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Electron paramagnetic resonance and optical absorption spectra of manganese(II) ions in lead acetate glasses
Polyhedron Vol. 12, No. 12, pp. 1539-1543, Printed in Great Britain
1993 0
0277-5387/93 S6.W+ .I0 1993 Pergamon Press Ltd
ELECTRON PARAMAGNETIC RESONANCE AND OPTICAL ABSORPTION SPECTRA OF MANGANESE(H) IONS IN LEAD ACETATE GLASSES A. VENKATASUBBAIAI-I,
J. LAKSHMANA
RAO* and S. V. J. LAKSHMAN
Department of Physics, S.V. University, Tirupati 517 502, India and B. SREEDHAR
School of Physics, University of Hyderabad, Hyderabad 500 134, India (Received 27 August 1992 ; accepted 8 February 1993) Abstract-EPR and optical absorption studies have been made on lead acetate glasses containing l-10 mol% MnSO, - H20. The EPR spectra of Mn2+ ion-doped lead acetate glasses exhibit a sextet centred at g = 2.02. Remarkable changes have been observed in the EPR spectrum with changes in temperature, particularly at the glass transition temperature (T, = 317 K). The optical absorption spectrum at room temperature shows five bands characteristic of Mn2+ ions in an octahedral symmetry. The crystal field parameter (Dq) and the Racah interelectronic repulsion parameters (B and C) have been evaluated. EPR and optical absorption studies reveal that the site symmetry around the transition metal ion is octahedral.
Interest in the research in common glasses and other less-common non-crystalline solids containing transition metal ions has grown in recent years, because of their potential use as optical fibres and efficient laser hosts. EPR and optical absorption spectra of transition metal ions in glasses have made it possible not only to interpret the energy levels involved in the observed transitions, but also to comment on the chemical and structural environment about the metal ion centre. Glass-forming molten acetates have been studied extensively as these provide a bridge between essentially polymeric and ionic systems. ’ These glasses can be prepared over a wide range of glass compositions and these embrace cations of widely different chemistries.2 This range includes (1) single component systems, e.g. LiOAc or Pb(OAc), ; and (2) various mixtures, e.g. KOAc-Ca(OAc), and KOAc-Pb(OAc)2. Of particular interest to us is the lead acetate glass, because its glass transition
*Author to whom correspondence should be addressed.
temperature (Tg) lies within the range of the EPR measurements at different temperatures (123423 K). In the present investigation we describe the results of EPR and optical absorption spectra of Mn2+ ions in lead acetate glasses through the glass transition temperature (T,). EXPERIMENTAL Glasses were prepared by melting appropriate amounts of Analar grade lead acetate with Mn SO4 * H20 in an electric furnace at a temperature 50 K higher than that of the liquid. The melt was then quenched between two porcelain plates. EPR spectra were recorded at 300 K at 9.205 GHz on a JEOL FElX EPR spectrometer with a magnetic field modulation of 100 KHz. The magnetic field was scanned between 2250 and 4250 G. The EPR spectra were also recorded at different temperatures (123-423 K). The ‘temperature was varied using a JES-VT-IA variable-temperature controller capable of regulating the temperature
1539
1540
A. VENKATASUBBAIAH
from 523 to 103 K. A temperature stability of f 1 K was easily obtained by waiting for half an hour at the set temperature before recording the spectrum. Optical absorption spectra were recorded using a Perkin-Elmer 55 1 spectrophotometer in the wavelength region 25@-650 nm at 300 K. The wave number accuracy for sharp bands is f 5 cm- ‘. RESULTS AND DISCUSSION
In undoped glasses no EPR signal is detected. When Mn*+ ions are introduced into the lead acetate glasses, all investigated’samples exhibit absorption lines. Figure 1 shows the EPR spectrum of Mn2+ ions in lead actate glasses at 300 K. The spectrum shows the characteristic six-line hyperfine splitting spectrum centred at g = 2.02 f 0.02. There is also a relatively weak absorption centred at g = 4.3 f 0.2. EPR spectra at different temperatures (123-423 K) were also recorded to study the temperature dependence of the intensify of the resonance line at g = 2.02. The intensity of the resonance line at g = 2.02 decreased gradua& with increases in the temperature from 123 K, in proportion to the Boltzmann factor. A marked difference in the intensity of the resonance line at g ‘= 2.02 was observed as the glass transition temperature was approached. The decrease in the intensity of the transition became more rapid at T - Tg and the sextet almost disappeared leaving behind a single broad line. EPR spectra at different temperatures are shown in Fig. 2. From Fig. 2 it is clear that the intensity of the resonance line at g = 2.02 decreased with increases
1000 2900
I
2000 I 3100
I
et al.
in temperature. The observed intensity of the resonance line at g = 2.02 at different temperatures is compared to the Boltzmann factor, i.e. l/T, and is shown in Fig. 3. For a derivative spectrum the measurement associated with the area under the absorption curve (i.e. intensity) is the peak-to-peak height x width2. The optical absorption spectrum measured at room temperature (300 K) for Mn2+ ions (10 mol %) in lead acetate glass is shown in Fig. 4. From the position and the number of observed bands the authors assumed Mn2+ ions in an octahedral symmetry. The optical absorption spectrum at 300 K consists of five bands centred at 17,235, 19,450, 24,445, 26,660 and 27,624 cm-’ and they are denoted as A, B, C, D and E, respectively. The bands C and E are sharp, whereas bands A and B are broad and band D is moderately sharp. Among the sharp bands band C is very sharp. When the number of t,, electrons is the same in both the ground and excited states the energy expressions for the transitions are independent of Dq and the ligand field bands appear to be shaq3 The sharp band C is, therefore, assigned to the transition 6A,,(S) + 4A,,(G) ; 4Eg(G) as the energy expression is independent of Dq. The broad bands A and B are assigned to the transitions 6ALS(S)+ 4T1,(G) and 6A ,,(S) + 4T2g(G), respectively. The bands at D and E are assigned to the transitions ‘jAIg(S) --) 4T2,(D) and 6AIg(S) + 4E,(D), respectively. The EPR spectrum of Mn2+ ions in glasses are characterized by an intense resonance at g = 2.0 with a hyperfine structure, an absorption at g = 4.3 and a distinct shoulder at g = 3.34. The EPR spectra of manganese obtained for lead
I 3300
I
I 3500
I
G(GAIJSS)
Fig. 1. EPR spectrum of Mn’+ ions in lead acetate glass at 300 K.
37c
Spectra of Mn” ions in lead acetate glasses
1541
c
I
I
2900
I
I
3100
3300
_) t-i
I
3500
3700
(GAUSS)
Fig. 2. EPR spectrum of MI-I’+ ions in lead acetate glass at different temperatures.
acetate glasses are qualitatively very similar to those reported for Mn’+ ions in various glasses. &’ ’ Due to this similarity one may conclude that Mn*+ ions are responsible for the EPR spectra. The EPR spectrum comprising a sextet is analysed using the following expression for resonant
41
H,,, = Ho-Aom-A~(35-4m2)/8H,
where Ho = hv/g,,j? and m = - 512, -312, . . . +5/2 ; go is the isotropic g factor and A is the isotropic interaction parameter. The isotropic sig-
I 2
I 6
4 +
Fig. 3. Temperature
fields :
(K-l)
I 6
x 1O-3
dependence of the intensity of the resonance line at g = 2.02 at different temperatures is compared .to the Bobmann factor, i.e. l/T.
A. VENKATASUBBAIAH
1542
WAVENUMBER 28563 I
et al.
(cm-‘)
22216 I
18177 I
1538 IO
1.6
1.4 UI .z 3 ,x 1.2 e C P ; z 1.0 tr: cn 2 i
0.8
u t o
U 0.6
0.4
0.2
0
I
350
I
I
550
450 WAVELENGTH
65( 3
(nm)
Fig. 4. Optical absorption spectrum of 10 mol % Mn’+ ions in lead acetate glass at room temperature (300 K).
nal at g = 2.02 is due to Mn2+ ions in an environment close to octahedral :symmetry. The value of A = 90 G is consistent with Mn2+ ions in an octahedral coordination.‘0,‘2-15 The magnitude of the hyperfine splitting constant A provides a measure of covalency between the Mn2+ ion and its ligands. Van WieringerL6 empirically determined a positive correlation between A and the ionicity of the manganeseligand bond. On this basis it was found that Mn*+ ions in lead acetate glasses are quite ionic (A = 90 G), consistent with Mn2+ ions in alkali borate5 and K2S04ZnS04” glasses (A = 92 G). The decrease in the signal intensity of the resonance line at g = 2.02 with an increase in temperature from 123 K could be due to an efficient
spin-lattice relaxation. A closer inspection reveals that the decrease in the intensity of the transitions becomes more: rapid as the glass transition temperature, TB= 317 K,’ is approached. Similar observations were found for Mn2+ ions in the EPR studies of non-silicate inorganic glasses by Parthasarathy et al.’ as the glass transition temperature (TB) of sulphate (460 K), phosphate (469 K) and PbO-PbC12 (480 K) is approached. The glass transition temperature (T,) for lead acetate glasses is 3 17 K, ’ at which temperature the authors observed a sudden decrease in the intensity of the resonance line at g = 2.02, as is shown in Fig. 3. By diagonalizing the energy matrices” for the d5 configuration the crystal-field parameter (Dq) and the Racah interelectronic repulsion parameters (B
1543
Spectra of Mn” ions in lead acetate glasses Table 1. Experimental data and analysis of the absorption spectrum of Mn2+ ions in lead acetate glass at room temperature : B = 600, C = 3395, Dq = 1000, a=76cm-’
Absorption peak
Transition 6AI,(S) --)
Observed wave number wavelength (cm- ‘) (mn)
Calculated wave number (cm-7
A B C
4T,,(G) 4T,,(G) _~.
580 514
17,235 19,450
17,232 19,350
4A,,(G) ; 4-W3
409
24,445
24,445
D E
4T,,(D) 4E,(D)
375 362
26,660 27,624
26,360 27,420
and C) have been evaluated. In the analysis of the optical absorption spectrum Tree’s correction” parameter, CI,has been incorporated, which is equal to 76 cm-’ in order to obtain a good fit to the observed and calculated values. The observed and calculated energies along with the assignments are given in Table 1.
In the present work the value of the interelectronic repulsion parameter, B = 600 cm- ’ (free ion value is 960 cm- I),” was obtained, which indicates that the bonding is moderately ionic. This is also confirmed from the EPR spectra.
CONCLUSIONS
The EPR and optical absorption spectra of Mn’+ ions in lead acetate glasses have been investigated. EPR spectra at different temperatures reveal that there is a rapid fall in the intensity near the glass transition temperature Tg. The EPR and optical absorption studies of Mn2+ ions in lead acetate glasses show that the site symmetry around the transition metal ion is octahedral. Acknowledgements-The authors thank the Department of Science and Technology, New Delhi, for the purchase of an ESR spectrometer. One of the authors (BS) thanks the Council for Scientific and Industrial Research, New Delhi, for the award of a Research Associateship.
REFERENCES 1. M. D. Ingram, G. G. Lewis and J. A. Duffy, J. Phys. Chem. 1972.76. 1035.
2. R. F. Bartholomew and S. S. Lewek, J. Am. Ceram. sot. 1970, 53, 445. 3. C. J. Ballhausen, Introduction to LigandField Theory. McGraw-Hill, New York (1962). 4. V. Cemy, B. Petrova and M. Frumar, J. Non-Cryst. Solids 1990,125, 17. 5. D. L. Griscom and R. E. Griscom, J. Chem. Phys. 1967,47,2711. 6. R. C. Nicklin, H. A. Farach and C. P. Poole Jr, J. Chem. Phys. 1976,65,2998. 7. I. Watanabe, Y. Inagaki and T. Shirnizu, J. NonCryst. Solids 1976,22, 109. 8. R. Parthasarathy, K. J. Rao and C. N. R. Rao, Chem. Phys. 1982,68,393. 9. L. D. Bogomolova, E. G. Grechko, N. A. Krasilnikova and V. V. Sakharov, J. Non-Cryst. Soli& 1985,69,299. 10. J. Lakshmana Rao, B. Sreedhar, Y. C. Ratnakar and S. V. J. Lakshman, J. Non-Cryst. Solids 1987, 92, 175. 11. E. A. Harris, Phys. Chem. Glasses 1987,28, 196. 12. H. W. De Wijn and R. F. van Balderen, J. Chem. Phys. 1967,46, 1381. 13. S. Bamier, M. Guittard, M. Wintenberger and J. Flahaut, J. Non-Cryst. Solids 1983,56, 319. 14. J. M. Dance, J. J. Videan and J. Portier, J. NonCryst. Solids 1986, 86, 88. 15. J. Lakshmana Rao, G. L. Narendra, B. Sreedhar and S. V. J. Lakshman, Phys. Stat. Sol. (b) 1989, 153, 257. 16. J. S. Van Wieringer, Disc. Faraday Sot. 1955, 19, 118. 17. H. G. K. Sundar and K. J. Rao, J. Non-Cryst. Solids 1982,50, 137. 18. Y. Tanabe and S. Sugano, J. Phys. Sot. Japan 1954, 9, 753. 19. R. E. Trees, Phys. Reu. 1951,83, 756. 20. B. N. Figgis, Introduction to Ligand Field, Wiley, New Delhi (1976).
1993 0
0277-5387/93 S6.W+ .I0 1993 Pergamon Press Ltd
ELECTRON PARAMAGNETIC RESONANCE AND OPTICAL ABSORPTION SPECTRA OF MANGANESE(H) IONS IN LEAD ACETATE GLASSES A. VENKATASUBBAIAI-I,
J. LAKSHMANA
RAO* and S. V. J. LAKSHMAN
Department of Physics, S.V. University, Tirupati 517 502, India and B. SREEDHAR
School of Physics, University of Hyderabad, Hyderabad 500 134, India (Received 27 August 1992 ; accepted 8 February 1993) Abstract-EPR and optical absorption studies have been made on lead acetate glasses containing l-10 mol% MnSO, - H20. The EPR spectra of Mn2+ ion-doped lead acetate glasses exhibit a sextet centred at g = 2.02. Remarkable changes have been observed in the EPR spectrum with changes in temperature, particularly at the glass transition temperature (T, = 317 K). The optical absorption spectrum at room temperature shows five bands characteristic of Mn2+ ions in an octahedral symmetry. The crystal field parameter (Dq) and the Racah interelectronic repulsion parameters (B and C) have been evaluated. EPR and optical absorption studies reveal that the site symmetry around the transition metal ion is octahedral.
Interest in the research in common glasses and other less-common non-crystalline solids containing transition metal ions has grown in recent years, because of their potential use as optical fibres and efficient laser hosts. EPR and optical absorption spectra of transition metal ions in glasses have made it possible not only to interpret the energy levels involved in the observed transitions, but also to comment on the chemical and structural environment about the metal ion centre. Glass-forming molten acetates have been studied extensively as these provide a bridge between essentially polymeric and ionic systems. ’ These glasses can be prepared over a wide range of glass compositions and these embrace cations of widely different chemistries.2 This range includes (1) single component systems, e.g. LiOAc or Pb(OAc), ; and (2) various mixtures, e.g. KOAc-Ca(OAc), and KOAc-Pb(OAc)2. Of particular interest to us is the lead acetate glass, because its glass transition
*Author to whom correspondence should be addressed.
temperature (Tg) lies within the range of the EPR measurements at different temperatures (123423 K). In the present investigation we describe the results of EPR and optical absorption spectra of Mn2+ ions in lead acetate glasses through the glass transition temperature (T,). EXPERIMENTAL Glasses were prepared by melting appropriate amounts of Analar grade lead acetate with Mn SO4 * H20 in an electric furnace at a temperature 50 K higher than that of the liquid. The melt was then quenched between two porcelain plates. EPR spectra were recorded at 300 K at 9.205 GHz on a JEOL FElX EPR spectrometer with a magnetic field modulation of 100 KHz. The magnetic field was scanned between 2250 and 4250 G. The EPR spectra were also recorded at different temperatures (123-423 K). The ‘temperature was varied using a JES-VT-IA variable-temperature controller capable of regulating the temperature
1539
1540
A. VENKATASUBBAIAH
from 523 to 103 K. A temperature stability of f 1 K was easily obtained by waiting for half an hour at the set temperature before recording the spectrum. Optical absorption spectra were recorded using a Perkin-Elmer 55 1 spectrophotometer in the wavelength region 25@-650 nm at 300 K. The wave number accuracy for sharp bands is f 5 cm- ‘. RESULTS AND DISCUSSION
In undoped glasses no EPR signal is detected. When Mn*+ ions are introduced into the lead acetate glasses, all investigated’samples exhibit absorption lines. Figure 1 shows the EPR spectrum of Mn2+ ions in lead actate glasses at 300 K. The spectrum shows the characteristic six-line hyperfine splitting spectrum centred at g = 2.02 f 0.02. There is also a relatively weak absorption centred at g = 4.3 f 0.2. EPR spectra at different temperatures (123-423 K) were also recorded to study the temperature dependence of the intensify of the resonance line at g = 2.02. The intensity of the resonance line at g = 2.02 decreased gradua& with increases in the temperature from 123 K, in proportion to the Boltzmann factor. A marked difference in the intensity of the resonance line at g ‘= 2.02 was observed as the glass transition temperature was approached. The decrease in the intensity of the transition became more rapid at T - Tg and the sextet almost disappeared leaving behind a single broad line. EPR spectra at different temperatures are shown in Fig. 2. From Fig. 2 it is clear that the intensity of the resonance line at g = 2.02 decreased with increases
1000 2900
I
2000 I 3100
I
et al.
in temperature. The observed intensity of the resonance line at g = 2.02 at different temperatures is compared to the Boltzmann factor, i.e. l/T, and is shown in Fig. 3. For a derivative spectrum the measurement associated with the area under the absorption curve (i.e. intensity) is the peak-to-peak height x width2. The optical absorption spectrum measured at room temperature (300 K) for Mn2+ ions (10 mol %) in lead acetate glass is shown in Fig. 4. From the position and the number of observed bands the authors assumed Mn2+ ions in an octahedral symmetry. The optical absorption spectrum at 300 K consists of five bands centred at 17,235, 19,450, 24,445, 26,660 and 27,624 cm-’ and they are denoted as A, B, C, D and E, respectively. The bands C and E are sharp, whereas bands A and B are broad and band D is moderately sharp. Among the sharp bands band C is very sharp. When the number of t,, electrons is the same in both the ground and excited states the energy expressions for the transitions are independent of Dq and the ligand field bands appear to be shaq3 The sharp band C is, therefore, assigned to the transition 6A,,(S) + 4A,,(G) ; 4Eg(G) as the energy expression is independent of Dq. The broad bands A and B are assigned to the transitions 6ALS(S)+ 4T1,(G) and 6A ,,(S) + 4T2g(G), respectively. The bands at D and E are assigned to the transitions ‘jAIg(S) --) 4T2,(D) and 6AIg(S) + 4E,(D), respectively. The EPR spectrum of Mn2+ ions in glasses are characterized by an intense resonance at g = 2.0 with a hyperfine structure, an absorption at g = 4.3 and a distinct shoulder at g = 3.34. The EPR spectra of manganese obtained for lead
I 3300
I
I 3500
I
G(GAIJSS)
Fig. 1. EPR spectrum of Mn’+ ions in lead acetate glass at 300 K.
37c
Spectra of Mn” ions in lead acetate glasses
1541
c
I
I
2900
I
I
3100
3300
_) t-i
I
3500
3700
(GAUSS)
Fig. 2. EPR spectrum of MI-I’+ ions in lead acetate glass at different temperatures.
acetate glasses are qualitatively very similar to those reported for Mn’+ ions in various glasses. &’ ’ Due to this similarity one may conclude that Mn*+ ions are responsible for the EPR spectra. The EPR spectrum comprising a sextet is analysed using the following expression for resonant
41
H,,, = Ho-Aom-A~(35-4m2)/8H,
where Ho = hv/g,,j? and m = - 512, -312, . . . +5/2 ; go is the isotropic g factor and A is the isotropic interaction parameter. The isotropic sig-
I 2
I 6
4 +
Fig. 3. Temperature
fields :
(K-l)
I 6
x 1O-3
dependence of the intensity of the resonance line at g = 2.02 at different temperatures is compared .to the Bobmann factor, i.e. l/T.
A. VENKATASUBBAIAH
1542
WAVENUMBER 28563 I
et al.
(cm-‘)
22216 I
18177 I
1538 IO
1.6
1.4 UI .z 3 ,x 1.2 e C P ; z 1.0 tr: cn 2 i
0.8
u t o
U 0.6
0.4
0.2
0
I
350
I
I
550
450 WAVELENGTH
65( 3
(nm)
Fig. 4. Optical absorption spectrum of 10 mol % Mn’+ ions in lead acetate glass at room temperature (300 K).
nal at g = 2.02 is due to Mn2+ ions in an environment close to octahedral :symmetry. The value of A = 90 G is consistent with Mn2+ ions in an octahedral coordination.‘0,‘2-15 The magnitude of the hyperfine splitting constant A provides a measure of covalency between the Mn2+ ion and its ligands. Van WieringerL6 empirically determined a positive correlation between A and the ionicity of the manganeseligand bond. On this basis it was found that Mn*+ ions in lead acetate glasses are quite ionic (A = 90 G), consistent with Mn2+ ions in alkali borate5 and K2S04ZnS04” glasses (A = 92 G). The decrease in the signal intensity of the resonance line at g = 2.02 with an increase in temperature from 123 K could be due to an efficient
spin-lattice relaxation. A closer inspection reveals that the decrease in the intensity of the transitions becomes more: rapid as the glass transition temperature, TB= 317 K,’ is approached. Similar observations were found for Mn2+ ions in the EPR studies of non-silicate inorganic glasses by Parthasarathy et al.’ as the glass transition temperature (TB) of sulphate (460 K), phosphate (469 K) and PbO-PbC12 (480 K) is approached. The glass transition temperature (T,) for lead acetate glasses is 3 17 K, ’ at which temperature the authors observed a sudden decrease in the intensity of the resonance line at g = 2.02, as is shown in Fig. 3. By diagonalizing the energy matrices” for the d5 configuration the crystal-field parameter (Dq) and the Racah interelectronic repulsion parameters (B
1543
Spectra of Mn” ions in lead acetate glasses Table 1. Experimental data and analysis of the absorption spectrum of Mn2+ ions in lead acetate glass at room temperature : B = 600, C = 3395, Dq = 1000, a=76cm-’
Absorption peak
Transition 6AI,(S) --)
Observed wave number wavelength (cm- ‘) (mn)
Calculated wave number (cm-7
A B C
4T,,(G) 4T,,(G) _~.
580 514
17,235 19,450
17,232 19,350
4A,,(G) ; 4-W3
409
24,445
24,445
D E
4T,,(D) 4E,(D)
375 362
26,660 27,624
26,360 27,420
and C) have been evaluated. In the analysis of the optical absorption spectrum Tree’s correction” parameter, CI,has been incorporated, which is equal to 76 cm-’ in order to obtain a good fit to the observed and calculated values. The observed and calculated energies along with the assignments are given in Table 1.
In the present work the value of the interelectronic repulsion parameter, B = 600 cm- ’ (free ion value is 960 cm- I),” was obtained, which indicates that the bonding is moderately ionic. This is also confirmed from the EPR spectra.
CONCLUSIONS
The EPR and optical absorption spectra of Mn’+ ions in lead acetate glasses have been investigated. EPR spectra at different temperatures reveal that there is a rapid fall in the intensity near the glass transition temperature Tg. The EPR and optical absorption studies of Mn2+ ions in lead acetate glasses show that the site symmetry around the transition metal ion is octahedral. Acknowledgements-The authors thank the Department of Science and Technology, New Delhi, for the purchase of an ESR spectrometer. One of the authors (BS) thanks the Council for Scientific and Industrial Research, New Delhi, for the award of a Research Associateship.
REFERENCES 1. M. D. Ingram, G. G. Lewis and J. A. Duffy, J. Phys. Chem. 1972.76. 1035.
2. R. F. Bartholomew and S. S. Lewek, J. Am. Ceram. sot. 1970, 53, 445. 3. C. J. Ballhausen, Introduction to LigandField Theory. McGraw-Hill, New York (1962). 4. V. Cemy, B. Petrova and M. Frumar, J. Non-Cryst. Solids 1990,125, 17. 5. D. L. Griscom and R. E. Griscom, J. Chem. Phys. 1967,47,2711. 6. R. C. Nicklin, H. A. Farach and C. P. Poole Jr, J. Chem. Phys. 1976,65,2998. 7. I. Watanabe, Y. Inagaki and T. Shirnizu, J. NonCryst. Solids 1976,22, 109. 8. R. Parthasarathy, K. J. Rao and C. N. R. Rao, Chem. Phys. 1982,68,393. 9. L. D. Bogomolova, E. G. Grechko, N. A. Krasilnikova and V. V. Sakharov, J. Non-Cryst. Soli& 1985,69,299. 10. J. Lakshmana Rao, B. Sreedhar, Y. C. Ratnakar and S. V. J. Lakshman, J. Non-Cryst. Solids 1987, 92, 175. 11. E. A. Harris, Phys. Chem. Glasses 1987,28, 196. 12. H. W. De Wijn and R. F. van Balderen, J. Chem. Phys. 1967,46, 1381. 13. S. Bamier, M. Guittard, M. Wintenberger and J. Flahaut, J. Non-Cryst. Solids 1983,56, 319. 14. J. M. Dance, J. J. Videan and J. Portier, J. NonCryst. Solids 1986, 86, 88. 15. J. Lakshmana Rao, G. L. Narendra, B. Sreedhar and S. V. J. Lakshman, Phys. Stat. Sol. (b) 1989, 153, 257. 16. J. S. Van Wieringer, Disc. Faraday Sot. 1955, 19, 118. 17. H. G. K. Sundar and K. J. Rao, J. Non-Cryst. Solids 1982,50, 137. 18. Y. Tanabe and S. Sugano, J. Phys. Sot. Japan 1954, 9, 753. 19. R. E. Trees, Phys. Reu. 1951,83, 756. 20. B. N. Figgis, Introduction to Ligand Field, Wiley, New Delhi (1976).