# EPR and Optical Absorption Spectral Investigations of Cu2+ in Bi2O3-ZnO-B2O3-Li2O Glasses

Authors: Shashidhar Bale; Syed Rahman
Journal: ISRN Spectroscopy
 (2012)

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## Abstract

Electron paramagnetic resonance and optical absorption studies of Bi2O3-ZnO-B2O3-Li2O glasses were made by introducing Cu2+ as spin probe. The EPR spectra of Cu2+ in all the glass samples recorded in the X-band frequency have similar spectral features. The variation in glass composition influences the spin Hamiltonian parameters calculated from the spectra. The spin Hamiltonian parameters indicate that the Cu2+ ions are coordinated with six ligand atoms in a distorted octahedron elongated along one of the axes and the ground state of the Cu2+ is dx2−y2 orbital. The optical absorption spectra exhibited a broad band corresponding to d-d transition bands of Cu2+ ion. The values of bonding parameters indicate a covalent nature for the in-plane σ bonding and pure ionic nature for the in-plane and out-of-plane π bonding. The theoretical optical basicity parameter values were evaluated, and it was observed that the value of Γσ increases whereas Γπ
decreases with the increase in optical basicity.


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## Body

## 1. Introduction

Electron paramagnetic resonance (EPR) studies of transition metal ions in oxide glasses is of scientific interest and gives information concerning the state of the ligands, the glass structure, nature of bonding, and site symmetry around metal ion [1–4]. The electron paramagnetic resonance (EPR) spectroscopic technique was first applied to glassy materials by Sands [1]. Unconventional glasses containing Bi2O3 as glass former are of great interest because of their potential applications in industry and many allied areas [5–7]. EPR investigations of Cu2+ ions in glasses are interesting and have received a considerable attention due to the sensitivity of parameters to local symmetry and have been studied in wide variety of glasses [8–13]. Optical absorption of transition metal (TM) ions in glasses is influenced by host structure into which the transition metal ions are incorporated. In oxide glasses, the TM ions mostly form coordination complexes with doubly charged oxygen as the ligands. By correlating the EPR and optical absorption spectra, one can obtain information regarding the bond parameters that determine the metal-ligand bond in the glasses.In this paper EPR and optical absorption properties of 1 mole % copper-doped (60-x)Bi2O3-25ZnO-15B2O3-xLi2O glasses (0≤x≤15) are studied. The influence of Li2O on the EPR and optical absorption parameters will be discussed. The variation of these parameters will be correlated with optical basicity of the glasses.


## 2. Experimental

In the present study copper-(1 mole %) doped glass samples of compositions (60-x)Bi2O3-25ZnO-15B2O3-xLi2O (0≤x≤15) were prepared by melt quench technique using reagent grade chemicals Bi2O3, ZnO, H3BO3, Li2CO3, and CuO. The mixture of these chemicals is taken in porcelain crucibles and was calcinated at 450°C for 1 h and then melted at 1100–1200°C depending on the glass composition. The liquids were agitated for 1 h to ensure homogeneity in the mixture. The clear liquid was quickly cast in a stainless steel mould kept at 200°C and pressed with another steel disc to obtain glass. All samples were transparent and greenish in colour. The glasses were chemically stable and nonhygroscopic. Thus, the obtained glasses were annealed at 200°C for 12 h to remove thermal stress and strain.The room temperature EPR spectra of powdered glass samples were recorded using a JEOL-1X-EPR spectrometer in the range 2200 G–4200 G operating in the X-band and employing a field modulation of 100 kHz. DPPH was used as the standardg marker for the determination of magnetic field.The optical absorption spectra of the present glass samples were recorded at room temperature using a double-beam Shimadzu spectrometer (model UV-3100) in the wavelength range 500–800 nm. The uncertainty in the observed wave length is about±1 nm.


## 3. Results and Discussion

### 3.1. EPR Spectra

The room temperature EPR spectra of the present glasses containing Cu2+ are shown in Figure 1. Each spectrum was analysed using the spin Hamiltonian:
(1)H=g||βHzSz+g⊥β(HxSx+HySy)+A||IzSz+A⊥(IxSx+IySy),
where the symbols have their usual meaning [14]. From the figure it is observed that the obtained absorption spectra are asymmetric, characteristic of Cu2+ (3d9) ions in axially distorted octahedral symmetric sites. It is found that the spectra keep their overall aspect in the entire composition range suggesting high structural stability of the glassy matrix to accept Cu2+ ions. The spectra show the hyperfine structure due to the interaction of the unpaired electron spin with the nuclear one, I=3/2, characteristic of Cu2+. The hyperfine structure is resolved in the parallel band of the spectra, and the perpendicular component is unresolved. Three hyperfine lines were observed on the parallel features of the spectrum. The variations in the hyperfine line widths can be attributed to the fluctuations in the coordination sphere surrounding the probe Cu2+ ion [15]. The values of spin Hamiltonian parameters g||, g⊥, A||, and A⊥ were estimated from the spectra and are given in Table 1. The estimated values of g|| and g⊥ satisfy the relationship g||>g⊥>ge (=2.0023) characteristic of Cu2+ ions coordinated with six ligand atoms in a distorted octahedron, elongated along one of the axes and the ground state of the Cu2+ is dx2-y2 orbital. This configuration satisfies the conditions for applying the analysis developed by Maki and McGarvey [16] and modified by Kivelson and Neiman [17]. The change in spin Hamiltonian parameters with composition can be attributed to the variation of ligand field around the probe ion.Table 1
EPR and optical parameters of the glass system (60-x)Bi2O3-25ZnO-15B2O3-xLi2O.
Parametersx=15x=10x=5x=0g||2.3372.3322.3332.312g⊥2.1272.1272.0792.056A||×10-4(cm-1)16011410899A⊥×10-4(cm-1)49604855λ (nm)717712719727ΔExy (cm−1)13947140441390813755α20.8750.7430.7070.651β120.8030.9390.9800.986β20.6560.7970.8370.909Γσ (%)27566376Γπ (%)391232Λth0.9540.9630.9730.982Figure 1
EPR spectra of Cu2+ in (60-x)Bi2O3-25ZnO-15B2O3-xLi2O glasses.


### 3.2. Optical Absorption Spectra

Figure2 presents the optical absorption spectra of Cu2+ ions in the present glasses. A single absorption band in the near-infrared region was observed for all samples. This band in the near-infrared region can be identified as d-d transition band due to Cu2+ ions [18] and can be described in terms of ligand field theory [19]. In glasses it is assumed that due to vitreous state disorder, no site is perfectly cubic. Therefore, tetragonal distortions are endemic to the vitreous state, which leads to the splitting of energy levels. It is observed that the elongated structures are usually more energetically favoured than the compressed ones [20]. Hence, in the present investigation the observed asymmetric band is due to overlap of B1g2→A1g2 and B1g2→B2g2 transitions. Most of the authors [16, 17, 21, 22] assigned the observed optical peak to the B1g2→B2g2 transition (ΔExy) and have used this value in the evaluation of the bond parameters.Figure 2
Optical absorption spectra of (60-x)Bi2O3-25ZnO-15B2O3-xLi2O glasses.


### 3.3. Bond Parameters

To determine the bonding coefficients of Cu2+, one needs to know the absorption frequencies from the optical absorption bands. In a given glass, ΔExy is the frequency of an intense absorption band of Cu2+ in the optical spectrum. By correlating EPR and optical absorption data, one can evaluate the bonding coefficients of Cu2+. The bonding between the Cu2+ ion and its ligands can be described in terms of the covalency parameters α2,  β2, and β12 where α2 describes the in-plane σ bonding with the copper dx2-y2 orbital, β2 describes the out-of-plane π bonding with the dxz and dyz orbitals, and the β12 parameter is a measure of the in-plane π bonding with the dxy orbital. The values of α2 lie between 0.5 and 1, the limits of pure covalent and pure ionic bonding, respectively. The terms β2 and β12 can be interpreted similarly.The bonding parameters were evaluated using the equations given below [23]:(2)α2=|A||P|+(g||-2)+37(g⊥-2)+0.04,β12=[(g||ge)-1]ΔExy3312α2,β2=[(g⊥ge)-1]ΔExz,yz828α2,
where P is the dipolar hyperfine coupling parameter, ΔExy is the energy corresponding to the transition B1g2→B2g2, and λ is the spin-orbit coupling constant (λ = −828 cm−1).The corresponding value ofΔExz,yz was calculated using the approximation [24](3)ΔExz,yz=1656k2(g⊥-ge),
where k2 is the orbital reduction factor (=0.77).The normalized covalencies of Cu (II)-O in-plane bonds ofσ and π symmetry are expressed in terms of bonding coefficients α2 and β12 as follows [16]:(4)Γσ=200(1-S)(1-α2)(1-2S)%,Γπ=200(1-β12)%,
where S is the overlap integral (Soxy = 0.076).The calculated values ofα2, β12, β2, Γσ, and Γπ are presented in Table 1. The normalized covalency of Cu(II)-O in-plane bonding of π symmetry (Γπ) indicates the basicity of the oxide ion. The values of α2,β12, and β2 indicate a covalent nature for the in-plane σ bonding and pure ionic nature for the in-plane and out-of-plane π bonding.


### 3.4. Optical Basicity

The optical basicity(Λ) of an oxide medium as proposed by Duffy and Ingram [25] is a numerical expression of the average electron donor power of the oxide species constituting the medium. It is used as a measure of the acid-base properties of oxides, glasses, alloys, molten salts, and so forth. Theoretical optical basicity (Λth), for the present glasses, can be calculated using the following equation, which is based on the approach proposed by Duffy [26]:(5)Λth=XLi2OΛLi2O+XZnOΛZnO+XBi2O3ΛBi2O3+XB2O3ΛB2O3,
where XLi2O, XZnO, XBi2O3, and XB2O3 are the contents of individual oxides in mole %. ΛLi2O,ΛZnO, ΛBi2O3, and ΛB2O3 are the theoretical optical basicity values assigned to oxides present in the glass. The values ΛLi2O=1, ΛZnO=8.2, ΛBi2O3=1.19, and ΛB2O3=0.42 are used in the present study [26, 27]. The Λth values calculated using the above equation for the present glasses are given in Table 1. It can be seen from Table 1 that the theoretical optical basicity increases slightly with increasing Bi2O3 content, which can be attributed to the high polarizability of bismuth ions. Similar observations were reported earlier [6, 28]. The variation of Γσ and Γπ with optical basicity is represented in Figure 3. It is observed that there is an overall increase in Γσ whereas Γπ decreases with the increase in optical basicity. The optical basicity can also be used to classify the covalent/ionic ratios of the glasses since an increase in the optical basicity indicates decreasing covalency. Therefore, in the present investigation, with the increase in Bi2O3 content, optical basicity increases and hence the covalency between Cu2+ ions and oxygen ligands decreases, which is clear from Γπ values. Similar observation was reported by [11, 29].Figure 3
Variation ofΓσ and Γπ with optical basicity (Λ).


## 3.1. EPR Spectra

The room temperature EPR spectra of the present glasses containing Cu2+ are shown in Figure 1. Each spectrum was analysed using the spin Hamiltonian:
(1)H=g||βHzSz+g⊥β(HxSx+HySy)+A||IzSz+A⊥(IxSx+IySy),
where the symbols have their usual meaning [14]. From the figure it is observed that the obtained absorption spectra are asymmetric, characteristic of Cu2+ (3d9) ions in axially distorted octahedral symmetric sites. It is found that the spectra keep their overall aspect in the entire composition range suggesting high structural stability of the glassy matrix to accept Cu2+ ions. The spectra show the hyperfine structure due to the interaction of the unpaired electron spin with the nuclear one, I=3/2, characteristic of Cu2+. The hyperfine structure is resolved in the parallel band of the spectra, and the perpendicular component is unresolved. Three hyperfine lines were observed on the parallel features of the spectrum. The variations in the hyperfine line widths can be attributed to the fluctuations in the coordination sphere surrounding the probe Cu2+ ion [15]. The values of spin Hamiltonian parameters g||, g⊥, A||, and A⊥ were estimated from the spectra and are given in Table 1. The estimated values of g|| and g⊥ satisfy the relationship g||>g⊥>ge (=2.0023) characteristic of Cu2+ ions coordinated with six ligand atoms in a distorted octahedron, elongated along one of the axes and the ground state of the Cu2+ is dx2-y2 orbital. This configuration satisfies the conditions for applying the analysis developed by Maki and McGarvey [16] and modified by Kivelson and Neiman [17]. The change in spin Hamiltonian parameters with composition can be attributed to the variation of ligand field around the probe ion.Table 1
EPR and optical parameters of the glass system (60-x)Bi2O3-25ZnO-15B2O3-xLi2O.
Parametersx=15x=10x=5x=0g||2.3372.3322.3332.312g⊥2.1272.1272.0792.056A||×10-4(cm-1)16011410899A⊥×10-4(cm-1)49604855λ (nm)717712719727ΔExy (cm−1)13947140441390813755α20.8750.7430.7070.651β120.8030.9390.9800.986β20.6560.7970.8370.909Γσ (%)27566376Γπ (%)391232Λth0.9540.9630.9730.982Figure 1
EPR spectra of Cu2+ in (60-x)Bi2O3-25ZnO-15B2O3-xLi2O glasses.


## 3.2. Optical Absorption Spectra

Figure2 presents the optical absorption spectra of Cu2+ ions in the present glasses. A single absorption band in the near-infrared region was observed for all samples. This band in the near-infrared region can be identified as d-d transition band due to Cu2+ ions [18] and can be described in terms of ligand field theory [19]. In glasses it is assumed that due to vitreous state disorder, no site is perfectly cubic. Therefore, tetragonal distortions are endemic to the vitreous state, which leads to the splitting of energy levels. It is observed that the elongated structures are usually more energetically favoured than the compressed ones [20]. Hence, in the present investigation the observed asymmetric band is due to overlap of B1g2→A1g2 and B1g2→B2g2 transitions. Most of the authors [16, 17, 21, 22] assigned the observed optical peak to the B1g2→B2g2 transition (ΔExy) and have used this value in the evaluation of the bond parameters.Figure 2
Optical absorption spectra of (60-x)Bi2O3-25ZnO-15B2O3-xLi2O glasses.


## 3.3. Bond Parameters

To determine the bonding coefficients of Cu2+, one needs to know the absorption frequencies from the optical absorption bands. In a given glass, ΔExy is the frequency of an intense absorption band of Cu2+ in the optical spectrum. By correlating EPR and optical absorption data, one can evaluate the bonding coefficients of Cu2+. The bonding between the Cu2+ ion and its ligands can be described in terms of the covalency parameters α2,  β2, and β12 where α2 describes the in-plane σ bonding with the copper dx2-y2 orbital, β2 describes the out-of-plane π bonding with the dxz and dyz orbitals, and the β12 parameter is a measure of the in-plane π bonding with the dxy orbital. The values of α2 lie between 0.5 and 1, the limits of pure covalent and pure ionic bonding, respectively. The terms β2 and β12 can be interpreted similarly.The bonding parameters were evaluated using the equations given below [23]:(2)α2=|A||P|+(g||-2)+37(g⊥-2)+0.04,β12=[(g||ge)-1]ΔExy3312α2,β2=[(g⊥ge)-1]ΔExz,yz828α2,
where P is the dipolar hyperfine coupling parameter, ΔExy is the energy corresponding to the transition B1g2→B2g2, and λ is the spin-orbit coupling constant (λ = −828 cm−1).The corresponding value ofΔExz,yz was calculated using the approximation [24](3)ΔExz,yz=1656k2(g⊥-ge),
where k2 is the orbital reduction factor (=0.77).The normalized covalencies of Cu (II)-O in-plane bonds ofσ and π symmetry are expressed in terms of bonding coefficients α2 and β12 as follows [16]:(4)Γσ=200(1-S)(1-α2)(1-2S)%,Γπ=200(1-β12)%,
where S is the overlap integral (Soxy = 0.076).The calculated values ofα2, β12, β2, Γσ, and Γπ are presented in Table 1. The normalized covalency of Cu(II)-O in-plane bonding of π symmetry (Γπ) indicates the basicity of the oxide ion. The values of α2,β12, and β2 indicate a covalent nature for the in-plane σ bonding and pure ionic nature for the in-plane and out-of-plane π bonding.


## 3.4. Optical Basicity

The optical basicity(Λ) of an oxide medium as proposed by Duffy and Ingram [25] is a numerical expression of the average electron donor power of the oxide species constituting the medium. It is used as a measure of the acid-base properties of oxides, glasses, alloys, molten salts, and so forth. Theoretical optical basicity (Λth), for the present glasses, can be calculated using the following equation, which is based on the approach proposed by Duffy [26]:(5)Λth=XLi2OΛLi2O+XZnOΛZnO+XBi2O3ΛBi2O3+XB2O3ΛB2O3,
where XLi2O, XZnO, XBi2O3, and XB2O3 are the contents of individual oxides in mole %. ΛLi2O,ΛZnO, ΛBi2O3, and ΛB2O3 are the theoretical optical basicity values assigned to oxides present in the glass. The values ΛLi2O=1, ΛZnO=8.2, ΛBi2O3=1.19, and ΛB2O3=0.42 are used in the present study [26, 27]. The Λth values calculated using the above equation for the present glasses are given in Table 1. It can be seen from Table 1 that the theoretical optical basicity increases slightly with increasing Bi2O3 content, which can be attributed to the high polarizability of bismuth ions. Similar observations were reported earlier [6, 28]. The variation of Γσ and Γπ with optical basicity is represented in Figure 3. It is observed that there is an overall increase in Γσ whereas Γπ decreases with the increase in optical basicity. The optical basicity can also be used to classify the covalent/ionic ratios of the glasses since an increase in the optical basicity indicates decreasing covalency. Therefore, in the present investigation, with the increase in Bi2O3 content, optical basicity increases and hence the covalency between Cu2+ ions and oxygen ligands decreases, which is clear from Γπ values. Similar observation was reported by [11, 29].Figure 3
Variation ofΓσ and Γπ with optical basicity (Λ).


## 4. Conclusions

From the valuesg||>g⊥>ge (2.0023), it is concluded that the ground state of Cu2+ ions in all the samples under study in the present paper is dx2-y2 orbital (B1g2 state) and the site symmetry around Cu2+ ion is tetragonally distorted octahedral sites.From the nature and position of the absorption band observed, it can be inferred that the band corresponds toB1g2→B2g2 transition.The values of the bonding parameters indicate that the in-plane sigma bonding is covalent whereas the in-plane and out-of-planeπ bonding is ionic in nature. The change in the spin Hamiltonian parameters with Li2O content is attributed to the change in local structure.The value ofΓσ increases whereas Γπ decreases with the increase in optical basicity and hence the covalency between Cu2+ ions and oxygen ligands decreases.



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*Source: 583015-2012-06-21.xml*