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Electron paramagnetic resonance and thermal behaviour of lithium potassium borate glasses containing Cu2+ ions
Optical Materials 12 (1999) 47±52
Electron paramagnetic resonance and thermal behaviour of lithium potassium borate glasses containing Cu2 ions R.M. Krishna a
a,* ,
J.J. Andre a, V.P. Seth b, S. Khasa b, S.K. Gupta
c
Institut Charles Sadron, CNRS, 6, rue Boussingault, 67083 Strasbourg, France b Department of Physics, M.D. University, Rohtak 124001, India c EPR Group, National Physical Laboratory, New Delhi 110012, India Received 1 June 1998; accepted 30 October 1998
Abstract The structure and thermal behaviour of lithium potassium borate glasses, containing 2 mol% Cu2 ions, have been studied by means of electron paramagnetic resonance (EPR) and dierential scanning calorimetry (DSC). From the observed EPR spectra, the spin-Hamiltonian parameters have been evaluated which are independent of the temperature from room temperature to liquid helium temperature and it is observed that the spin-Hamiltonian parameters (SHP) are dependent on the concentration of alkali ions present in the glass system. The observed SHP shows that Cu2 ions in lithium potassium borate glasses are present in a distorted octahedral environment. Characteristic glass transition temperatures, Tg , have also been measured for these borate glasses and it is found that the Tg decreases with increasing lithium content upto 10 mol% and then increases for further lithium content. The theoretical optical basicity, Kth , of the glasses has also been evaluated and it is observed that the changes in optical basicity values are independent of the changes in SHP. Ó 1999 Published by Elsevier Science Inc. All rights reserved.
1. Introduction Alkali borate glasses are of great technological interest because of their fast ionic conduction and potential use as solid electrolytes in batteries and other electrochemical devices [1,2]. Structural studies of alkali borate glasses have been reported by several investigators [1±5], and have proved very informative data on the nature of the glass network, i.e., the type and concentration of a variety of boron±oxygen arrangements. The proper-
* Corresponding author. Present address: Department of Chemistry, University of Houston, Houston, TX 77204 5641, USA.
ties of an alkali oxide glass show a strong nonlinear behaviour when one kind of alkali ion is gradually replaced by another. Earlier Ahmed and Abbas [6] reported the spectral absorption, refractive index, density and molar volume of lithium±sodium, lithium±potassium and sodium± potassium borate glasses. As is known, the EPR technique can provide important information about the microscopic local environment of paramagnetic transition (TM) metal ions in glasses which in turn is governed by their structure. To our knowledge, the EPR study of Cu2 ions in lithium potassium borate glasses has not yet been reported. In the present paper, we would like to obtain structural information about Cu2 ions in lithium potassium borate glasses by
0925-3467/99/$ ± see front matter Ó 1999 Published by Elsevier Science Inc. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 8 ) 0 0 0 7 2 - X
48
R.M. Krishna et al. / Optical Materials 12 (1999) 47±52
EPR technique for the selected glass composition as xLi2 O á (30 ÿ x) K2 O á 70B2 O3 0 6 x 6 25. Besides the microscopic information obtained from EPR, we relate thermodynamical eects from DSC studies; indeed the glass transition temperature, Tg , re¯ects the structural changes on a macroscopic level in the copper containing glasses and supercooled liquids. 2. Experimental procedure 2.1. Glass preparation Glasses were prepared by co-grinding the appropriate amounts of analar grade powders of anhydrous Li2 O, K2 O and B2 O3 . The base glass composition was taken as xLi2 O á (30ÿx) K2 O á 70B2 O3 0 6 x 6 25. Table 1 gives the composition in mol% of the glasses studied in the present study. To each bach 2 mol% of Cu2 was added in the form of CuO as the paramagnetic probe. Melting was performed in porcelain crucibles at temperatures in the range of 900±1000°C for 30 min depending on composition. The clear, bubble free melts were splat quenched between two polished carbon blocks to yield glass specimens with good surfaces, which were used for the present study without any further surface treatment. A part of each sample was annealed at 200°C for two hours to study the changes in the measurements of annealed and unannealed samples. 2.2. EPR measurements First derivative EPR spectra of the crushed glasses doped with 2 mol% of Cu2 ions were reTable 1 Composition of the Glass studied in the xLi2 O á (30 ÿ x) K2 O á 70B2 O3 system x content
System
Composition (mol%)
0 5 10 15 20 25
KB LKB1 LKB2 LKB3 LKB4 LKB5
30K2 O±70B2 O3 5Li2 O±25K2 O±70B2 O3 10Li2 O±20K2 O±70B2 O3 15Li2 O±15K2 O±70B2 O3 20Li2 O±10K2 O±70B2 O3 25Li2 O±5K2 O±70B2 O3
corded at room temperature (300 K) and at liquid helium temperature (4.3 K) on a Bruker ER 100 and Varian E-112 spectrometers working at Xband frequencies and employing 100 kHz ®eld modulation. For low temperature measurements, the samples were installed in the TE102 cavity of the Bruker ER 100 spectrometer equipped with an Oxford ESP 300 continuous ¯ow helium cryostat. The temperature was independently checked by a gold±iron chromel thermocouple at the sample position. The uncertainty in the measurement of g and A values is 0.002 and 0.50 ´ 10-4 cmÿ1 , respectively. 2.3. DSC measurements Glass characteristic temperatures were measured on a Perkin-Elmer dierential scanning calorimetry (DSC-4). Sample handling as well as thermal measurements were done under a ¯owing nitrogen atmosphere. All measurements were made on approximately 20±30 mg samples using graphite pans for both glass and reference samples. Sample thickness was always greater than 3 mm thickness. A heating rate of 10°C/min was used and samples were scanned over wide temperature ranges, typically from 300°C to 550°C. Only one Tg was observed for each sample. The Tg were taken on the second order transitions observed as a step change in baseline, i.e., at where the curve is midway between the two tangents. The maximum estimated experimental error in Tg is 2°C. 3. Results and analysis 3.1. EPR spectra No EPR signal was detected in the spectra of undoped glass samples. When Cu2 ions are introduced into the lithium/potassium borate glasses (see Table 1), all investigated samples exhibit EPR absorption peaks. Fig. 1 shows the spectra of 2 mol% Cu2 ions in dierent lithium/potassium borate glasses (x 0, 15 and 25 mol%) at 300 K. The spectrum at 4.3 K did not show much dierence except that the lines were sharper and more intense. The spectra have structures which are
R.M. Krishna et al. / Optical Materials 12 (1999) 47±52
49
3.2. Thermal behaviour by DSC
Fig. 1. Typical room temperature X-band EPR spectra of Cu2 ions in xLi2 O á (30 ÿ x) K2 O á 70B2 O3 glasses 0 6 x 6 25. (x 0, 15 and 25 mol% of Li2 O content).
characteristic of a hyper®ne interaction arising from an unpaired electron with the 63 Cu and 65 Cu (the natural abundances are 69% and 31% respectively) nuclei whose nuclear spin is 3/2 and the observed spectrum is typical of Cu2 ions in oxide glasses [7]. It exhibits a resolved hyper®ne structure (hfs) for the parallel part of the spectrum and partially resolved hfs for the perpendicular part. When the concentration of Li2 O increases, the resolution of perpendicular hfs becomes poorer and the intensity of the complete spectrum increases (Fig. 1). The ®eld positions of the parallel peaks depend on the glass composition. EPR spectra of the annealed samples are identical with corresponding unannealed samples. From the observed EPR spectra, the SHP have been evaluated and are presented in Table 2.
A typical DSC traces of xLi2 O á (30 ÿ x) K2 O á 70B2 O3 unannealed and annealed glass samples recorded at heating rate of 10°C/min is shown in Fig. 2(a), (b) respectively. The characteristic glass transition temperatures are presented in Table 2 for both annealed and unannealed glass samples. Tg for x 0 to 25 mol% of Li2 O (in other words 30ÿx mol% of K2 O) glasses are observed to ®rst decrease and then increases when K2 O is gradually replaced by Li2 O. Similar variations were observed in the alkali and mixed alkali oxide [8,9] and lithium alkali borate [10] glass systems. The DSC traces of the annealed samples are identical with corresponding unannealed spectra. 4. Discussion The EPR spectra of Cu2 ions in all investigated samples can be described by the components gk and g? of the g-tensor and components of Ak and A? of the hyper®ne tensor [11] H gk bHz Sz g? b Hx Sx Hy Sy Ak Sz Iz A? Sx Ix Sy Iy ;
1
where the symbols have their usual meaning, the quadrupole and nuclear Zeeman interactions are ignored. Two sets, each consisting of four resonance peaks, are usually attributed as parallel and perpendicular hyper®ne resonance peaks. The peak positions are related to the principal values of the g and A tensors as follows [12]: hm gk bHk mAk 15=4 ÿ m2 A2? =2gk bHk ;
2
Table 2 Spin-Hamiltonian parameters, glass transition temperatures and optical basicity values of Cu2 ions in xLi2 O á (30 ÿ x)K2 O á 70B2 O3 glasses x content
gk
g?
Ak (10ÿ4 cmÿ1 )
A? (10ÿ4 cmÿ1 )
Kth
Tg (UA)
0 5 10 15 20 25
2.327 2.328 2.330 2.339 2.339 2.339
2.070 2.069 2.069 2.069 2.069 2.069
136.60 137.53 137.53 138.45 138.45 138.45
32.31 34.15 34.15 35.07 35.07 35.07
0.485 0.482 0.478 0.475 0.472 0.469
455 444 442 454 462 480
Unannealed.
Annealed.
(°C)
Tg (A) 455 444 440 454 461 480
(°C)
50
R.M. Krishna et al. / Optical Materials 12 (1999) 47±52
Fig. 2. Typical DSC traces with heating rate of 10°C/min showing the glass transition temperature (Tg ) for unannealed and annealed xLi2 O á (30 ÿ x) K2 O á 70B2 O3 glass samples. (x 0, 5, 10, 15, 20 and 25 mol% of Li2 O content).
Li2 O, whereas g? is almost constant irrespective of alkali concentration. An octahedral site with tetragonally elongated oxygen would give gk > g? > 2:0023 and Ak > A? [7,13]. The values of the SHP obtained in the present work agree with this relationship and are close to those of similar copper complexes reported in the literature [7,13,14]. It is, therefore, con®rmed that the copper ions in lithium potassium borate glasses exist as Cu2 ions in tetragonally elongated oxygen octahedra and in the ground state of Cu2 ions is the unpaired electron dhx2 ÿy 2 i orbital. The spectra at 4.3 K were very similar to those at room temperature, except that the individual hyper®ne lines were sharper, which is ascribed to a spin-lattice relaxation eect. The correlation between Tg and glass structure has been studied extensively. Generally, Tg of an oxide glass increases with bond strength, cross-link density and closeness packing of the glass [15,16]. There is a tendency for the value of Tg to decrease with decreasing single bond strength in alkali and alkaline earth [17] metaphosphate glasses. This indicates that the single bond strength is one of the origins to control the glass transition temperature. Koritala et al. [10] extensively studied the glass transition temperature in lithium alkali borate glasses and reported that the Tg is highest for the Li±Na system followed by Li±K and Li±Rb and is lowest for Li±Cs borate glasses at low alkali contents, whereas there is an apparent reversal of the
hm g? bH? mA? 15=4 ÿ m2 A2? A2k =4g? bH? ;
3
where m is the nuclear magnetic quantum number of the copper nucleus. The resolution of the parallel peak at m ÿ3/2 is very poor, while the perpendicular peak at m 1/2 could not be observed because of the overlap of the spectrum with the central resonance line. However, from the observed positions of the other resonance peaks and using Eqs. (2) and (3), the values of gk , g? , Ak and A? have been evaluated and are given in Table 2. From Table 2, it is clear that gk , Ak and A? are ®rst gradually increase upto x 10 mol% and then do not change when K2 O was gradually replaced by
Fig. 3. Variation of Tg for an unannealed and annealed lithium potassium borate glass samples doped with 2 mol% Cu2 ions. (Solid lines are only a guide for eye).
R.M. Krishna et al. / Optical Materials 12 (1999) 47±52
trend at higher alkali contents. These dierences were explained in terms of structural changes occurring in the glasses. As shown in Fig. 3, the tendency of the present Tg values agrees with that of the above observation. In the present system, we consider the relationship between Tg and atomic mass of the alkali (network modi®ers) atoms proposed by Zhang et al. [8]. By reference to oxide glass, large cations act as network modi®ers and they in¯uence directly the glass transition because this solid to liquid state transition involves the breaking of the weakest chemical bonds. As the network corresponds to a stronger framework, glass transition involves a modi®cation of the chemical bonds between modifying cations and network anions. The heavier network modi®ers are more eective in reducing Tg than lighter ones. Indeed Tg decreases in the present study upto x 10 mol% of Li2 O content because of the in¯uence of heavier network modi®er (K2 O) concentration. As Li2 O (lighter network modi®er) concentration increases further the eect of lithium ions is more predominant than the potassium ions which causes the increase in Tg as observed in the present study. In annealed glass samples also similar trends have been observed and there is no signi®cant change in the Tg values within experimental error as presented in Fig. 3. The EPR results also support this tendency. It is also possible to calculate the value of theoretical optical basicity of the glasses, Kth , as follows by using expression [18] Kth
n X i
Zi ri =ci ;
4
where Zi is the total oxidation number of the cation i, ri is the molar ratio of the cation i to the total number of oxides and ci is the basicity moderating parameter. ci for the cation is given as follows: ci 1:36 xi ÿ 0:26;
5
where xi is the Pauling electronegativity of the cation [19]. The values of optical basicity calculated following this way are also included in Table 2. It is observed that at constant mol% B2 O3 content, the SHP are independent of the change in
51
the Kth value. Optical basicity serves in the ®rst approximation as a measure of the ability of oxygen to donate a negative charge to the probe ion [20]. It can be observed from Table 2 that the Kth value for copper ions decreases from KB to LKB5 glass samples. This means that the ability of ligand to donate the negative charge to the probe ion (Cu2 ion) decreases from KB glass to LKB5 glass.
5. Conclusion The EPR spectra of transition metal ions (Cu2 ) in x Li2 O á (30 ÿ x) K2 O á 70B2 O3 glasses have many features common with those of the copper complexes obtained in oxide glasses. Our results are in agreement with the site symmetry around Cu2 ions in the tetragonally elongated oxygen octahedra and in the ground state of Cu2 ions is the unpaired electron dhx2 ÿy 2 i orbital. The SHP are independent of the temperature and also independent of the change in the optical basicity with the alkali content. The decrease and increase in the Tg indicate that the network modi®ers (lithium and potassium) are responsible for it, depending on their concentration in the glass system.
Acknowledgements One of the Authors (RMK) would like to thank Department of Science & Technology, New Delhi, India and CNRS, France for the award of Young Scientist Project and Research Scientist grants, respectively.
References [1] H.L. Tuller, D.P. Button, D.R. Uh lmann, J. Non-cryst. Solids 40 (1980) 93. [2] Y. Yito, K. Miyauchiand, T. Oi, J. Non-cryst. solids 57 (1983) 389. [3] K. Singh, J. Ratnam, V.K. Deshpande, Solid State Ionics 20±30 (1988) 821. [4] H. Kawazoe, H. Hosono, T. Kanazawa, J. Non-cryst. Solids 29 (1978) 173. [5] H. Kawazoe, H. Hosono, H. Kokumai, J. Nishii, T. Kanazawa, J. Non-cryst. Solids 40 (1980) 291.
52
R.M. Krishna et al. / Optical Materials 12 (1999) 47±52
[6] A.A. Ahmed, A.F. Abbas, J. Non-cryst. Solids 1 (1969) 235. [7] R.H. Sands, Phys. Rev. 99 (1955) 1222. [8] M. Zhang, S. Mancini, W. Bresser, P. Boolchand, J. Noncryst. Solids 151 (1992) 149. [9] D.E. Day, J. Non-cryst. Solids 21 (1976) 343. [10] S. Koritala, K. Farooqui, M. Aatigato, S. Feller, S. Kambeyanda, S. Ghosh, E.I. Kamitsos, G.D. Chryssikos, A.P. Patsis, J. Non-cryst. Solids 134 (1991) 277. [11] A. Abragam, M.H.L. Pryce, Proc. Roy. Soc. (London) A205 (1951) 135. [12] B. Bleany, K.D. Bowers, D.J.E. Ingram, Proc. Roy. Soc. (London) A228 (1955) 147.
[13] H. Imagawa, Phys. Stat. Solidi 30 (1968) 469. [14] R.M. Krishna, J.J. Andre, R.P. Pant, V.P. Seth, J. Noncryst. Solids (1997) (in press). [15] N.H. Ray, J. Non-cryst. Solids 15 (1974) 423. [16] N.H. Ray, Ionic Polymers, in: L. Holliday (Ed.), Applied Science, London, 1975, p. 369. [17] Hand Book of Glass Data, part B (Elsevier, Amsterdam, 1985). [18] J.A. Duy, M.D. Ingram, J. Inorg. Nucl. Chem. 37 (1975) 1203. [19] L. Pauling, The Nature of the Chemical Bond, 3rd ed., Cornell university press, 1960, p. 93. [20] A. Klonkowski, Phys. Chem. glasses 26 (1985) 11.
Electron paramagnetic resonance and thermal behaviour of lithium potassium borate glasses containing Cu2 ions R.M. Krishna a
a,* ,
J.J. Andre a, V.P. Seth b, S. Khasa b, S.K. Gupta
c
Institut Charles Sadron, CNRS, 6, rue Boussingault, 67083 Strasbourg, France b Department of Physics, M.D. University, Rohtak 124001, India c EPR Group, National Physical Laboratory, New Delhi 110012, India Received 1 June 1998; accepted 30 October 1998
Abstract The structure and thermal behaviour of lithium potassium borate glasses, containing 2 mol% Cu2 ions, have been studied by means of electron paramagnetic resonance (EPR) and dierential scanning calorimetry (DSC). From the observed EPR spectra, the spin-Hamiltonian parameters have been evaluated which are independent of the temperature from room temperature to liquid helium temperature and it is observed that the spin-Hamiltonian parameters (SHP) are dependent on the concentration of alkali ions present in the glass system. The observed SHP shows that Cu2 ions in lithium potassium borate glasses are present in a distorted octahedral environment. Characteristic glass transition temperatures, Tg , have also been measured for these borate glasses and it is found that the Tg decreases with increasing lithium content upto 10 mol% and then increases for further lithium content. The theoretical optical basicity, Kth , of the glasses has also been evaluated and it is observed that the changes in optical basicity values are independent of the changes in SHP. Ó 1999 Published by Elsevier Science Inc. All rights reserved.
1. Introduction Alkali borate glasses are of great technological interest because of their fast ionic conduction and potential use as solid electrolytes in batteries and other electrochemical devices [1,2]. Structural studies of alkali borate glasses have been reported by several investigators [1±5], and have proved very informative data on the nature of the glass network, i.e., the type and concentration of a variety of boron±oxygen arrangements. The proper-
* Corresponding author. Present address: Department of Chemistry, University of Houston, Houston, TX 77204 5641, USA.
ties of an alkali oxide glass show a strong nonlinear behaviour when one kind of alkali ion is gradually replaced by another. Earlier Ahmed and Abbas [6] reported the spectral absorption, refractive index, density and molar volume of lithium±sodium, lithium±potassium and sodium± potassium borate glasses. As is known, the EPR technique can provide important information about the microscopic local environment of paramagnetic transition (TM) metal ions in glasses which in turn is governed by their structure. To our knowledge, the EPR study of Cu2 ions in lithium potassium borate glasses has not yet been reported. In the present paper, we would like to obtain structural information about Cu2 ions in lithium potassium borate glasses by
0925-3467/99/$ ± see front matter Ó 1999 Published by Elsevier Science Inc. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 8 ) 0 0 0 7 2 - X
48
R.M. Krishna et al. / Optical Materials 12 (1999) 47±52
EPR technique for the selected glass composition as xLi2 O á (30 ÿ x) K2 O á 70B2 O3 0 6 x 6 25. Besides the microscopic information obtained from EPR, we relate thermodynamical eects from DSC studies; indeed the glass transition temperature, Tg , re¯ects the structural changes on a macroscopic level in the copper containing glasses and supercooled liquids. 2. Experimental procedure 2.1. Glass preparation Glasses were prepared by co-grinding the appropriate amounts of analar grade powders of anhydrous Li2 O, K2 O and B2 O3 . The base glass composition was taken as xLi2 O á (30ÿx) K2 O á 70B2 O3 0 6 x 6 25. Table 1 gives the composition in mol% of the glasses studied in the present study. To each bach 2 mol% of Cu2 was added in the form of CuO as the paramagnetic probe. Melting was performed in porcelain crucibles at temperatures in the range of 900±1000°C for 30 min depending on composition. The clear, bubble free melts were splat quenched between two polished carbon blocks to yield glass specimens with good surfaces, which were used for the present study without any further surface treatment. A part of each sample was annealed at 200°C for two hours to study the changes in the measurements of annealed and unannealed samples. 2.2. EPR measurements First derivative EPR spectra of the crushed glasses doped with 2 mol% of Cu2 ions were reTable 1 Composition of the Glass studied in the xLi2 O á (30 ÿ x) K2 O á 70B2 O3 system x content
System
Composition (mol%)
0 5 10 15 20 25
KB LKB1 LKB2 LKB3 LKB4 LKB5
30K2 O±70B2 O3 5Li2 O±25K2 O±70B2 O3 10Li2 O±20K2 O±70B2 O3 15Li2 O±15K2 O±70B2 O3 20Li2 O±10K2 O±70B2 O3 25Li2 O±5K2 O±70B2 O3
corded at room temperature (300 K) and at liquid helium temperature (4.3 K) on a Bruker ER 100 and Varian E-112 spectrometers working at Xband frequencies and employing 100 kHz ®eld modulation. For low temperature measurements, the samples were installed in the TE102 cavity of the Bruker ER 100 spectrometer equipped with an Oxford ESP 300 continuous ¯ow helium cryostat. The temperature was independently checked by a gold±iron chromel thermocouple at the sample position. The uncertainty in the measurement of g and A values is 0.002 and 0.50 ´ 10-4 cmÿ1 , respectively. 2.3. DSC measurements Glass characteristic temperatures were measured on a Perkin-Elmer dierential scanning calorimetry (DSC-4). Sample handling as well as thermal measurements were done under a ¯owing nitrogen atmosphere. All measurements were made on approximately 20±30 mg samples using graphite pans for both glass and reference samples. Sample thickness was always greater than 3 mm thickness. A heating rate of 10°C/min was used and samples were scanned over wide temperature ranges, typically from 300°C to 550°C. Only one Tg was observed for each sample. The Tg were taken on the second order transitions observed as a step change in baseline, i.e., at where the curve is midway between the two tangents. The maximum estimated experimental error in Tg is 2°C. 3. Results and analysis 3.1. EPR spectra No EPR signal was detected in the spectra of undoped glass samples. When Cu2 ions are introduced into the lithium/potassium borate glasses (see Table 1), all investigated samples exhibit EPR absorption peaks. Fig. 1 shows the spectra of 2 mol% Cu2 ions in dierent lithium/potassium borate glasses (x 0, 15 and 25 mol%) at 300 K. The spectrum at 4.3 K did not show much dierence except that the lines were sharper and more intense. The spectra have structures which are
R.M. Krishna et al. / Optical Materials 12 (1999) 47±52
49
3.2. Thermal behaviour by DSC
Fig. 1. Typical room temperature X-band EPR spectra of Cu2 ions in xLi2 O á (30 ÿ x) K2 O á 70B2 O3 glasses 0 6 x 6 25. (x 0, 15 and 25 mol% of Li2 O content).
characteristic of a hyper®ne interaction arising from an unpaired electron with the 63 Cu and 65 Cu (the natural abundances are 69% and 31% respectively) nuclei whose nuclear spin is 3/2 and the observed spectrum is typical of Cu2 ions in oxide glasses [7]. It exhibits a resolved hyper®ne structure (hfs) for the parallel part of the spectrum and partially resolved hfs for the perpendicular part. When the concentration of Li2 O increases, the resolution of perpendicular hfs becomes poorer and the intensity of the complete spectrum increases (Fig. 1). The ®eld positions of the parallel peaks depend on the glass composition. EPR spectra of the annealed samples are identical with corresponding unannealed samples. From the observed EPR spectra, the SHP have been evaluated and are presented in Table 2.
A typical DSC traces of xLi2 O á (30 ÿ x) K2 O á 70B2 O3 unannealed and annealed glass samples recorded at heating rate of 10°C/min is shown in Fig. 2(a), (b) respectively. The characteristic glass transition temperatures are presented in Table 2 for both annealed and unannealed glass samples. Tg for x 0 to 25 mol% of Li2 O (in other words 30ÿx mol% of K2 O) glasses are observed to ®rst decrease and then increases when K2 O is gradually replaced by Li2 O. Similar variations were observed in the alkali and mixed alkali oxide [8,9] and lithium alkali borate [10] glass systems. The DSC traces of the annealed samples are identical with corresponding unannealed spectra. 4. Discussion The EPR spectra of Cu2 ions in all investigated samples can be described by the components gk and g? of the g-tensor and components of Ak and A? of the hyper®ne tensor [11] H gk bHz Sz g? b Hx Sx Hy Sy Ak Sz Iz A? Sx Ix Sy Iy ;
1
where the symbols have their usual meaning, the quadrupole and nuclear Zeeman interactions are ignored. Two sets, each consisting of four resonance peaks, are usually attributed as parallel and perpendicular hyper®ne resonance peaks. The peak positions are related to the principal values of the g and A tensors as follows [12]: hm gk bHk mAk 15=4 ÿ m2 A2? =2gk bHk ;
2
Table 2 Spin-Hamiltonian parameters, glass transition temperatures and optical basicity values of Cu2 ions in xLi2 O á (30 ÿ x)K2 O á 70B2 O3 glasses x content
gk
g?
Ak (10ÿ4 cmÿ1 )
A? (10ÿ4 cmÿ1 )
Kth
Tg (UA)
0 5 10 15 20 25
2.327 2.328 2.330 2.339 2.339 2.339
2.070 2.069 2.069 2.069 2.069 2.069
136.60 137.53 137.53 138.45 138.45 138.45
32.31 34.15 34.15 35.07 35.07 35.07
0.485 0.482 0.478 0.475 0.472 0.469
455 444 442 454 462 480
Unannealed.
Annealed.
(°C)
Tg (A) 455 444 440 454 461 480
(°C)
50
R.M. Krishna et al. / Optical Materials 12 (1999) 47±52
Fig. 2. Typical DSC traces with heating rate of 10°C/min showing the glass transition temperature (Tg ) for unannealed and annealed xLi2 O á (30 ÿ x) K2 O á 70B2 O3 glass samples. (x 0, 5, 10, 15, 20 and 25 mol% of Li2 O content).
Li2 O, whereas g? is almost constant irrespective of alkali concentration. An octahedral site with tetragonally elongated oxygen would give gk > g? > 2:0023 and Ak > A? [7,13]. The values of the SHP obtained in the present work agree with this relationship and are close to those of similar copper complexes reported in the literature [7,13,14]. It is, therefore, con®rmed that the copper ions in lithium potassium borate glasses exist as Cu2 ions in tetragonally elongated oxygen octahedra and in the ground state of Cu2 ions is the unpaired electron dhx2 ÿy 2 i orbital. The spectra at 4.3 K were very similar to those at room temperature, except that the individual hyper®ne lines were sharper, which is ascribed to a spin-lattice relaxation eect. The correlation between Tg and glass structure has been studied extensively. Generally, Tg of an oxide glass increases with bond strength, cross-link density and closeness packing of the glass [15,16]. There is a tendency for the value of Tg to decrease with decreasing single bond strength in alkali and alkaline earth [17] metaphosphate glasses. This indicates that the single bond strength is one of the origins to control the glass transition temperature. Koritala et al. [10] extensively studied the glass transition temperature in lithium alkali borate glasses and reported that the Tg is highest for the Li±Na system followed by Li±K and Li±Rb and is lowest for Li±Cs borate glasses at low alkali contents, whereas there is an apparent reversal of the
hm g? bH? mA? 15=4 ÿ m2 A2? A2k =4g? bH? ;
3
where m is the nuclear magnetic quantum number of the copper nucleus. The resolution of the parallel peak at m ÿ3/2 is very poor, while the perpendicular peak at m 1/2 could not be observed because of the overlap of the spectrum with the central resonance line. However, from the observed positions of the other resonance peaks and using Eqs. (2) and (3), the values of gk , g? , Ak and A? have been evaluated and are given in Table 2. From Table 2, it is clear that gk , Ak and A? are ®rst gradually increase upto x 10 mol% and then do not change when K2 O was gradually replaced by
Fig. 3. Variation of Tg for an unannealed and annealed lithium potassium borate glass samples doped with 2 mol% Cu2 ions. (Solid lines are only a guide for eye).
R.M. Krishna et al. / Optical Materials 12 (1999) 47±52
trend at higher alkali contents. These dierences were explained in terms of structural changes occurring in the glasses. As shown in Fig. 3, the tendency of the present Tg values agrees with that of the above observation. In the present system, we consider the relationship between Tg and atomic mass of the alkali (network modi®ers) atoms proposed by Zhang et al. [8]. By reference to oxide glass, large cations act as network modi®ers and they in¯uence directly the glass transition because this solid to liquid state transition involves the breaking of the weakest chemical bonds. As the network corresponds to a stronger framework, glass transition involves a modi®cation of the chemical bonds between modifying cations and network anions. The heavier network modi®ers are more eective in reducing Tg than lighter ones. Indeed Tg decreases in the present study upto x 10 mol% of Li2 O content because of the in¯uence of heavier network modi®er (K2 O) concentration. As Li2 O (lighter network modi®er) concentration increases further the eect of lithium ions is more predominant than the potassium ions which causes the increase in Tg as observed in the present study. In annealed glass samples also similar trends have been observed and there is no signi®cant change in the Tg values within experimental error as presented in Fig. 3. The EPR results also support this tendency. It is also possible to calculate the value of theoretical optical basicity of the glasses, Kth , as follows by using expression [18] Kth
n X i
Zi ri =ci ;
4
where Zi is the total oxidation number of the cation i, ri is the molar ratio of the cation i to the total number of oxides and ci is the basicity moderating parameter. ci for the cation is given as follows: ci 1:36 xi ÿ 0:26;
5
where xi is the Pauling electronegativity of the cation [19]. The values of optical basicity calculated following this way are also included in Table 2. It is observed that at constant mol% B2 O3 content, the SHP are independent of the change in
51
the Kth value. Optical basicity serves in the ®rst approximation as a measure of the ability of oxygen to donate a negative charge to the probe ion [20]. It can be observed from Table 2 that the Kth value for copper ions decreases from KB to LKB5 glass samples. This means that the ability of ligand to donate the negative charge to the probe ion (Cu2 ion) decreases from KB glass to LKB5 glass.
5. Conclusion The EPR spectra of transition metal ions (Cu2 ) in x Li2 O á (30 ÿ x) K2 O á 70B2 O3 glasses have many features common with those of the copper complexes obtained in oxide glasses. Our results are in agreement with the site symmetry around Cu2 ions in the tetragonally elongated oxygen octahedra and in the ground state of Cu2 ions is the unpaired electron dhx2 ÿy 2 i orbital. The SHP are independent of the temperature and also independent of the change in the optical basicity with the alkali content. The decrease and increase in the Tg indicate that the network modi®ers (lithium and potassium) are responsible for it, depending on their concentration in the glass system.
Acknowledgements One of the Authors (RMK) would like to thank Department of Science & Technology, New Delhi, India and CNRS, France for the award of Young Scientist Project and Research Scientist grants, respectively.
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