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Spectroscopic and electrical studies of sodium-digermanate glasses containing iron oxide
Journal of Non-Crystalline Solids 79 (1986) 275-284 North-Holland, Amsterdam
275
SPECTROSCOPIC AND ELECTRICAL STUDIES OF SODIUM-DIGERMANATE GLASSES CONTAINING IRON OXIDE
E.E. K H A W A J A t ,
M. S A K H A W A T H U S S A I N 2 a n d M . N . K H A N 1
Department of Physics 1 and Chemistry e, University of Petroleum and Minerals, Dhahran, Saudi Arabia
Received 13 August 1984 Revised manuscript received 2 April 1985
The infrared, optical absorption spectra and electrical conductivity of sodium-digermanate glasses containing iron oxide have been studied as a function of iron content. Addition of iron does not introduce any new absorption band in the infrared spectrum of pure sodium-digermanate glass. A small shift of the existing absorptions toward lower wavenumbers is observed. These are in agreement with the spectra of Fe203-Na20-2SiO reported earlier. The optical absorption spectra also indicated that the non-bridging oxygen present in pure sodium-digermanate glass was unaffected by the addition of Fe203 in a small quantity. The DC conductivity measurements revealed "mixed conduction" phenomenon in which ionic as well as electronic conduction occur in the glass.
1. Introduction T h e optical a n d I R spectral studies of vitreous GeO2 a n d a l k a l i - d i g e r m a n a t e glass have b e e n the subject of several recent studies [1]. T h e r e has b e e n some d o u b t a b o u t the U V t r a n s m i s s i o n cut off b u t investigations b y different a u t h o r s are in a g r e e m e n t c o n c e r n i n g i n f r a r e d spectra. A n u m b e r of c o m p o s i t i o n s in a l k a l i - d i g e r m a n a t e systems show that the highest a b s o r p t i o n b a n d at a b o u t 880 c m -1 shifts to longer wavelengths with an increasing a m o u n t of alkali. T h e a d d i t i o n of F e 2 0 3 to s o d i u m - d i s i l i c a t e glass systems d i d n o t i n t r o d u c e a n y new a b s o r p t i o n b a n d in the i n f r a r e d s p e c t r u m of p u r e s o d i u m - d i s i l i c a t e glass [2]. O n l y a general shift t o w a r d lower w a v e n u m b e r s was observed. These o b s e r v a t i o n s were consistent with the recently p r o p o s e d structural m o d e l [3] for the b o n d i n g of F e 3+ ions in the i r o n - s o d i u m - s i l i c a t e glass system, W e have e x t e n d e d the studies relating to iron d o p i n g of s o d i u m - d i s i l i c a t e glasses to s o d i u m - d i g e r m a n a t e systems a n d r e p o r t here the infrared, U V a n d visible spectra, electrical m e a s u r e m e n t s a n d crystallization studies of F e 2 0 3d o p e d s o d i u m - d i g e r m a n a t e glasses. 0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
276
E.E. Khawaja et al. / Spectroscopic and electrical studies of sodium-digermanate glasses
2. Experimental All glasses containing various amounts of Fe203 ( [ N a 2 0 - 2 G e O 2 ] l _ x [Fe203] x where x = 0.05, 0.1 and 0.15) were prepared from analytical grade material melted in alumina crucibles at about 1100°C. The melts were stirred occasionally using an alumina rod, and were finally poured on a clean stainless steel plate and cast into the shape of a disc of 20 mm diameter and 2 - 4 mm thickness. The glass samples were polished using diamond paste down to a minimum grit size of 0.1 /~m. Samples having thickness in the range 0.7 to 4 mm were used for optical absorption measurements in the region of the low absorption coefficient, a, less than about 102 cm -a. High absorption measurements were made for blown films of thickness I to 10/~m, which was measured using a Sigma comparator. In both high and low absorption regions, measurements were made from two samples of different thickness in order to eliminate the reflection effects. The ultravio!et and visible data were recorded using a Varian DMS-90 spectrometer. The films for infrared measurements were obtained by a blowing technique using an alumina tube. The as-blown thin glass film was mounted on a specially designed machinable glass ceramic specimen holder. The holder was directly placed into an annealing furnace as well as, mounted in the sample compartment of the infrared spectrophotometer. Infrared spectra in the range 200-2000 cm -1 were obtained by using a Perkin-Elmer 180 Infrared spectrophotometer. Sodium germanate glass films without Fe203 doping were highly hygroscopic and were annealed for an hour at about 110°C prior to infrared measurements. Some of the blown films were annealed for about 12 h in a furnace maintained at 350°C in order to introduce crystallization. The examination of microstructural changes after heat treatment was conducted by means of a JEOL JSEM-200 transmission electron microscope. For electrical measurements gold electrodes were deposited by vacuum evaporation on thick glass discs. One side had a gold circular electrode of of 1 cm diameter while the other side had a slightly smaller electrode surrounded by an annular guard-ring electrode. The electrical measurements were made by a standard technique. For DC measurements the current was measured by means of a Keithley 610 electrometer and a Keithley 240A high voltage supply.
3. Results and discussion The infrared absorption spectra for the pure sodium-digermanate system and glasses containing various amounts of Fe203 are shown in figs. 1 and 2. The general shape of the absorption pattern for undoped sodium-digermanates resembles that reported by Murthy and Kirby [3] for binary alkali-digermanate glasses with highest absorption bands shifted to lower wavenumbers with an increasing amount of alkali. Similar to the peak assign-
E.E. Khawaja et al. / Spectroscopic and electrical studies of sodium-digerrnanate glasses
277
WAVELENGTH (MICRONS) •
9
10
12
15
20
30
50
I
I
I
t
i
I
i
1.2
uJ z gc O ,,¢ ee
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1.2 klJ Z
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1000
800 600 400 WAVENUMBER (CM -1)
Fig. 1. The infrared absorption spectra of (a) sodium-digermanate glass without iron doping, and (b) containing 5 mol.% of Fe203 both without annealing.
ments of Park and Chen [2] in sodium silicate systems, the absorption peak at 830 cm -1 in [Na20.2GEO2] glasses is assigned to the asymmetric stretching mode of the bridged oxygens, (i.e. G e - O - G e bond), whereas the peak at 765 cm-1 is due to the stretching mode of the non-bridging oxygens (i.e. G e - O bond). A secondary band occurring near 570 cm-1 can also be attributed to G e - O - G e symmetric stretching mode. The shoulder near 500 cm -1 and a peak at 325 cm-1, are related to the doubly- and triply-degenerated bending mode of bridged oxygen atoms. The infrared spectra for various iron doped glasses containing 5, 10, and 15 mol.% of Fe203 are quite similar in shape to
278
E.E. Khawaja et al. / Spectroscopic and electrical studies of sodium-digermanate glasses
9
10
I
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WAVELENGTH(MICRONS) 12 15 20 30 I
|
I
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50 I
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WAVENUMBER(CM - 1 )
Fig. 2. Comparison of the infrared absorption spectra of a 15 mol.% iron doped glass taken before (a) and after (b) annealing at 350°C for 12 h.
the spectrum of the undoped sodium-digermanate glass without the appearance of any new peaks. Only a slight shift toward the lower wavenumbers is induced by Fe203. The observed vibration modes in the pure sodium-digermanate as well as in the iron doped glasses can be explained using the Lath et al [4] model which is based upon the random network concept of Warren [5]. Similar to the structure of sodium-silicates, two types of oxygens can be assumed to exist in the sodium-digermanates: (i) the bridging oxygens linking two Ge atoms, (ii) the non-bridging oxygens bonded to a single Ge atom. The percentage of new
E.E. Khawaja et al. / Spectroscopicand electricalstudies of sodium-digermanate glasses 279
G e - O - F e bonding, resulting from the addition of Fe203, is very small. Since the ratio of bridging and non-bridging oxygens remains unchanged, no new peaks are expected to appear in iron containing sodium-digermanate glasses. A slight shift in the absorption bands toward lower wavenumbers by increasing the Fe203 content in the N a 2 0 - G e O 2 matrix is probably because ionic bonding between oxygen and iron is weaker than the partial covalent bonding between oxygen and germanium. Similar downward shifts were observed by the addition of N a 2 0 into a pure GeO 2 matrix. In order to study crystallization, a thin film of sodium-digermanate glass containing 15 mol.% of F%O 3 was annealed for 12 h at 350°C. The annealed sample did not display any change in the infrared spectrum, as depicted in fig. 2. This is contrary to the iron containing sodium-disilicate glasses in which annealing of the samples resulted [2] in the appearance of a new peak which increased in magnitude with increasing annealing time. The appearance and growth of a new infrared peak in iron containing N a 2 0 - S i O 2 systems was explained on the basis of some type of microstructural change occurring in these systems after heat treatment. The microstructures were further confirmed by scanning electron micrographs of these systems. A comparison of scanning electron micrographs of NazO-Ge20 systems before and after heat treatment was also indicative of microstructural changes in iron doped sodium-digermanates, although no new absorption bands were apparent (fig. 2) in annealed samples. 3.1. Optical absorption measurements
Fig. 3 shows the spectral dependence of a in [Na20- 2GeO2]0.95[Fe203]0.05 in a wide spectral region. In fig. 4 high absorption spectra for different compositions are shown. No significant changes in the absorption spectra were noticed in the annealed samples in which some microstructure formation was observed from electron microscopy. If the Fe 3+ ion is in tetrahedral symmetry then ligand field theory [6] predicts the 4/'4(G ) band in the region of wavelength of about 500 nm (2.48 eV), 4 / 5 ( 6 ) at about 446 nm (2.78 eV), 4F1 and 4/'3(6 ) doublets at about 427 nm (2.9 eV). In fig. 3, the peak at about 2.75 eV may be assigned to 4/'5(G ) and the shoulder at about 3 eV to 4/'~ and 4/'4(G ) doublet. The missing 4F4(G ) band may be too weak to be observed. It may be noted that 4/'4(6 ) could not be observed in potassium silicate glass [7] while 4/'5(G ) was much broader than that shown in fig. 3. Higher ligand field bands are obscured by a strong charge transfer band (fig. 3). The optical absorption measurements thus, agree with tetrahedral symmetry of Fe 3+ ion in sodium germanate glasses. Results [5] for alkali silicate glasses also indicate predominantly tetrahedral coordination for Fe 3+. On the other hand, Fe 3+ appears to prefer an octahedral environment in phosphate glasses [1]. Absorption in the near infra-red was very small, suggesting almost negligible Fe 2+ content in the glasses [8].
280
E.E. Khawaja et al. / Spectroscopic and electrical studies of sodium-digermanate glasses l
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lo 3
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Fig. 3. The optical absorption spectra of [Na20-2GeOz]o.95[Fe202]o.05 shoulder corresponding to a 4/1 and 4F3(G) doublet.
glass. The arrow shows a
The charge transfer band centered at about 4.75 eV (fig. 4) increases with Fe content in the glass and is absent in the glasses which do not contain Fe, indicating the band to be associated with transitions that involve Fe. The corresponding band in sodium silicate glasses has been observed by a number of authors [1]. The UV centered transition due to the Fe3+-oxygen interaction provides a major contribution to the green-to-brown color of ordinary bottle glass. In the high energy region there appears to be another band centered at about 5.7 eV (fig. 3). The magnitude of this band seems to be independent of the Fe content in the glasses. Therefore this band cannot be due to transitions which involve Fe. The low absorption edge (a less than about 10 cm - t ) of this band as a function of N a 2 0 content in sodium germanate glasses has been studied by Khan and Khawaja [9]. They found that the edge shifts to lower
E.E. Khawaja et al. / Spectroscopic and electrical studies of sodium-digermanate glasses , o ~-
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Fig. 4. The optical absorption spectra for three glasses (a) [Na20-2GeO2], (b) [Na20. 2GeO2]0.95[Fez03 ]0.05and (c) [Na2O- 2OeO2]0.85[Fe203]015. energies as the N a 2 0 concentration increased. Following Sigel [10], we may suggest that non-bridging oxygens were involved with this absorption mechanism. The non-bridging oxygen is well known in soda glasses; the sodium ion is present interstitially, compensated by a negatively charged non-bridging oxygen in which the lone pair orbital is occupied by two electrons. The non-bridging oxygen in this state is thought to produce a deep donor level, responsible for the shift of the absorption edge of sodium germanate glass to lower frequencies than for GeO 2 [9]. If the above suggestion is true and since the magnitude of the band (fig. 4) is independent of Fe content in the glasses, it may be concluded that the non-bridging oxygen present in sodium-digermanate glass remain almost unchanged upon the addition of Fe in small quantity. This supports the IR measurements. 3.2. D C conductivity measurements
Fig. 5 shows the temperature dependence of DC conductivity. It is clear that conductivity of undoped sodium germanate glass is higher than those doped with Fe. In the doped glasses, the conductivity increases with the Fe content
282
E.E. Khawaja et al. / Spectroscopic and electrical studies of sodium-digermanate glasses !
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Fig. 5. Semilogarithmic plot of conductivity as a function of the reciprocal temperature for a series of glasses. (a) [Naz-2GeO2], (b) [Na20-2GeOz]o.95[Fe203]0.05, (c) [Na2-2GeOz]o.o9[Fe203]0.1o and (d) [NazO.2GeOz]o.85[Fe2Os]oj 5.
while at high temperature it seems that doping has no appreciable effect. Time dependence of current at an applied voltage of 20 V was studied for periods upto 2 h for specimens of different composition and held at room temperature; this revealed a polarization effect due to ionic conductivity.
E.E. Khawaja et at,. / Spectroscopic and electrical studies of sodium-digermanate glasses 283
A decrease of conductivity of sodium-digermanate glasses upon doping with ion could possibly be explained by a "blocking effect" of the Fe 3÷ ion on the overall mobility of N a ions, considering the larger ionic radii of Fe ions compared with the N a ions. A similar argument was considered by Mackenzie [11] where the conductivity of S i O 2 - N a z - M O glasses was established to the ionic. The observed decrease in conductivity with increasing concentration of MO (MO for MgO, PbO, BaO and CaO) was attributed to the low mobility and effective blocking by the divalent ions of the Na ions. Khawaja et al. [12] observed a similar effect in G e O 2 - N a 2 0 - C u O glasses. The increase of conductivity (at low temperatures) in the Fe doped glasses with Fe content could possibly be explained by an increase in electronic conductivity with Fe. Thus it may be concluded that G e O 2 - N a 2 0 - F e 2 0 3 glasses exhibit a "mixed conduction" phenomenon in which ionic conduction as well as electronic conduction occur in the glass.
4. Conclusion In agreement with the observations of Park and Chen [2], the addition of Fe20 3 did not introduce any new absorption bands in the infrared spectrum of pure sodium-digermanate glass. Optical absorption measurements are explained on the basis of: (a) the tetrahedral symmetry of the Fe 3÷ ion in sodium-digermanate glass, (b) a charge transfer band centered at 4.75 eV was dependent on the Fe20 3 concentration in the glass and (c) the UV band centered at 5.7 eV was independent of Fe20 3 content in the glass. The 5.7 eV band was associated with transitions involving non-bridging oxygens. The infrared measurements show that the non-bridging oxygens present in the sodium-digermanate glass were not affected by the addition of Fe20 3 in small quantities. The DC conductivity measurements were consistent with the "mixed conduction" phenomenon in which ionic as well as electronic conduction occur in the glass. Annealing of the glass sample doped with 15 mol.% of Fe20 ~ revealed microstructural changes. The research support from the U P M Research Committee during the summer of 1984 and some assistance by Mr S. Fayyaz in infrared measurements are gratefully acknowledged.
References [1] [2] [3] [4]
J. Wong and C.A. Angell, Glass Structure by Spectroscopy(Dekker, New York, 1976) p. 103. J.W. Park and H. Chen, J. Non-Cryst. Solids 40 (1980) 515. M.K. Murthy and E.M, Kirby, Phys. Chem. Glasses 5 (1964) 144. D.F. Lam, B.W. Veal, H. Chem and G.S. Knapp, in: Proc. Symp. Science underlying Radioactive Waste Management (Mat. Res. Soc. Boston, Mass., 1978). [5] B.E. Warren and J. Brisco, J. Am. Ceram. Soc. 21 (1938) 259.
284
E.E. Khawaja et al. / Spectroscopic and electrical studies of sodium-digermanate glasses
[6] T. Bates, Modem Aspects of the Vitreous State, ed., J.D. Mackenzie (Butterworths, London, 1962) p. 244. [7] V.V. Varguine and T.I. Weinberg, IV e Congres International du verre (1955). [8] C.R. Bamford, Colour Generation and Control in Glass (Elsevier, Amsterdam, 1977) p. 35. [9] M.N. Khan and E.E. Khawaja, Phys. Stat. Sol. (a)74 (1982) 273. [10] G.H. Sigel Jr, J. Phys. Chem. Solids 32 (1971) 2373. [11] J.D. Mackenzie, Electrical Conductivity in Ceramics and Glass, Part B, ed., N.M. Tallan (Dekker, New York, 1974) p. 573. [12] E.E. Khawaja, M.A. Khan, M.N. Khan, A.S.W. Li and J.S. Hwang, J. Mat. Sci. Lett. 3 (1984) 593.
275
SPECTROSCOPIC AND ELECTRICAL STUDIES OF SODIUM-DIGERMANATE GLASSES CONTAINING IRON OXIDE
E.E. K H A W A J A t ,
M. S A K H A W A T H U S S A I N 2 a n d M . N . K H A N 1
Department of Physics 1 and Chemistry e, University of Petroleum and Minerals, Dhahran, Saudi Arabia
Received 13 August 1984 Revised manuscript received 2 April 1985
The infrared, optical absorption spectra and electrical conductivity of sodium-digermanate glasses containing iron oxide have been studied as a function of iron content. Addition of iron does not introduce any new absorption band in the infrared spectrum of pure sodium-digermanate glass. A small shift of the existing absorptions toward lower wavenumbers is observed. These are in agreement with the spectra of Fe203-Na20-2SiO reported earlier. The optical absorption spectra also indicated that the non-bridging oxygen present in pure sodium-digermanate glass was unaffected by the addition of Fe203 in a small quantity. The DC conductivity measurements revealed "mixed conduction" phenomenon in which ionic as well as electronic conduction occur in the glass.
1. Introduction T h e optical a n d I R spectral studies of vitreous GeO2 a n d a l k a l i - d i g e r m a n a t e glass have b e e n the subject of several recent studies [1]. T h e r e has b e e n some d o u b t a b o u t the U V t r a n s m i s s i o n cut off b u t investigations b y different a u t h o r s are in a g r e e m e n t c o n c e r n i n g i n f r a r e d spectra. A n u m b e r of c o m p o s i t i o n s in a l k a l i - d i g e r m a n a t e systems show that the highest a b s o r p t i o n b a n d at a b o u t 880 c m -1 shifts to longer wavelengths with an increasing a m o u n t of alkali. T h e a d d i t i o n of F e 2 0 3 to s o d i u m - d i s i l i c a t e glass systems d i d n o t i n t r o d u c e a n y new a b s o r p t i o n b a n d in the i n f r a r e d s p e c t r u m of p u r e s o d i u m - d i s i l i c a t e glass [2]. O n l y a general shift t o w a r d lower w a v e n u m b e r s was observed. These o b s e r v a t i o n s were consistent with the recently p r o p o s e d structural m o d e l [3] for the b o n d i n g of F e 3+ ions in the i r o n - s o d i u m - s i l i c a t e glass system, W e have e x t e n d e d the studies relating to iron d o p i n g of s o d i u m - d i s i l i c a t e glasses to s o d i u m - d i g e r m a n a t e systems a n d r e p o r t here the infrared, U V a n d visible spectra, electrical m e a s u r e m e n t s a n d crystallization studies of F e 2 0 3d o p e d s o d i u m - d i g e r m a n a t e glasses. 0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
276
E.E. Khawaja et al. / Spectroscopic and electrical studies of sodium-digermanate glasses
2. Experimental All glasses containing various amounts of Fe203 ( [ N a 2 0 - 2 G e O 2 ] l _ x [Fe203] x where x = 0.05, 0.1 and 0.15) were prepared from analytical grade material melted in alumina crucibles at about 1100°C. The melts were stirred occasionally using an alumina rod, and were finally poured on a clean stainless steel plate and cast into the shape of a disc of 20 mm diameter and 2 - 4 mm thickness. The glass samples were polished using diamond paste down to a minimum grit size of 0.1 /~m. Samples having thickness in the range 0.7 to 4 mm were used for optical absorption measurements in the region of the low absorption coefficient, a, less than about 102 cm -a. High absorption measurements were made for blown films of thickness I to 10/~m, which was measured using a Sigma comparator. In both high and low absorption regions, measurements were made from two samples of different thickness in order to eliminate the reflection effects. The ultravio!et and visible data were recorded using a Varian DMS-90 spectrometer. The films for infrared measurements were obtained by a blowing technique using an alumina tube. The as-blown thin glass film was mounted on a specially designed machinable glass ceramic specimen holder. The holder was directly placed into an annealing furnace as well as, mounted in the sample compartment of the infrared spectrophotometer. Infrared spectra in the range 200-2000 cm -1 were obtained by using a Perkin-Elmer 180 Infrared spectrophotometer. Sodium germanate glass films without Fe203 doping were highly hygroscopic and were annealed for an hour at about 110°C prior to infrared measurements. Some of the blown films were annealed for about 12 h in a furnace maintained at 350°C in order to introduce crystallization. The examination of microstructural changes after heat treatment was conducted by means of a JEOL JSEM-200 transmission electron microscope. For electrical measurements gold electrodes were deposited by vacuum evaporation on thick glass discs. One side had a gold circular electrode of of 1 cm diameter while the other side had a slightly smaller electrode surrounded by an annular guard-ring electrode. The electrical measurements were made by a standard technique. For DC measurements the current was measured by means of a Keithley 610 electrometer and a Keithley 240A high voltage supply.
3. Results and discussion The infrared absorption spectra for the pure sodium-digermanate system and glasses containing various amounts of Fe203 are shown in figs. 1 and 2. The general shape of the absorption pattern for undoped sodium-digermanates resembles that reported by Murthy and Kirby [3] for binary alkali-digermanate glasses with highest absorption bands shifted to lower wavenumbers with an increasing amount of alkali. Similar to the peak assign-
E.E. Khawaja et al. / Spectroscopic and electrical studies of sodium-digerrnanate glasses
277
WAVELENGTH (MICRONS) •
9
10
12
15
20
30
50
I
I
I
t
i
I
i
1.2
uJ z gc O ,,¢ ee
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0.9
"~ 0.6 t.u z
0.3
1.2 klJ Z
0.9
,¢ ,,~ 0.6 Z
"1 0.3
1000
800 600 400 WAVENUMBER (CM -1)
Fig. 1. The infrared absorption spectra of (a) sodium-digermanate glass without iron doping, and (b) containing 5 mol.% of Fe203 both without annealing.
ments of Park and Chen [2] in sodium silicate systems, the absorption peak at 830 cm -1 in [Na20.2GEO2] glasses is assigned to the asymmetric stretching mode of the bridged oxygens, (i.e. G e - O - G e bond), whereas the peak at 765 cm-1 is due to the stretching mode of the non-bridging oxygens (i.e. G e - O bond). A secondary band occurring near 570 cm-1 can also be attributed to G e - O - G e symmetric stretching mode. The shoulder near 500 cm -1 and a peak at 325 cm-1, are related to the doubly- and triply-degenerated bending mode of bridged oxygen atoms. The infrared spectra for various iron doped glasses containing 5, 10, and 15 mol.% of Fe203 are quite similar in shape to
278
E.E. Khawaja et al. / Spectroscopic and electrical studies of sodium-digermanate glasses
9
10
I
I
WAVELENGTH(MICRONS) 12 15 20 30 I
|
I
I
50 I
1.2 U Z ,< m 0 (n m ,< n< i,i z ,,J
0.9
0.6
0.3
1.2
0.9
0.6
0.3
12oo
1()o0
8()0
660
460
260
WAVENUMBER(CM - 1 )
Fig. 2. Comparison of the infrared absorption spectra of a 15 mol.% iron doped glass taken before (a) and after (b) annealing at 350°C for 12 h.
the spectrum of the undoped sodium-digermanate glass without the appearance of any new peaks. Only a slight shift toward the lower wavenumbers is induced by Fe203. The observed vibration modes in the pure sodium-digermanate as well as in the iron doped glasses can be explained using the Lath et al [4] model which is based upon the random network concept of Warren [5]. Similar to the structure of sodium-silicates, two types of oxygens can be assumed to exist in the sodium-digermanates: (i) the bridging oxygens linking two Ge atoms, (ii) the non-bridging oxygens bonded to a single Ge atom. The percentage of new
E.E. Khawaja et al. / Spectroscopicand electricalstudies of sodium-digermanate glasses 279
G e - O - F e bonding, resulting from the addition of Fe203, is very small. Since the ratio of bridging and non-bridging oxygens remains unchanged, no new peaks are expected to appear in iron containing sodium-digermanate glasses. A slight shift in the absorption bands toward lower wavenumbers by increasing the Fe203 content in the N a 2 0 - G e O 2 matrix is probably because ionic bonding between oxygen and iron is weaker than the partial covalent bonding between oxygen and germanium. Similar downward shifts were observed by the addition of N a 2 0 into a pure GeO 2 matrix. In order to study crystallization, a thin film of sodium-digermanate glass containing 15 mol.% of F%O 3 was annealed for 12 h at 350°C. The annealed sample did not display any change in the infrared spectrum, as depicted in fig. 2. This is contrary to the iron containing sodium-disilicate glasses in which annealing of the samples resulted [2] in the appearance of a new peak which increased in magnitude with increasing annealing time. The appearance and growth of a new infrared peak in iron containing N a 2 0 - S i O 2 systems was explained on the basis of some type of microstructural change occurring in these systems after heat treatment. The microstructures were further confirmed by scanning electron micrographs of these systems. A comparison of scanning electron micrographs of NazO-Ge20 systems before and after heat treatment was also indicative of microstructural changes in iron doped sodium-digermanates, although no new absorption bands were apparent (fig. 2) in annealed samples. 3.1. Optical absorption measurements
Fig. 3 shows the spectral dependence of a in [Na20- 2GeO2]0.95[Fe203]0.05 in a wide spectral region. In fig. 4 high absorption spectra for different compositions are shown. No significant changes in the absorption spectra were noticed in the annealed samples in which some microstructure formation was observed from electron microscopy. If the Fe 3+ ion is in tetrahedral symmetry then ligand field theory [6] predicts the 4/'4(G ) band in the region of wavelength of about 500 nm (2.48 eV), 4 / 5 ( 6 ) at about 446 nm (2.78 eV), 4F1 and 4/'3(6 ) doublets at about 427 nm (2.9 eV). In fig. 3, the peak at about 2.75 eV may be assigned to 4/'5(G ) and the shoulder at about 3 eV to 4/'~ and 4/'4(G ) doublet. The missing 4F4(G ) band may be too weak to be observed. It may be noted that 4/'4(6 ) could not be observed in potassium silicate glass [7] while 4/'5(G ) was much broader than that shown in fig. 3. Higher ligand field bands are obscured by a strong charge transfer band (fig. 3). The optical absorption measurements thus, agree with tetrahedral symmetry of Fe 3+ ion in sodium germanate glasses. Results [5] for alkali silicate glasses also indicate predominantly tetrahedral coordination for Fe 3+. On the other hand, Fe 3+ appears to prefer an octahedral environment in phosphate glasses [1]. Absorption in the near infra-red was very small, suggesting almost negligible Fe 2+ content in the glasses [8].
280
E.E. Khawaja et al. / Spectroscopic and electrical studies of sodium-digermanate glasses l
I
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i
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lo 3
0 o 0 0
o lo 2
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Fig. 3. The optical absorption spectra of [Na20-2GeOz]o.95[Fe202]o.05 shoulder corresponding to a 4/1 and 4F3(G) doublet.
glass. The arrow shows a
The charge transfer band centered at about 4.75 eV (fig. 4) increases with Fe content in the glass and is absent in the glasses which do not contain Fe, indicating the band to be associated with transitions that involve Fe. The corresponding band in sodium silicate glasses has been observed by a number of authors [1]. The UV centered transition due to the Fe3+-oxygen interaction provides a major contribution to the green-to-brown color of ordinary bottle glass. In the high energy region there appears to be another band centered at about 5.7 eV (fig. 3). The magnitude of this band seems to be independent of the Fe content in the glasses. Therefore this band cannot be due to transitions which involve Fe. The low absorption edge (a less than about 10 cm - t ) of this band as a function of N a 2 0 content in sodium germanate glasses has been studied by Khan and Khawaja [9]. They found that the edge shifts to lower
E.E. Khawaja et al. / Spectroscopic and electrical studies of sodium-digermanate glasses , o ~-
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Fig. 4. The optical absorption spectra for three glasses (a) [Na20-2GeO2], (b) [Na20. 2GeO2]0.95[Fez03 ]0.05and (c) [Na2O- 2OeO2]0.85[Fe203]015. energies as the N a 2 0 concentration increased. Following Sigel [10], we may suggest that non-bridging oxygens were involved with this absorption mechanism. The non-bridging oxygen is well known in soda glasses; the sodium ion is present interstitially, compensated by a negatively charged non-bridging oxygen in which the lone pair orbital is occupied by two electrons. The non-bridging oxygen in this state is thought to produce a deep donor level, responsible for the shift of the absorption edge of sodium germanate glass to lower frequencies than for GeO 2 [9]. If the above suggestion is true and since the magnitude of the band (fig. 4) is independent of Fe content in the glasses, it may be concluded that the non-bridging oxygen present in sodium-digermanate glass remain almost unchanged upon the addition of Fe in small quantity. This supports the IR measurements. 3.2. D C conductivity measurements
Fig. 5 shows the temperature dependence of DC conductivity. It is clear that conductivity of undoped sodium germanate glass is higher than those doped with Fe. In the doped glasses, the conductivity increases with the Fe content
282
E.E. Khawaja et al. / Spectroscopic and electrical studies of sodium-digermanate glasses !
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Fig. 5. Semilogarithmic plot of conductivity as a function of the reciprocal temperature for a series of glasses. (a) [Naz-2GeO2], (b) [Na20-2GeOz]o.95[Fe203]0.05, (c) [Na2-2GeOz]o.o9[Fe203]0.1o and (d) [NazO.2GeOz]o.85[Fe2Os]oj 5.
while at high temperature it seems that doping has no appreciable effect. Time dependence of current at an applied voltage of 20 V was studied for periods upto 2 h for specimens of different composition and held at room temperature; this revealed a polarization effect due to ionic conductivity.
E.E. Khawaja et at,. / Spectroscopic and electrical studies of sodium-digermanate glasses 283
A decrease of conductivity of sodium-digermanate glasses upon doping with ion could possibly be explained by a "blocking effect" of the Fe 3÷ ion on the overall mobility of N a ions, considering the larger ionic radii of Fe ions compared with the N a ions. A similar argument was considered by Mackenzie [11] where the conductivity of S i O 2 - N a z - M O glasses was established to the ionic. The observed decrease in conductivity with increasing concentration of MO (MO for MgO, PbO, BaO and CaO) was attributed to the low mobility and effective blocking by the divalent ions of the Na ions. Khawaja et al. [12] observed a similar effect in G e O 2 - N a 2 0 - C u O glasses. The increase of conductivity (at low temperatures) in the Fe doped glasses with Fe content could possibly be explained by an increase in electronic conductivity with Fe. Thus it may be concluded that G e O 2 - N a 2 0 - F e 2 0 3 glasses exhibit a "mixed conduction" phenomenon in which ionic conduction as well as electronic conduction occur in the glass.
4. Conclusion In agreement with the observations of Park and Chen [2], the addition of Fe20 3 did not introduce any new absorption bands in the infrared spectrum of pure sodium-digermanate glass. Optical absorption measurements are explained on the basis of: (a) the tetrahedral symmetry of the Fe 3÷ ion in sodium-digermanate glass, (b) a charge transfer band centered at 4.75 eV was dependent on the Fe20 3 concentration in the glass and (c) the UV band centered at 5.7 eV was independent of Fe20 3 content in the glass. The 5.7 eV band was associated with transitions involving non-bridging oxygens. The infrared measurements show that the non-bridging oxygens present in the sodium-digermanate glass were not affected by the addition of Fe20 3 in small quantities. The DC conductivity measurements were consistent with the "mixed conduction" phenomenon in which ionic as well as electronic conduction occur in the glass. Annealing of the glass sample doped with 15 mol.% of Fe20 ~ revealed microstructural changes. The research support from the U P M Research Committee during the summer of 1984 and some assistance by Mr S. Fayyaz in infrared measurements are gratefully acknowledged.
References [1] [2] [3] [4]
J. Wong and C.A. Angell, Glass Structure by Spectroscopy(Dekker, New York, 1976) p. 103. J.W. Park and H. Chen, J. Non-Cryst. Solids 40 (1980) 515. M.K. Murthy and E.M, Kirby, Phys. Chem. Glasses 5 (1964) 144. D.F. Lam, B.W. Veal, H. Chem and G.S. Knapp, in: Proc. Symp. Science underlying Radioactive Waste Management (Mat. Res. Soc. Boston, Mass., 1978). [5] B.E. Warren and J. Brisco, J. Am. Ceram. Soc. 21 (1938) 259.
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E.E. Khawaja et al. / Spectroscopic and electrical studies of sodium-digermanate glasses
[6] T. Bates, Modem Aspects of the Vitreous State, ed., J.D. Mackenzie (Butterworths, London, 1962) p. 244. [7] V.V. Varguine and T.I. Weinberg, IV e Congres International du verre (1955). [8] C.R. Bamford, Colour Generation and Control in Glass (Elsevier, Amsterdam, 1977) p. 35. [9] M.N. Khan and E.E. Khawaja, Phys. Stat. Sol. (a)74 (1982) 273. [10] G.H. Sigel Jr, J. Phys. Chem. Solids 32 (1971) 2373. [11] J.D. Mackenzie, Electrical Conductivity in Ceramics and Glass, Part B, ed., N.M. Tallan (Dekker, New York, 1974) p. 573. [12] E.E. Khawaja, M.A. Khan, M.N. Khan, A.S.W. Li and J.S. Hwang, J. Mat. Sci. Lett. 3 (1984) 593.