ARTICLE pubs.acs.org/JPCA
Effect of Bi2O3 Addition on Electron Paramagnetic Resonance, Optical Absorption, and Conductivity in Vanadyl-Doped Li2O-K2O-Bi2O3-B2O3 Glasses M. Subhadra and P. Kistaiah* Department of Physics, University College of Science, Osmania University, Hyderabad-500007, India ABSTRACT: Glasses with composition 15Li2O-15K2O-xBi2O3-(65 - x)B2O3/5V2O5 (3 e x e 15) have been prepared by the conventional melt quench technique. The electron paramagnetic resonance spectra of VO2þ in these glasses have been recorded in the X-band frequency (≈9.3 GHz) at room temperature. The spin Hamiltonian parameters and covalency rates were evaluated. It was found that the V4þ ions exist as vanadyl (VO2þ) ions and are in an octahedral coordination with a tetragonal compression. The covalency rates (1 - R2) and (1 - γ2) indicate moderate covalency for the σ- and πbonds. It was observed that the spin-Hamiltonian parameters depend slightly on the relative concentration of Bi2O3. The optical properties of this glass system are studied from the optical absorption spectra recorded in the wavelength range 200-800 nm. The fundamental absorption edge has been identified from the optical absorption spectra. The values of optical band gap for indirect allowed transitions have been determined using available theories. The direct current electrical conductivity, σ, has been measured in the temperature range 373-573 K. The conductivity decreases with the increase in Bi2O3 concentration. This has been discussed in terms of the decrease in the number of mobile ions and their mobility. An attempt is made to correlate the EPR, optical, and electrical results and to find the effect of Bi2O3 content on these parameters.
1. INTRODUCTION Electron paramagnetic resonance (EPR) spectroscopy is a powerful experimental technique that can be used to get information about the structural and dynamic phenomena of materials. EPR studies of glasses containing transition metal (TM) ions have been used to obtain information regarding the glassy network and to identify the site symmetry around the TM ion. The transition metal ions can be used to probe the glass structure, because their outer d electron orbital functions have rather broad radial distributions and their responses to surrounding cations are very sensitive.1 The changes in the composition of the glass may change the local environment of the TM ion incorporated into the glass leading to the ligand field changes, which may be reflected in EPR spectra. Several studies have been made on the EPR spectra of TM ions in oxide glasses.2-9 The borate glasses doped with TM ions have attracted a great deal of attention because of their potential applications in the development of new tunable solid-state lasers, solar-energy converters, and fiber-optic communication devices.10 The EPR spectrum of VO2þ in glasses is rich in hyperfine structure due to the 51V nucleus (nuclear spin I = 7/2) and is easily observable in most of the glass systems at room temperature.11-17 Bismuth-based glasses have been extensively investigated because of their interesting technological applications due to their optical properties. These glasses are of scientific interest due to their long infrared (IR) cut off, optical nonlinearity, and important applications in the field of glass ceramics. Several authors have investigated the properties of bismuth glasses and different ideas were r 2011 American Chemical Society
presented for its role in glass structure.18-21 Bi2O3 content plays a vital role in tailoring the optical properties in these glasses. It is found that the optical band gap decreases with the increase in Bi2O3 content.22-25 These glasses can be used as ultrafast optical switches, photonic devices, plasma display panels, and dielectric layers. It has been established that alkali borate glasses can be used as solid electrolytes. These glassy electrolytes are of significance because of their inherent advantages such as isotropic conductivity, ease of preparation, better thermal stability, and large available composition ranges. It has been shown that the addition of Bi2O3 leads to an improvement in the chemical durability and thermal stability of oxide glasses.26 In view of the potential applications of the vanadyl-doped alkali bismuth borate glasses, it is worthwhile to have some understanding of the spectroscopic and conductivity properties of such multicomponent glasses. The main objective of the present work is to study the effects of the added Bi2O3 on the optical and electrical properties at room temperature of these glasses. The studies on electrical properties pave the way for estimating the insulating character of the glasses, whereas the spectroscopic investigations give the information on the position and oxidation states of the vanadium ions in the glass network. This in turn helps to assess the suitability of these glasses for practical applications. To the best of our knowledge, such types of work have not been reported in the literature for these glasses. Received: July 13, 2010 Published: January 19, 2011 1009
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Table 1. Glass Compositions of the Samples in Bi Series Glass System composition in mol % Sl. No.
sample code
x (mol %)
Li2O
K2O
Bi2O3
B2O3
V2O5
1
Bi-3
3
15
15
3
62
5
2
Bi-5
5
15
15
5
60
5
3 4
Bi-7 Bi-10
7 10
15 15
15 15
7 10
58 55
5 5
5
Bi-12
12
15
15
12
53
5
6
Bi-15
15
15
15
15
50
5
2. EXPERIMENTAL SECTION Glasses with composition 15Li2O-15K2O-xBi2O3-(65 - x)B2O3/5V2O5 (3 e x e 15) have been prepared by a conventional melt quench technique. For glass preparation, analar grade reagents of Bi2O3, H3BO3, Li2CO3, K2CO3, and V2O5 were taken in appropriate proportions in accordance with the above formulas and ground together to constitute a 10 g batch. The ground mixture was taken in porcelain crucibles and melted slowly in an electrical muffle furnace under ordinary atmospheric conditions. The temperature of the furnace was first increased to 400 C to remove water vapor, carbon dioxide, and so on, and then the temperature was increased to about 1050-1100 C, depending on the glass composition. The melt was kept at this temperature for 45 min in the furnace during which it was occasionally stirred to ensure homogeneity. The homogenized melt was poured on to a preheated steel mold plate to avoid breaking of the samples due to thermal strains and pressed it with another plate quickly to get disk shaped samples of 1-2 mm thickness. These glass samples were then immediately transferred to another furnace kept at 350 C and annealed for 3 h to remove the thermal strains and cooled to room temperature. The compositions of the glasses studied in the present investigation are given in Table 1. The concentration of Bi2O3 was limited to 15 mol % inside the host glass matrix because the glass formation gets harder beyond this limit. This concentration is the maximum load of Bi2O3 in our glass system without devitrification. It is possible that corrosive melts at high temperatures can attack the porcelain made crucibles. However, the integrity of the porcelain crucibles used in this study after quenching the melt was found to be intact. Further, there are numerous reports in the literature wherein the glasses were prepared at the temperature ranges operated here27,28 and no chemical analysis on the samples was carried out and the set composition was assumed. The amorphous nature of the glass samples was confirmed using powder X-ray diffraction (XRD). The XRD spectra did not show any sharp peaks but showed only broad humps typical of amorphous materials. The densities of annealed glass samples were determined through the method of apparent weight loss in xylene. The detailed characterization results were given elsewhere.29 The EPR spectra were recorded at room temperature by using an EPR spectrometer (JEOL- FE-1X) operating in the X-band frequency (≈9.205 GHz) with a field modulation of 100 kHz. The magnetic field was scanned from 200 to 500 mT. The modulation width was set at 0.1 mT and the microwave power used was 5 mW. Polycrystalline diphenyl picryl hydrazyl with g = 2.0023 was used as a standard for the determination of spin Hamiltonian parameters (SHP). The optical absorption spectra of these glasses were recorded at room temperature (300 K) in the UV region to measure the optical
absorption edges by using a UV Elmer Lambda 750 spectrophotometer in the wavelength region 200-800 nm. Glass samples of thickness about 0.4 mm were grinded to smooth surface and finely polished with the help of emery paper and cloth for recording the optical absorption spectra. To measure the d.c. conductivity, samples in the form of discs of nearly 1 mm uniform thickness were chosen. Colloidal silver paint was used as an electrode material. The silver painted samples were sandwiched between two copper electrodes and then fitted in designed holder connected to a programmable furnace with heating rate 2 C/min. The temperature of the samples was measured using a chromel-alumel thermocouple kept very near to the glass specimen. To verify reproducibility and minimize errors the experiment was repeated and data collected over several experiments have been averaged. Conductivity measurements were made by the standard technique,30 that is, two terminal method over a temperature range from about 373-573 K, first by increasing the temperature and then by decreasing it. A constant voltage of 1.5 V was applied across the sample and the current was measured by using a Keithley 614 electrometer. To minimize the polarization effects, the d.c. voltage was applied for a very short period ( A^.3,5,48 Hochstrasser11 reached the same conclusion by comparing the measured spin Hamiltonian parameters with the ones previously reported for V4þ and VO2þ in other glass matrices. A detailed analysis of g-tensor in the presence of trigonal symmetry was given by Gladney and Swalen.49 The values of spin Hamiltonian parameters obtained in the present study satisfy this relationship and are comparable to the values of other vanadyl complexes reported.3,11-17 It is therefore concluded that V4þ in the present glass samples exists as VO2þ ions in octahedral coordination with tetragonal compression. The symmetry of the vanadyl complex is C4v and the ground state of 3d1 ion is dxy. Tables 2 and 3 show that the values of g , g^, and A0^ increase with an increase in the mol % of Bi2O3, whereas the values of A , A^, and A0 decrease. The value of ((Δg )/(Δg^)), a measure of tetragonal distortion, is found to increase with the increase in x up to 10 mol %, and thereafter, it shows a small decrease. This gives an indication that the tetragonal distortion around the V4þ complex increases up to x = 10 mol %, and with a further increase in x, the octahedral symmetry is found to be increasing. This is attributed to the structural changes occurring in the glass network due to the introduction of bismuth. Bismuth acts partly as network modifier and partly as network former. The increase in the tetragonal distortion is supported by the increase in the value of A0^. When the value of A0^ increases, the interaction between the unpaired electron and nucleus increases and the 3dxy orbital contracts. This results in the increase of the tetragonal distortion. Also, from Table 3 it is observed that the values of (1 - R2) and (1 - γ2) are small, indicating moderate covalency for the σ- and π-bonds. 4.2. Optical Absorption. From the analysis of optical absorption spectra it is found that optical absorption edge is not sharply defined in the glass system under study. This is an indication of its amorphous nature. From Table 4 it is observed that the optical band gap values for indirect transitions vary between 2.77 and 3.02 eV and they are found to decrease with increasing x. Similar variations were observed in some bismuth borate glasses.23,50 In the present glass system the absorption edges and cutoff wavelengths for different compositions are tabulated in Table 4. It is observed that the fundamental absorption edge and cutoff wavelength shift toward red with the increase in Bi2O3 content. The decrease in Eopt with increasing Bi2O3 content corresponds with the red shift in λc. This suggests that the non bridging oxygen (NBO) ions are increasing with increasing concentration of Bi2O3. The NBO ions contribute to valence band maximum. When a metal-oxygen bond is broken, the bond energy is released and the non bridging orbitals have higher energies than bonding orbitals.51 Increase in the concentration of NBO ions results in shifting of valence band maximum to higher energies and thus reduce the band gap. The values of Urbach energies are found to vary between )
Table 4. Cut Off Wave Length, Absorption Edge, and Optical Energy Gap of Indirect Transition and Urbach Energy of Bi Series Glass System
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Table 5. DC Conductivity (σ) and Activation Energy (W) of Bi Series Glass System glass system
x (mol %)
σ at 503 K (Ω-1 cm-1)
σ at 553 K (Ω-1 cm-1)
σ at 583 K (Ω-1 cm-1)
W (eV)
Bi-3
3
5.13 10-9
3.01 10-8
8.89 10-8
0.79
-9
-8
Bi-5
5
3.48 10
2.74 10
6.85 10-8
0.85
Bi-7
7
3.34 10-9
2.09 10-8
5.05 10-8
0.95
Bi-10
10
1.77 10-9
7.08 10-9
1.95 10-8
1.08
Bi-12
12
1.70 10-9
5.10 10-9
1.70 10-8
1.23
Bi-15
15
1.50 10-9
3.00 10-9
1.05 10-8
1.34
)
belong to C4v symmetry with dxy ground state. There is an expansion of the 3dxy orbital of unpaired electron in the vanadyl ion with the increase in Bi2O3 content. The values of covalency rates (1 - R2) and (1 - γ2) indicate a moderate covalency for the σ- and π-bonds, respectively. The value of (Δg /g|) increases up to x = 10 mol % and thereafter shows a small decrease. This indicates an increase in tetragonal distortion around vanadium complex up to x = 10 mol % and thereafter an increase in octahedral symmetry. The optical absorption studies revealed that the optical absorption edges are not sharply defined a characteristic feature of the amorphous nature of the prepared glass samples. The analysis of optical absorption data reveals that the optical energy gap, Eopt, decreases with an increase in Bi2O3 content due to an increase in the formation of nonbridging oxygens. The DC conductivity of the samples increases with an increase in temperature due to an increase in the mobility of the ions. The DC conductivity decreases and the activation energy increases with the increase in Bi2O3 content because of a decrease in the number of mobile ions and their mobility. The DC conductivity is ionic in nature and there is no electronic contribution to it because of the addition of Bi2O3 in the present glass system. The EPR and optical spectroscopic studies have allowed us to follow the evaluation of the glass structure of present glass system. The EPR spectral analysis revealed that the addition of Bi2O3 to the borate glasses causes a change in the symmetry around the vanadium ion, indicating a change in the magnetic interactions between these ions. This is responsible for the change in magnetic hyperfine constants. The analysis of optical absorption spectra revealed that the addition of Bi2O3 to the borate glasses causes a change in the form of absorption edge and shifting of valence band maximum into the band gap, resulting in the decrease in the optical band gap energy. The DC conductivity studies revealed that the mobility of ions increases with temperature and at a given temperature decreases with the Bi2O3 content. The decrease in the mobility with increasing x is attributed to the interactions of mobile ions with the (BiO6)3and (BiO5)2- units. The observed Eopt values and the shift in the absorption edge with increasing x indicate the suitability of these glasses to optical device applications. The small value of conductivity and its decrease with increasing x indicates the suitability of these glasses to electrical insulating applications.
2
3 Figure 6. Variation of activation energy (W) with concentration of Bi2O3 (x) in Bi series of glass system.
0.11 and 0.16 eV showing a small increase with increasing x. This results in the increase in the disorder in the glass network and shifting of valence band maximum to higher energies with increasing Bi2O3 content. 4.3. DC Conductivity. It is clear from Figure 5 that the DC conductivity increases with a rise in temperature and decreases with the addition of Bi2O3 content at a given temperature. The activation energy shows the reverse behavior. The increase in conductivity follows the Arrhenius law which is attributed to the increase in mobility of ions with temperature. The decrease in the conductivity with increasing Bi2O3 content could be interpreted in terms of a decrease in mobility.52,53 and the proportion of lithium ions available for conduction because Liþ ions may be interacting with [BiO6]3- and [BiO5]2- units rather than taking part in conduction . In other words, one can say that the increase of Bi2O3 shows a “blocking effect” on the migration of mobile ions rather than making any contribution in the form of electronic conductivity. Similar results were also observed by Agarwal et al.13,15 in vanadyl-doped Bi2O3 3 K2O 3 B2O3 and Bi2O3 3 Li2O 3 B2O3 glass samples, respectively.
5. CONCLUSIONS EPR, optical, and electrical studies have been carried out to explore the role of Bi2O3 content in the 15Li2O-15K2OxBi2O3-(65 - x)B2O3/5V2O5 glasses. The following conclusions are drawn from these studies. 1 The intensity of the EPR spectra of the samples is found to decrease with the increase in the concentration of Bi2O3. This may be due to the decrease in the number of V4þ ions responsible for EPR spectra. The V4þ ions exist as VO2þ ions in octahedral coordination with a tetragonal compression and
4
5
’ AUTHOR INFORMATION Corresponding Author
*Fax: þ91-40-27090020. E-mail:
[email protected]. 1016
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’ ACKNOWLEDGMENT M.S. thanks UGC for financial assistance through Research Fellowships in Science for Meritorious Students (RFSMS) scheme. The authors thank the referees for their critical comments and useful suggestions. The authors also thank S.V. University, Tirupati, for providing EPR and Prof. M. N. Chary, Dept. of Physics, O.U., for providing DC conductivity measurements. ’ REFERENCES (1) Wong, J.; Angell, C. A. Glass Structure by Spectroscopy; Marcel Dekker: New York, 1976. (2) Anshu; Rani, S.; Agarwal, A.; Sanghi, S.; Kishore, N.; Seth, V. P. Indian J. Pure Appl. Phys. 2008, 46, 382. (3) Hecht, H. G.; Johnson, T. S. J. Chem. Phys. 1967, 46, 23. (4) Sreedhar, B.; Indira, P.; Bhatnagar, A. K.; Kazuo, K. J. Non-Cryst. Solids 1994, 167, 106. (5) Sreedhar, B.; Laxman Rao, J.; Laxman, S. V. J. J. Non-Cryst. Solids 1990, 116, 111. (6) Agarwal, A.; Seth, V. P.; Gahlot, P. S.; et al. J. Phys. Chem. Solids 2003, 64, 2281. (7) Toyuki, H.; Akagi, S. Phys. Chem. Glasses 1972, 13, 15. (8) Shareefuddin, Md.; Jamal, Md.; Narasimha Chary, M. J. NonCryst. Solids 1996, 201, 95. (9) Shareefuddin, Md.; Jamal, Md.; Vanaja, K.; Iyengar, A.; Narasimha Chary, M. J. Mater. Sci. Lett. 1995, 14, 646. (10) Kreidl, N. J. J. Non-Cryst. Solids 1990, 123, 377. (11) Hochstrasser, G. Phys. Chem. Glasses 1966, 7, 178. (12) Paul, A.; Assabghy, F. J. Mater. Sci. 1975, 10, 613. (13) Agarwal, A.; Seth, V. P.; Gahlot, P. S.; Khasa, S.; Chand, P. Mater. Chem. Phys. 2004, 85, 215. (14) Gahlot, P. S.; Agarwal, A.; Seth, V. P.; Sanghi, S.; Gupta, S. K.; Arora, M. Spectrochim. Acta, Part A 2005, 61, 1189. (15) Agarwal, A.; Seth, V. P.; Gahlot, P. S.; Khasa, S.; Arora, M.; Gupta, S. K. J. Alloys Compd. 2004, 377, 225. (16) Sreekanth Chakradhar, R. P.; Murali, A.; Lakshmana Rao, J. Phys. B 2000, 293, 108. (17) Sreekanth Chakradhar, R. P.; Ramesh, K. P.; Lakshmana Rao, J.; Ramakrishna Mater. Res. Bull. 2005, 40, 1028. (18) Randall, J. T.; Rooksby, H. P. J. Soc. Glass Technol. 1933, 30, 287. (19) Heynes, M. S. R.; Rawson, H. J. Soc. Glass Technol. 1957, 41, 347. (20) Janakirama Rao, B. V. J. Am. Ceram. Soc. 1962, 45, 555. (21) Hiryama, C.; Subbarao, E. C. Phys. Chem. Glasses 1962, 3, 111. (22) Stebbins, J. F.; Zhao, P.; Kroeker, S. Solid State Nucl. Magn. Reson. 2000, 16, 9. (23) Sindhu, S.; Sanghi, S.; Agarwal, A.; Seth, V. P.; Kishore, N. Mater. Chem. Phys. 2005, 90, 83. (24) Saddeek, Y. B.; Shaaban; Essam, R.; Moustafa, E. S.; Moustafa, H. M. Phys. B 2008, 403, 2399. (25) Saddeek, Y. B.; Yahia, I. S.; Aly, K. A.; Dobrowolski, W. Solid State Sci. 2010, 12, 1426. (26) Liu, H. S.; Shih, P. Y.; Chin, T. S. Phys. Chem. Glasses 1997, 38, 123. (27) Harish Bhat, M.; Kandaval, M.; Ganguli, M.; Rao, K. J. Bull. Mater. Sci. 2004, 27, 189. (28) Mohammad, M. I.; Abd-Allah, Kh.; Hasaan, M. Y. Egypt J. Solids 2004, 27, 299. (29) Subhadra, M.; Kistaiah, P. Spectrochim. Acta, Part A 2010. (30) Khasa, S.; Seth, V. P.; Prakash, D.; Chand, P. Radiat. Eff. Defects Solids 1997, 140, 197. (31) Bleaney, B.; Bowers, K. D.; Pryce, M. H. L. Proc. R. Soc. A 1955, 228, 147. (32) Khasa, S.; Seth, V. P.; Agarwal, A.; Krishna, R. M.; Gupta, S. K.; Chand, P. Mater. Chem. Phys. 2001, 72, 366. (33) Shareefuddin, Md.; Jamal, Md.; Ramdevudu, G.; Lakshmipati Rao, M.; Narashimha Chary, M. J. Non-Cryst. Solids 1999, 255, 228.
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