ISRN SpectroscopyVolume 2012 (2012), Article ID 583015, 5 pagesdoi:10.5402/2012/583015

Research Article

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

Shashidhar Bale and Syed Rahman

Department of Physics, Osmania University, Hyderabad 500 007, India

Received 25 February 2012; Accepted 28 March 2012

Academic Editors: W. A. Badawy, J. Casado, and M. Mączka

Copyright © 2012 Shashidhar Bale and Syed Rahman. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 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 bonding. The theoretical optical basicity parameter values were evaluated, and it was observed that the value of increases whereas 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-)Bi₂O₃-25ZnO-15B₂O₃-Li₂O glasses () 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-)Bi₂O₃-25ZnO-15B₂O₃-Li₂O () 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 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  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: 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, , 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 , , , and were estimated from the spectra and are given in Table 1. The estimated values of and satisfy the relationship (=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 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-)Bi₂O₃-25ZnO-15B₂O₃-Li₂O.

Parameters

2.337
2.332
2.333
2.312

2.127
2.127
2.079
2.056

160
114
108
99

49
60
48
55

(nm)
717
712
719
727

(cm⁻¹)
13947
14044
13908
13755

0.875
0.743
0.707
0.651

0.803
0.939
0.980
0.986

0.656
0.797
0.837
0.909

(%)
27
56
63
76

(%)
39
12
3
2

0.954
0.963
0.973
0.982

Figure 1: EPR spectra of Cu²⁺ in (60-)Bi₂O₃-25ZnO-15B₂O₃-Li₂O 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 and transitions. Most of the authors [16, 17, 21, 22] assigned the observed optical peak to the transition and have used this value in the evaluation of the bond parameters.

Figure 2: Optical absorption spectra of (60-)Bi₂O₃-25ZnO-15B₂O₃-Li₂O 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, 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 α ², , and where α ² describes the in-plane σ bonding with the copper orbital, describes the out-of-plane π bonding with the and orbitals, and the parameter is a measure of the in-plane bonding with the orbital. The values of α ² lie between 0.5 and 1, the limits of pure covalent and pure ionic bonding, respectively. The terms and can be interpreted similarly.

The bonding parameters were evaluated using the equations given below [23]: where is the dipolar hyperfine coupling parameter, is the energy corresponding to the transition , and λ is the spin-orbit coupling constant (λ = −828 cm⁻¹).

The corresponding value of was calculated using the approximation [24] where 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 α ² and as follows [16]: where is the overlap integral ( = 0.076).

The calculated values of α ², , , , and are presented in Table 1. The normalized covalency of Cu(II)-O in-plane bonding of π symmetry () indicates the basicity of the oxide ion. The values of ,, and 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 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 , for the present glasses, can be calculated using the following equation, which is based on the approach proposed by Duffy [26]: where , , , and are the contents of individual oxides in mole %. ,, , and are the theoretical optical basicity values assigned to oxides present in the glass. The values , , , and are used in the present study [26, 27]. The 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 and with optical basicity is represented in Figure 3. It is observed that there is an overall increase in whereas 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 values. Similar observation was reported by [11, 29].

Figure 3: Variation of and with optical basicity .

4. Conclusions

From the values (2.0023), it is concluded that the ground state of Cu²⁺ ions in all the samples under study in the present paper is orbital ( 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 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 increases whereas decreases with the increase in optical basicity and hence the covalency between Cu²⁺ ions and oxygen ligands decreases.

References

R. H. Sands, “Synchrotron oscillations induced by radiation fluctuations,” Physical Review, vol. 97, no. 2, pp. 470–473, 1955.

H. Imagava, “Polariton dispersion in biaxial and uniaxial crystals,” Physica Status Solidi, vol. 30, no. 2, pp. 449–454, 1968.

L. D. Bogomolowa, “The effects of copper impurity in vanadate and tungsten-phosphate glasses,” Journal of Non-Crystalline Solids, vol. 30, no. 3, pp. 379–383, 1979.

D. L. Griscom and R. E. Griscom, “Paramagnetic resonance of Mn²⁺ in glasses and compounds of the lithium borate system,” Journal of Chemical Physics, vol. 47, no. 8, pp. 2711–2722, 1967.

C. Stehle, C. Vira, D. Vira, D. Hogan, S. Feller, and M. Affatigato, “Optical and physical properties of bismuth borate glasses related to structure,” Physics and Chemistry of Glasses, vol. 39, no. 2, pp. 83–86, 1998.

S. Bale, N. S. Rao, and S. Rahman, “Spectroscopic studies of Bi₂O₃–Li₂O–ZnO–B₂O₃ glasses,” Solid State Sciences, vol. 10, no. 3, pp. 326–331, 2008.

S. Bale, M. Purnima, C. H. Srinivasu, and S. Rahman, “Vibrational spectra and structure of bismuth based quaternary glasses,” Journal of Alloys and Compounds, vol. 457, no. 1-2, pp. 545–548, 2008.

N. Srinivasa Rao, S. Bale, M. Purnima, K. Siva Kumar, and S. Rahman, “Optical absorption and electron spin resonance studies of Cu²⁺ in Li₂O–Na₂O–B₂O₃–As₂O₃ glasses,” Bulletin of Materials Science, vol. 28, no. 6, pp. 589–592, 2005.

B. Sumalatha, I. Omkaram, T. R. Rao, and Ch. L. Raju, “Alkaline earth zinc borate glasses doped with Cu²⁺ ions studied by EPR, optical and IR techniques,” Journal of Non-Crystalline Solids, vol. 357, no. 16-17, pp. 3143–3152, 2011.

I. Ardelean, M. Peteanu, R. Ciceo-Lucacel, and I. Bratu, “Structural investigation of CuO containing strontium-borate glasses by means of EPR and IR spectrometry,” Journal of Materials Science, vol. 11, no. 1, pp. 11–16, 2000.

G. Ramadevudu, M. Shareefuddin, N. Sunitha Bai, M. L. Rao, and M. N. Chary, “Electron paramagnetic resonance and optical absorption studies of Cu²⁺ spin probe in MgO–Na₂O–B₂O₃ ternary glasses,” Journal of Non-Crystalline Solids, vol. 278, no. 1–3, pp. 205–212, 2000.

S. Bale and S. Rahman, “Optical absorption and EPR studies on (70 − x)Bi₂O₃––Li₂O–30(ZnO–B₂O₃) (0 ≤ x ≤ 20) glasses,” Journal of Non-Crystalline Solids, vol. 355, no. 43-44, pp. 2127–2133, 2009.

O. Cozar, D. A. Magdas, and I. Ardelean, “EPR study of molybdenum-lead-phosphate glasses,” Journal of Non-Crystalline Solids, vol. 354, no. 10-11, pp. 1032–1035, 2008.

B. Bleaney, K. D. Bowers, and D. J. E. Ingram, “Paramagnetic resonance in diluted copper salts. I. Hyperfine structure in diluted copper tutton salts,” Proceedings of the Royal Society A, vol. 228, no. 1173, pp. 147–157, 1955.

H. Kawazoe, H. Hosono, and T. Kanazawa, “Electronic structure and properties of oxide glasses (I) π-electron distribution on alkali borate glass networks,” Journal of Non-Crystalline Solids, vol. 29, no. 2, pp. 159–171, 1978.

A. H. Maki and B. R. McGarvey, “Electron spin resonance in transition metal chelates. I. Copper (II) bis-acetylacetonate,” The Journal of Chemical Physics, vol. 29, no. 1, article 31, 4 pages, 1958.

D. Kivelson and R. Neiman, “ESR studies on the bonding in copper complexes,” The Journal of Chemical Physics, vol. 35, no. 2, 7 pages, 1961.

I. Siegel and J. A. Lorenc, “Paramagnetic resonance of copper in amorphous and polycrystalline GeO₂,” The Journal of Chemical Physics, vol. 45, no. 6, article 2315, 6 pages, 1966.

T. Bates, Modern Aspects of the Vitreous State, vol. 2, Butterworths, London, UK, 1962.

C. K. Jorgensen, “Comparative crystal field studies of some ligands and the lowest singlet state of paramagnetic nickel(II) complexes,” Acta Chemica Scandinavica, vol. 9, pp. 1362–1377, 1955.

A. H. Dietzel, “On the so-called mixed alkali effect,” Physics and Chemistry of Glasses, vol. 24, no. 6, pp. 172–180, 1983.

B. V. Raghavaiah, C. Laxmikanth, and N. Veeraiah, “Spectroscopic studies of titanium ions in PbO–Sb₂O₃–As₂O₃ glass system,” Optics Communications, vol. 235, no. 4–6, pp. 341–349, 2004.

G. van Veen, “Simulation and analysis of EPR spectra of paramagnetic ions in powders,” Journal of Magnetic Resonance, vol. 30, no. 1, pp. 91–109, 1978.

H. G. Hetch and T. S. Johnston, “Study of the structure of vanadium in soda—boric oxide glasses,” The Journal of Chemical Physics, vol. 46, no. 1, article 23, 12 pages, 1967.

J. A. Duffy and M. D. Ingram, “Establishment of an optical scale for lewis basicity in inorganic oxyacids, molten salts, and glasses,” Journal of the American Chemical Society, vol. 93, no. 24, pp. 6448–6454, 1971.

J. A. Duffy, “Electronic polarisability and related properties of the oxide ion,” Physics and Chemistry of Glasses, vol. 30, no. 1, pp. 1–4, 1989.

R. W. Young, J. A. Duffy, G. J. Hassall, and Z. Xu, “Use of optical basicity concept for determining phosphorus and sulfur slag-metal partitions,” Ironmaking and Steelmaking, vol. 19, no. 3, pp. 201–219, 1992.

S. Sindhu, S. Sanghi, A. Agarwal, V. P. Seth, and N. Kishore, “Effect of Bi₂O₃ content on the optical band gap, density and electrical conductivity of MO·Bi₂O₃·B₂O₃ (M = Ba, Sr) glasses,” Materials Chemistry and Physics, vol. 90, no. 1, pp. 83–89, 2005.

R. P. Sreekanth Chakradhar, B. Yasoda, J. L. Rao, and N. O. Gopal, “Mixed alkali effect in Li₂O–Na₂O–B₂O₃ glasses containing Fe₂O₃—an EPR and optical absorption study,” Materials Research Bulletin, vol. 41, no. 9, pp. 1646–1656, 2006.