25 mol% CuO suggests the presence of larger concentration of Cu+ ions in this glass.

The absorption coefficient is related to photon energy by the relation [27]:

$$\alpha = \, A \, \left( {h\nu - \, E_{\text{opt}} } \right)^{n} /h\nu$$
(1)

where A is constant, E opt is the energy gap and n is constant that depends on the mechanism of electron transition, wherein the direct transition, n, has (1/2 or 3/2) and indirect transition, n, has (2 or 3) depending on whether the transition is allowed or forbidden [28]. The value of E opt and n can be determined by drawing a relation between αhν 1/n and hν.

The best fit was found to be with n = 1/2, which is the characteristic behavior of indirect transition in all studied samples. The relation between E opt and CuO content is shown in Fig. 11. From Fig. 12, it can be seen that the optical gap decreases with the increase in copper content in glass samples.

Fig. 11
figure 11

Relation between optical energy gap and copper oxide content

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Fig. 12
figure 12

Relation between Urbach energy Et and CuO content

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The decrease in E opt with increasing copper content in lead borate glass samples could be related to the change in bridging oxygen BO to non-bridging oxygen NBO (good agreement with results of infrared), which binds excited electrons less firmly than bridging oxygen [29]. The ultraviolet transparency of oxide glasses has been investigated in terms of electronegativity of the ingredient oxygen atoms [30]. In glasses, negative charge on the NBOs is larger than that on the bridging oxygen. Increasing the iconicity of oxygen ions by converting them from BO to NBO ions decreases the band gap energy E opt. The concentration of NBOs in the glass matrix is higher [5]. This leads to increase in the degree of localization of electrons, thereby increasing the donor centers in the glass matrix. The presence of larger concentration of these donor centers decreases the optical band gap.

The width of the band tail was calculated using Urbach’s formula [31]

$$\ln \alpha = \, \ln B + \left( {h\nu/{\varvec{E}}_{{\bf t}} } \right)$$
(2)

where B is constant and E t is the width of the tail (localized states) in the optical band gap.

Figure 12 shows the relation between Urbach energy E t and CuO content. From Fig. 12, it can be observed that the Urbach energy E t increases with the increase in the copper content. Urbach energy, which corresponds to the width of localized states, is used to characterize the degree of disorder in amorphous and crystalline systems. Materials with large Urbach energy would have greater tendency to convert weak bonds into defects. As a result, the defect concentration could be determined by the measure of Urbach energy.

The dielectric constant can be calculated according to the relations [32]

$$\acute{\varepsilon} = n^{2} {-}k^{2}$$
(3)

and

$$\varepsilon = \, 2nk$$
(4)

where n is the refractive index and k is the extinction coefficient and calculated according Eq. (5)

$$k \, = \, \alpha \lambda /4\pi$$
(5)

[α is the absorption coefficient and λ is the wavelength]

Figure 13a, b shows the relation between έ and ε as a function of wavelength.

Fig. 13
figure 13

a, b Relation between έ and ε as a function of wavelength

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As shown in Fig. 13a, b, the electrooptical phenomena at the same wavelength (start and end bandpass) can be observed [24]. Lithium lead borate glass samples containing copper oxide are used to remove a region of the near-infrared spectrum from sunlight and to produce glasses which transmit in the green region.

The ultraviolet cutoff wavelength of glass samples is shifted toward the visible region by the addition of copper oxides to lithium lead borate. The addition of copper oxide formed the non-bridging oxygen in the glass network, which shifts the ultraviolet cutoff toward the visible region, forms longpass glass filter with cutoff wavelengths which vary with the non-bridging oxygen concentration and creates charge transfer bands, producing longpass filters with cutoffs in the range of 190 to 400 nm.

4 Conclusion

The trends of densities, conductivities and magnetic susceptibility are attributed to changes in the glass network structure. This behavior can be attributed to the transformation of copper ions from network modifier positions to network former positions. The conduction mechanism was found due to ionic and electronic conduction. And the magnetic susceptibility χ depends on the change of the role of copper ions in the sample’s structure due to the change in the copper glass-forming oxide. The optical properties indicate that the copper ions coexist in Cu+ state with Cu2+ state. The optical energy band gap E opt increases and the Urbach energy E tail decreases with the increase in the concentration of CuO up to 20 mol%. This leads to the increase in the degree of localization of electrons, thereby increasing the donor centers in the glass matrix. The glass samples containing copper are used as longpass filter and absorption glass.