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

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 𝑑_{𝑥2−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 Γ_𝜎 increases whereas Γ_𝜋 decreases with the increase in optical basicity.

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 standard 𝑔 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: ℋ=𝑔_||𝛽𝐻_𝑧𝑆_𝑧+𝑔_⟂𝛽𝐻_𝑥𝑆_𝑥+𝐻_𝑦𝑆_𝑦+𝐴_||𝐼_𝑧𝑆_𝑧+𝐴_⟂𝐼_𝑥𝑆_𝑥+𝐼_𝑦𝑆_𝑦,(1) 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_⟂>𝑔_𝑒 (=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 𝑑_{𝑥2−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.

Figure 1 

EPR spectra of Cu2+ in (60-𝑥)Bi2O3-25ZnO-15B2O3-𝑥Li2O glasses.

3.2. Optical Absorption Spectra

Figure 2 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 ²𝐵_1𝑔→²𝐴_1𝑔 and ²𝐵_1𝑔→²𝐵_2𝑔 transitions. Most of the authors [16, 17, 21, 22] assigned the observed optical peak to the ²𝐵_1𝑔→²𝐵_2𝑔 transition (Δ𝐸_𝑥𝑦) 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, Δ𝐸_𝑥𝑦 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 𝛽²¹ where α² describes the in-plane σ bonding with the copper 𝑑_{𝑥2−𝑦2} orbital, 𝛽² describes the out-of-plane π bonding with the 𝑑_𝑥𝑧 and 𝑑_𝑦𝑧 orbitals, and the 𝛽²¹ parameter is a measure of the in-plane 𝜋 bonding with the 𝑑_𝑥𝑦 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 𝛽²¹ can be interpreted similarly.

The bonding parameters were evaluated using the equations given below [23]:𝛼²=||||𝐴^||𝑃||||+𝑔^||−2+37𝑔^⟂𝛽−2+0.04,₂₁=𝑔^||𝑔_𝑒−13Δ31𝐸2𝛼^𝑥𝑦2𝛽₂,=𝑔^⟂𝑔_𝑒−1Δ8𝐸28𝛼^{𝑥𝑧,𝑦𝑧2},(2) where 𝑃 is the dipolar hyperfine coupling parameter, Δ𝐸_𝑥𝑦 is the energy corresponding to the transition ²𝐵_1𝑔→²𝐵_2𝑔, and λ is the spin-orbit coupling constant (λ = −828 cm⁻¹).

The corresponding value of Δ𝐸_{𝑥𝑧,𝑦𝑧} was calculated using the approximation [24]Δ𝐸_{𝑥𝑧,𝑦𝑧}=1656𝑘²𝑔_⟂−𝑔_𝑒,(3) where 𝑘² is the orbital reduction factor (=0.77).

The normalized covalencies of Cu (II)-O in-plane bonds of σ and π symmetry are expressed in terms of bonding coefficients α² and 𝛽²¹ as follows [16]:Γ_𝜎=200(1−𝑆)1−𝛼²(1−2𝑆)%,Γ^𝜋=2001−𝛽²¹%,(4) where 𝑆 is the overlap integral (𝑆_𝑜𝑥𝑦 = 0.076).

The calculated values of α², 𝛽²¹, 𝛽², Γ_𝜎, and Γ_𝜋 are presented in Table 1. The normalized covalency of Cu(II)-O in-plane bonding of π symmetry (Γ_𝜋) indicates the basicity of the oxide ion. The values of 𝛼²,𝛽²¹, 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 (Λ_th), for the present glasses, can be calculated using the following equation, which is based on the approach proposed by Duffy [26]:Λ_th=𝑋_Li2OΛ_Li2O+𝑋_ZnOΛ_ZnO+𝑋_Bi2O3Λ_Bi2O3+𝑋_B2O3Λ_B2O3,(5) where 𝑋_Li2O, 𝑋_ZnO, 𝑋_Bi2O3, and 𝑋_B2O3 are the contents of individual oxides in mole %. Λ_Li2O,Λ_ZnO, Λ_Bi2O3, and Λ_B2O3 are the theoretical optical basicity values assigned to oxides present in the glass. The values Λ_Li2O=1, Λ_ZnO=8.2, Λ_Bi2O3=1.19, and Λ_B2O3=0.42 are used in the present study [26, 27]. The Λ_th 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 Γ_𝜎 and Γ_𝜋 with optical basicity is represented in Figure 3. It is observed that there is an overall increase in Γ_𝜎 whereas Γ_𝜋 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 Γ_𝜋 values. Similar observation was reported by [11, 29].

Figure 3 

Variation of Γ𝜎 and Γ𝜋 with optical basicity (Λ).

4. Conclusions

From the values 𝑔_||>𝑔_⟂>𝑔_𝑒 (2.0023), it is concluded that the ground state of Cu²⁺ ions in all the samples under study in the present paper is 𝑑_{𝑥2−𝑦2} orbital (²𝐵_1𝑔 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 to ²𝐵_1𝑔→²𝐵_2𝑔 transition.

The values of the bonding parameters indicate that the in-plane sigma bonding is covalent whereas the in-plane and out-of-plane π 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 of Γ_𝜎 increases whereas Γ_𝜋 decreases with the increase in optical basicity and hence the covalency between Cu²⁺ ions and oxygen ligands decreases.