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Dielectric Dispersion in CuO Doped ZnF2–PbO–TeO2 Glasses
V. Ravi Kumar, N. Veeraiah, S. Buddhudu, V. Jaya Tyaga Raju
To cite this version:
V. Ravi Kumar, N. Veeraiah, S. Buddhudu, V. Jaya Tyaga Raju. Dielectric Dispersion in CuO Doped ZnF2–PbO–TeO2 Glasses. Journal de Physique III, EDP Sciences, 1997, 7 (5), pp.951-961.
�10.1051/jp3:1997167�. �jpa-00249632�
J. Phys. III IYance 7 (1997) 951-961 MAY 1997, PAGE 951
Dielectric Dispersion in CUD Doped ZnF2-PbO-Te02 Glasses
V. llavi Kumar (~)~ N. Veeraiah (~,*)~ S. Buddhudu (~) and V. Jaya Tyaga Raju (~)
(~) Department of Physics, Nagarjuna University P.G. Centre, Nuzvid-521 201 A.P., India (~) Department of Physics, Sri Venkateswara University, Tirupati-521 502 A.P., India (~) Department of Chemistry, Osmania University, Hyderbad-500 007 A-P-, India
(Received 2 May 1996, revised 20 August 1996 and 29 January 1997, accepted 6 February 1997)
PACS.61.43.Fs Glasses
PACS.77.22.Gm Dielectric loss and relaxation
Abstract. Dielectric constant e', loss tan d and conductivity a of 4SZnF2 -9PbO-(46 x) Te02 -xCuO glasses (with x values ranging from 0.2 to 0.6) are studied as a function of frequency
in the range 10~ to 10~ Hz and in the temperature range 30 to 200 °C. All these glasses are found to exhibit dipolar relaxation effects. The variation in e' and tan d with frequency and temperature are observed to be strongly dependent on the concentration of CUD. The observed relaxation effects are described by Classius-Mossotti Debye relation with a set of relaxation times. The possible nature of relaxing dipoles is discussed.
Introduction
The optical and electrical properties of various inorganic glasses containing different transition metal ions have been investigated by several workers [1-4]. ZnF2-PbO-Te02 glasses are well known due to their good transparency in the 3-18 pm region and were considered as the best materials for use as optical components such as IR domes, filters and laser windows.
Investigation on the dielectric properties of these glasses helps in estimating their insulating
character. Quite recently, we have reported the results of our studies concerning the effect of electric field applied m siti1with ionizing radiation on ~lectrical and optical properties of these glasses [5]. These studies have yielded valuable information regarding the insulating character of these glasses and also the defect processes that are taking place in them.
The transition metal ions such as copper dissolved inadvertently in ZnF2-PbO-Te02 glass
matrix even in very small quantities make these glasses coloured and have strong influence
over their insulating properties and optical transmission. To find out the detailed nature of such changes in the insulating properties brought by the Cu~+ ions in these glasses, it is felt important to have the data on dielectric constant, loss and ac conductivity of these glasses
after introducing Cu~+ in them. Moreover, investigation on the dielectric properties of the
glasses colored due to transition metal ions is of interest in itself. It is, therefore the aim of this paper to report the results of our measurements on dielectric constant (e'), loss (tand)
and ac conductivity (a) of ZnF2-PbO-Te02 glasses doped with CUO of the composition (*) Author for correspondence
@ Les #ditions de Physique 1997
(in wt%) 45ZnF2, 9PbO, (46 x)Te02 and xCuO (with x
= 0.2, 0A, 0.6), in'the frequency
range 10~ to 10~ Hz and in the temperature range 30 to 200 °C.
Experimental Methods
The glass formation region of the ZnF2-PbO-Te02 system is well known [6]. In the present investigation a particular composition [45ZnF2-9PbO-(46 x)Te02-xCuO] with varying CUO contents x = 0 (glass 1), 0.2 (glass 2), 0A (glass 3) 0.6 (glass4) was chosen. Our earlier study on a series of these glasses prepared with successive decrease in PbO and Te02 contents
simultaneously, with in the glass formation region, has indicated that the glasses prepared
with the composition 45ZnF2 -9PbO-46Te02 found to possess high mechanical strength, high
dielectric breakdown strength and low thermal expansion coefficient among the other glasses
of the series [6]. We have therefore chosen this composition for CUO doping. Appropriate
amounts of Analar grade reagents zinc fluoride, lead monoxide, tellurium oxide and copper
oxide powders were thoroughly mixed and melted in a platinum crucible at 600 °C for about 30 min until a bubble-free liquid was formed. The resultant melt was poured on a brass mould and subsequently annealed at 200 °C. X-ray powder diffraction was used to check the formation of any crystalline phase. The samples were then ground and finely polished., The final dimensions of the samples were about I x I x o-1 cm3. A thin ioating of silver paint
was applied (to the larger area faces) on either side of the samples to serve as electrodes.
The dielectric measurements were made on a GR capacitance bridge in the frequency range 10~ to 10~ Hz and a Radart Q-Meter in the frequency range 10~ to 10~ Hz. It may be
mentioned here that the dielectric measurements were repeated for some of the samples with
gold coating and the results obtained were almost identical with those of the silver painted samples. The optical absorption spectrum for these glasses was recorded at room temperature using Shimadzu-UV 3101 pc~ UV-Vis-NIR spectro-photometer and IR spectra using Perkin
elmer IR spectro-photometer.
Results
The dielectric constant at room temperature (30 °C) of pure ZnF2-PbO-Te02 glasses mea-
sured to be nearly 19.5 which is almost frequency independent. However this value is slightly
increased due to CUO doping and found to decrease with increasing frequency (Fig. I). The dielectric loss at room temperature has exhibited the similar behaviour.
The temperature dependence of e' at different frequencies for glass 4 (0.6%CUO) is presented
in Figure 2 and for different concentrations of CUO at 103 Hz in Figure 3. e' is found to exhibit a considerable increase at high temperature. This increase is more pronounced at lower frequencies. In addition, it is observed that the rate of increase of e' with temperature
at constant frequency is maximum for the glasses containing 0.6$l of CUO (Fig. 3).
The variation of dielectric loss tan d of ZnF2-PbO-Te02 glass containing 0.6$l CUO with temperature at different frequencies is shown in Figure 4; all these curves have distinct maxima with increasing frequency the temperature maximum of tan d shifts towards higher temp4ra-
tures indicating dipolar relaxation mechanism in these samples. The effect of CUO dopant
on the relaxation strength can be clearly understood from Figure 5, where tan d at 10~ Hz is
plotted uersils temperature for different compositions of CUO. As the concentration of CUO increases, the broadness as well as the maximum of the relaxation curves are found to increase.
Further it is observed that the temperature region of relaxation found to shift towards lower temperatures as the concentration of CUO is increased. Using the relation f
= foe~°'/~~ the effective activation energy W, for dipoles is computed for all the samples and presented in
N°S DIELECTRIC DISPERSION IN CUO DOPED ZnF2-PbO-Te02 953
121
~
~7 f'eqtwncy H2) --
Fig. 1.- Variation of dielectric constant e' with frequency at room temperature for
ZnF2-PbO-Te02 glasses doped with ix 0; (.) 0.2; Ill) 04; lo) 0.6 (wt%) of CUD.
32
sxio~
3 to 30
1,~5
6
2(
Temperature ( °c )
Fig. 2. Temperature dependence of e' at different frequencies in Hz for ZnF2-PbO-Te02 glasses containing 0.6 (wt%) of CUD.
Table I; the activation energy is found to decrease with increasing CUO content. As mentioned in the experimental methods measurements on gold coated samples have also indicated the
relaxation phenomenon at about the same frequency and temperature regions in these glassesj
this indicates the observed relaxation phenomenon in these glasses is due to dipolar effects in the glasses and not due to any electrode effects.
A typicil Cole-Cole diagram corresponding to a temperature of 80 °C within the region of relaxation is shown in Figure 6 for ZnF2-PbO-Te02 glasses containing 0.6$l of CUO; in the
126
i~
0 80 160
Tempemture °c ) -
Fig. 3 Variation of dielectric constant e' with temperature at 10~ Hz for different concentrations of CUD la) 0; 16) 0.2; (c) 0.4; Id) 0.6 (wt%)
Table I. Data on dielectric loss for Ci1O doped ZnF2-PbO-Te02 glasses.
concentration temperature region es eo~ activation a
of CUO (tand)ma~ of relaxation (°C) energy for (in rad)
(in wt%) dipoles (eV)
0 0.05 50 to l12 23.0 20.0 0.78 0
0.2 0.07 46 to 105 23.5 20.0 0.74 0.05
0.4 0.1 43 to 90 24.0 19.9 0.70 0.08
0.6 0.2 40 to 85 26.2 19.8 0.68 o-lo
same figure a similar plot is drawn for pure glasses at the same temperature. The diagrams show that the dielectric relaxation in these glasses is a Debye-type relaxation with a certain set of relaxation times T for the glasses doped with CUO. The points of intersection of the
circular arc with the abscissa in the Cole-Cole plot correspond to low frequency and high frequency dielectric constants (es and eo~). Similar plots at different temperatures within
N°5 DIELECTRIC DISPERSION IN CUD DOPED ZnF2-PbO-Te02 9SS
~~o
~ )
to
l~ji
~e
3c
~j2
'00 200
iempemture (°c
Fig. 4. Variation of dielectric loss tan d at different frequencies m Hz for ZnF2-PbO-Te02 glasses containing 0.6 (wt%) of CUD.
~~o
a b
c
i~
d
Lo§
i~
100 200
iempenJture (°c ~
Fig. S. Variation of dielectric loss tan d at 10~ Hz for d~lferent concentrations of CUD la) 0.6; 16) o4, jC) o.2( id) o jwt%).
UJ
g~
§
.
©
£ -
Fig 6. Cole-Cole diagram at 80 °C for ZnF2-PbO-Te02 glasses containing la) 0.6; 16) 0 (wt%)
of CUD.
the relaxation region have also been drawn for the glasses doped with other concentrations of CUO and the averaged values of es and eo~ obtained for each concentration are presented
in Table I. The parameter o characterizing the distribution function for T does not show any considerable change with temperature for a particular concentration of CUO; however for
different concentrations of CUO the values are appreciably different.
The ac conductivity a is calculated at different temperatures using the equation
a = ~Jeoe'tan d (where eo is vacuum dielectric constant) at different frequencies and the plots
of logo uersils 1IT are shown in Figure 7 for ZnF2-PbO-Te02 glasses doped with different
concentrations of CUO at a frequency of 10~ Hz. From these plots the activation energy for conduction in the high temperature region over which a near linear dependence of logo with
IIT could be observed is calculated and presented in Table II along with other pertinent data.
This activation energy is found to decrease with increase in CUO concentration.
We would also like to mention here that as the concentration of CUO increases beyond 0.6%
the glasses have become dark bluish green and the optical absorption in the visible range
(especially in the region 600 to 900 nm) has become extraordinarily high, and the infrared spectrum for the glasses containing CUO beyond 0.6% have also shown the marked decrease of transparency in the long wavelength region (Figs. 8 and 9). The dielectric loss too (in the
low frequency range) found to be extremely high and hence the study of dielectric properties of the glasses containing CUO beyond 0.6$l has not been reported.
Discussion
Tellurium oxide belongs to the intermediate class of glass forming oxides; it is an incipient glass
network former and as such does not readily form glass due to the fact that the octahedral Te-On polyhedron is highly rigid (when compared with the other glass forming oxides like
Ge02) to get the required distortion of Te-O bonds necessary for forming a stable random network. But it does form glass when mixed with modifiers like PbO [8]. Earlier neutron
scattering experiments [9] and Raman spectral studies [lo, iii on Te02 glasses containing
different modifiers have revealed that the basic building block of Te02 glass structure is a
trigonal bipyramid (Te04) [12-14].
N°5 DIELECTRIC DISPERSION IN CUD DOPED ZnF2-PbO-Te02 957
-6
TE y
~~ #
[ b
2.0 2.2 2.% 2 6 2 8
ix ill 11
~
Fig. 7. A C. conductivity a versus 1IT at 10~ Hz for different concentrations of CUD la) 0; 16) 0.2;
(c) 0A; Id) 0 6 (wt%)
Table II. Data on dielectric constant for Ci1O doped ZnF2-PbO-Te02 glasses.
Concentration T (°C) e Activation energy for
of CUO 5 x 10~ Hz 10~ Hz conduction (eV) from
(in wt$l) a uersils IIT
0 30 19.5 19.5 1.20
loo 20.0 19.8
200 22.5 20.5
0.2 30 20.9 20.6 1.15
loo 23.0 22.0
200 29.0 24.0
0A 30 21.3 21.2 1.07
100 24.0 22.3
200 24.5
0.6 30 22.5 22.3 1.00
100 26.3 22.9
200 26.1
~T2g~~E9
~n
b
~
x ~ a
e
k
~
#
w
< ~ Q
«
§w ~~~ ~~~
WAVELENGTH Inm ~
Fig 8. The optical absorption spectrum of ZnF2-PbO-Te02 glasses containing la) 0.6;
16) 1 (wt%) of CUO recorded at room temperature.
f~~ Q
~
X ~
~
WVENUMBER (cni')
Fig 9. Room temperature IR spectra of ZnF2-PbO-Te02 glasses la) pure glass, (b) glasses containing 0 6 and (c) (wt~) of CUD.
CUO is yet another modifier entering the ZnF2 -PbO-Te02 glass lattice network by breaking
up Te-O-Te bonds (the oxygens of CUO break the local symmetry while Cu~+ ions occupy interstitial positions) introduces co-ordinated defects known as dangling (broken) bonds in these glasses (Fig. lo). During this process there can be different ways of formation of dangling
bonds in the present glass: ii) the stable Te-0 and iii) the unstable Te-0 bonds which will later be modified to Te-O (or simply Te03+1) due to the contraction of one Te-O and the elongation of another Te-0 bonds. With increasing CUO content, cleavage of continuous
network leads to an increase of fraction of Te03+1 polyhedra. Further the elongation of Te-O bond of Te03+1 and its cleavage finally leads to the formation of trigonal prismatic Te03 units.
Thus the structure of the present glass network is an admixture of Te04, Te03+1 and Te03
units.
It is well-known that electronic, ionic, dipolar, and space charge polarization contribute to the dielectric constant. Among these the space charge polarization depends on the purity and
perfection of the glasses. Recollecting our data, the considerable increase in dielectric constant
as well as in loss at room temperature (30 °C) particularly at low frequencies observed for ZnF2-PbO-Te02 glasses doped with CUO may be ascribed to the defects produced in the
glass lattice which contribute to the space charge polarization.
N°S DIELECTRIC DISPERSION IN CUD DOPED ZnF2-PbO-Te02 959
, , , ,
b,
Fig. 10. A schematic illustration of the structural change in Te02 glass network from Te04 to
Te03+1 polyhedra due to modifier oxide. -Te; O Oxygen, arrow indicates modifier entry.
Generally, increasing the temperature of the glasses decreases the electronic part of the dielectric constant by about 3% for a temperature change of about 200 K. Similarly, it appears that the changes in the ionic polarization are not large. Even the presence of dipoles, according
to Debye's theory, should not influence significantly the e' values (beyond the relaxation region).
However in the present measurements of e' and tan d uersils temperature on these glasses,
we notice a large increase of e' and tan d with temperature; such a behaviour can only be attributed to space charge polarization due to the defects in these glasses [15,16]. Copper ion in the divalent state has a 3d9 outer electronic configuration; the free ion terms of this ion will be the same as those of a 3d~ system. When the ion is placed in a crystal field, the energy level splitting of d9 ion will be the same as those of d~ ion, but the ordering of levels gets
reversed. The Cu~+ ion in octahedral crystal field has electronic configuration ~t2ge(. This is equivalent to one electron missing from one eg orbital. Therefore the configuration gives rise to the term ~Eg as the ground state. When one of the electrons of the t2g orbital is promoted
to an eg orbital, the excited electronic configuration ~t2ge( gives rise to ~T2g upper state. Thus in a pure octahedral symmetry transition corresponding to ~Eg ~~ T2g is expected for Cu~+
ions. Optical absorption spectra shown in Figure 8 exhibits only one band in these glasses and that band is attributed to the above explained transition [17,18]. The absorption band in the IR spectrum at 660 cm~~ (Fig. 9) is due to the stretching of Te-O bond. The increase in the
absorption (at the bands) both in the visible and IR region with increase in CUO concentration indicates the presence of large concentration of bonding defects of the type mentioned earlier in ZnF2-PbO-Te02 glasses. The enhancement of space charge polarization which ultimately
caused the increase in e' and tan d values with temperature beyond the relaxation region is due to such defects.
Among the three constituents viz., ZnF21 PbO and Te02 of these glasses, the bonds of tellurium with oxygen are known to be polar in nature [19] and hence it is reasonable to attribute the observed dipolar effects in the pure ZnF2-PbO-Te02 glasses to the Te02 molecules. The presence of a set of relaxation times T for the dielectric relaxation in CUO doped ZnF2-PbO-Te02 glasses means, that the relaxation effects are due to several types of dipoles. Since the spreading of relaxation times is not observed in the pure glasses, the
spreading (of relaxation times) in the glasses containing CUO obviously is due to Cu~+ ions, in which Cu~+ ions together with a pair of cationic vacancies form dipoles and such dipoles are
the most probable relaxing defects which are able of orienting in the field direction in addition to the Te02 molecules in these glasses [20,21].
If it is assumed that the effective electric field in these glasses is a Lorentz field, the connection between the number N of the dipoles per unit volume, their dipole moment p, and the low and
W
~ mfi
~
Z
~
~
0 0.2 0.4 Q-b
cUo iwt%1-
Fig 11. Variation of the quantity 4xNp~/27k at 373 K with CUD concentration.
high frequency dielectric constants es and eo~ can be written according to modified Clausius- Mossotti Debye relation [22, 23]:
fi eo~ + 2 9 kT ~~~
Due to the fact that in the present glasses, the glass lattice ions and the dipoles can be approximately regarded as mere points and the concentration of dipoles is not abnormally high, one need not doubt the applicability of the above equation for these glasses.
After rearranging the terms, above equation will be modified to
(e~ + 2)(eo~ + 2)~ 27 k ~~~
The quantity N~J~ on right hand side of (2) is conventionally known as the strength of the
dipoles. Substituting the values of e~ and eo~ the quantity 4xN~J~/27 k is calculated at -373 K for different concentration of CUO and its dependence on CUO concentration is shown in
Figure Ii for these glasses. The rise in the curve confirms the presence of different types of dipoles in the ZnF2-PbO-Te02 glasses doped with CUO. The decrease in the effective
activation energy associated with dipoles with increasing CUO concentration in these glasses
indicates the increase in the lattice distortion with CUO concentration, thus making dipoles in the glass network more and more free.
In ZnF2-PbD-~eD2 glasses where the broad distribution of relaxation times T exists, the
ac conduction can be represented by a
= aoe~~. The mechanisms that could give rise to this form of ac conduction are: ii) the classical activation of carrier over a potential barrier
spreading two sites in which case (
= W/kT and (it) quantum mechanical tunneling of a carrier
through the potential barrier between the sites separated by a distance R, in this case (
= 2aR
