Role of chromium ions on the structure of LiF-SrO-B2O3 glass system by
means of spectroscopic and dielectric studies
4.1. Introduction
Vitreous B2O3 glasses are particularly interesting model systems as they exhibit
a variety of structural changes with alkali content. Borate glass is a promising host for
incorporation of Cr3+ ions [1-2]. The addition of fluorides, like
LiF to B2O3 glasses
further makes them suitable for radiation dosimetry applications [3]. SrO is a modifier
oxide which enters the glass network by breaking up the random network. Normally the
oxygens of such oxide break the local symmetry while the cations (Sr2+ions) occupy the
interstitial positions in the glass system [4]. Transition metal ions such as chromium, a
paramagnetic metal ion dissolved in glasses make them colored and have strong
influence over the insulating character and optical transmission of these glasses, Since
Cr2O3 also participates in the glass network forming with different structural units.
Chromium may exit in different oxidation states viz., Cr3+ (act as modifier with CrO6
structural units), Cr5+ and Cr6+ (enters as network former with CrO43- and CrO42structural units, respectively). In chromium, trivalent chromium (Cr3+) is the most
stable oxidation state. Therefore, it is widely used as a luminescent dopant and
luminescence sensitizer in different materials. The color of many natural and synthetic
gemstones like ruby, emerald, alexandrite, etc., is caused by Cr3+ ions. Moreover, Cr3+
doped systems are used in modern technologies, such as tunable solid-state lasers [5],
high temperature sensors [6, 7], and high-pressure calibrants [8].
114
By varying the concentration of Cr2O3 with respect to B2O3 the present
investigation to have a comprehensive understanding over the effect of chromium ions
on the insulating character of LiF-SrO-B2O3 glasses from a systematic study on
dielectric properties such as dielectric constant å', loss tanä, ac conductivity óac, in the
frequency range 103-106 Hz and in the temperature range 30-300 0C and dielectric
breakdown strength (in air medium) along with spectroscopic studies like optical
absorption, FT-IR and ESR spectra.
Six glasses in the quarternary system 30 LiF-10 SrO-(60-x) B2O3: x Cr2O3 with
the values of x ranging from 0 to 0.25mol% (in steps of 0.05 mol %) are synthesized.
The details of the composition are as follows.
C0: 30 LiF-10 SrO-60.0 B2O3
C5: 30 LiF -10 SrO-59.95 B2O3 : 0.05 Cr2O3
C10: 30 LiF -10 SrO-59.9 B2O3 : 0.10 Cr2O3
C15: 30 LiF -10 SrO-59.85 B2O3 : 0.15 Cr2O3
C20: 30 LiF -10 SrO-59.8 B2O3 : 0.20 Cr2O3
C25: 30 LiF -10 SrO-59.75 B2O3 : 0.25 Cr2O3
(all are in mol%)
The methods of preparation of the samples and the techniques adopted for recording Xray diffraction (XRD) pattern, optical absorption, FT-IR, etc., and for measuring
dielectric properties are same as that reported in our earlier chapters.
115
4.2. Brief review of the previous work on the glasses containing chromium ions
Kesavulu et al [9] have reported the results on mixed alkali effect (MAE) in
xLi2O-(30-x)Cs2O-69.25B2O3 (5≤x≤2.5) glasses doped with 0.75 mol% of Cr3+ ions
studied by electron paramagnetic resonance (EPR), optical absorption and
luminescence techniques. Bala Murali Krishna et al [10] have studied the role of
chromium ion valence states in ZnO-As2O3-Sb2O3 glass system by means of
spectroscopic and dielectric studies. The results have been analyzed when the
concentration of chromium ions is gradually increased; these ions seem to be exists
mostly in Cr3+ state. Pisarski et al [11] have reported transition metal (Cr3+) and rare
earth (Eu3+, Dy3+) ions used as a spectroscopic probe in compositional-dependent lead
borate glasses. They suggesting higher asymmetry and more covalent bonding
character between rare earth and oxygen ions. Sudhakar et al [12] have reported
vibrational spectral analysis of structural modifications of Cr2O3 containing
oxyfluoroborate glasses. The breaking and reforming of the boroxol ring is explained
from the Raman spectral studies of these glasses. Naga Raju et al [13] have
investigated the structural role of chromium ions on the improvement of insulating
character of ZnO-ZnF2-B2O3 glass system by means of dielectric, spectroscopic and
magnetic properties. The analyses of the results indicate that there is a substantial
enhancement of insulating strength of these glasses. Veerabhadra Rao et al [14]
investigated the dielectric dispersion in PbO-PbF2-B2O3 glass system doped with
Cr2O3. The results have been analysed in the light of different oxidation states of
chromium ions with aid of the data on differential scanning calorimetry, infrared,
optical absorption and ESR spectra. Laxmi Kanth et al [15] have studied spectroscopic
116
investigations on ZnF2-MO-TeO2 (MO = ZnO, CdO and PbO) glasses doped with
chromium ions. Venkateswara Rao and Veeraiah [16] reported study on certain
physical properties of R2O-CaF2- B2O3: Cr2O3 glasses and the analysis indicate the
presence of a part of the chromium ions in the Cr6+ state adopting network forming
positions. Ardelean et al [17] investigated EPR and magnetic susceptibility studies of
Cr2O3-Bi2O3-GeO2 glasses. In the composition of 40SiO2-30BaO-20B2O3-30CaO-10
M2O3 (M=Al, Cr, Y and La) glasses, Singh et al [18] reported that M2O3 plays an
important role in controlling the chemical durability and bioactivity of the glasses.
Shao -Yi Wu et al [19] have investigated the g factors and local structure for Cr5+
within Cr2O3 nanocrystals embedded in the silica glass matrix by theoretical studies
with the perturbation formulas and improved the previous treatments of EPR spectra
and the assignments of the optical transitions based on approximate tetragonal (D2d)
symmetry by inducing slight orthorhombic distortion in their work. By means of
spectroscopic and dielectric relaxation studies, Srinivasa Reddy et al [20] reported the
valence and coordination of chromium ions in ZnO-Sb2O3-B2O3 glass system. By the
analysis of thermo-optical properties of lithium aluminum silicate glasses doped with
Cr3+ ions, Fouad El-Diasty et al [21] correlated these properties with the structure and
the presence of nonbridging oxygen ions in these glasses. A review of the fundamentals
and recent research advances in optical properties of oxide glasses containing
chromium reported by Manal Abdel-Baki and Fauad El-Diasty [22]. With the bonding
parameters obtained from optical and EPR investigations, Ravikumar et al [23]
reported covalent nature for Cr3+ ion in chromium doped zinc phosphate glasses .
Sreekanth Chakradhar et al [24] reported the optical absorption and EPR structural
117
studies of chromium ions in alkali lead borotellurite glasses. Rodriguez-Mendoza et al
[25] have interpreted the absorption and emission spectra of the Cr3+ ions in tantalum
tellurite glasses considering the Cr3+ ions in an actahedral (Oh) environment. Ardelean
and Filip [26] have performed electron paramagnetic resonance and magnetic
susceptibility measurements on TeO2 based glasses containing transition metal ions
(Cr3+, Fe3+, Mn2+ and Cu2+) and concluded that the local neighbourhoods, structural
distributions, valence states and strength of magnetic interactions depend of the nature
and concentration of transition metal ions. Raghavaiah and Veeraiah [27] investigated
the dielectric and spectroscopic properties of 40 PbO-(60-x)Sb2O3-x As2O3:0.4 Cr2O3
(with x ranging from 10 to 55 mol%) and reported that lower concentrations of As2O3
glasses exhibits more favourable environment for the presence of larger concentrations
of laser-emitting Cr3+ ions in the composition. Koepke et al [28] have investigated the
presence of various states of chromium ions (Cr3+, Cr4+, Cr5+, Cr6+) in the ZrO2-Al2O3SiO2, Li2B4O7 glasses and in the silica sol-gel glass, and reported that the first two glass
systems are hosting the Cr3+ ions, while the sol-gel silica glass doesn’t. Strek et al [29]
have performed the EPR and optical measurements on Cr-doped silica sol-gel glasses
and concluded that the EPR signal is associated with Cr5+ ions at tetrahedral sites
whereas the emission is attributed to ligand-metal charge transfer transitions of Cr6+
ions coupled to Cr5+ and absence of Cr3+ ions in silica gel glasses.
118
4.3. Characterization
4.3.1. Physical parameters
From the measured values of density ñ and calculated average molar volume
Vm, various physical parameters such as chromium concentration Ni and inter ionic
distance of chromium ions Ri and polaron radius Rp of these glasses are evaluated using
the conventional formulae and presented in the Table 4.1
Table 4.1
Physical parameters of LiF-SrO-B2O3:Cr2O3 glasses.
Glass
Conc.
V2 O5
(mol%)
Density
Molar
Volume

3
Vm
(cm3)
(g/cm )
(±0.0001) (±0.001)
Conc. of
Cromium
ions Ni
(x1021
ions/cm3)
(±0.001)
Inter ionic
distance of
Cromium ions
Ri (Å)
(±0.001)
Polaron
radius Rp
(Å)
(±0.001)
C0
0
2.5400
23.589
--
--
--
C5
0.05
2.6453
22.668
1.328
9.097
3.665
C10
0.10
2.5427
23.603
2.552
7.318
2.949
C15
0.15
2.5482
23.563
3.833
6.390
2.575
C20
0.20
2.5614
23.460
5.135
5.796
2.336
C25
0.25
2.5629
23.456
6.417
5.381
2.168
4.4. Results
The compositions of as prepared LiF-SrB2O4 glasses doped with different
concentrations of Cr2O3 and some of their properties are given in Table 4.1. The
colorless LiF-SrB2O4 glass (C0) is turned to light green and then to dark green with
initial doping and with increase of Cr2O3 concentration in the glass matrix. Fig. 4.1
shows X-ray diffraction pattern of 0, 0.1 and 0.25 mol% of Cr2O3 doped (C0, C10 and
119
C25) glass samples. From the spectra, the absence of sharp brag peaks confirm the
amorphous nature of the present samples. Physical parameters such as density, molar
volume, inter ionic distance of chromium ions, polaran radius rp are helpful to describe
the concentration of structural units and transport properties as a function of modifier
oxide concentration. The density of C0 glass is found to be 2.540 g/cm3 and is
increased to2.5629 g/cm3 (C25) with gradual increase of doping of Cr2O3. Interestingly
the density of C5 glass sample (2.645) is higher than that of 0.1mol% of Cr2O3 doped
C10
glass
sample.
Intensity (arb.units)
C0
C10
C25
20
30
40
50
60
70
80
2
Fig. 6.1. X-ray diffraction patterns of the 30LiF-10SrO-(60-x) B2O3: xCr2O3 system
for x=0, 0.1, 0.25 mol%.
120
4.4.1. Optical absorption spectra
Fig. 4.2 shows the optical absorption spectra of as prepared glasses in the
wavelength region ranging from 250-800 nm. C0 glass (Cr2O3 free) has not shown any
absorption peak with 0.05 mol% doping of Cr2O3 two broad absorption bands centered
at 424, 620 nm are observed along with two feeble bands around 372 and 688 nm. In
addition a kink around 407 nm at shoulder of 424 peak also observed with further
addition of Cr2O3 the baseline of absorption is decreased with red shift of 424 nm peak
to 429nm and blue shift of 620 nm to 618nm, moreover the intensity of feeble band
around 688 nm is increased at the expense of 372 nm as one goes through higher
concentration.
1.2
Absorbance(arb.Units)
4
0.8
4
A2
T1(F)
407 424
372
429
4
4
A2
T2
620
C5
4
2
A2
T1
688
618
426
425
C25
0.4
619
618
425
C20
C15
C10
C0
618
0.0
400
600
Wavelength (nm)
Fig. 4.2. Optical absorption patterns of Cr2O3 doped LiF-SrO-B2O3 glasses.
800
121
The cutoff wavelength obtained for present glass samples from optical absorption
spectra are given in Table 4.2. It can be seen from Table 4.2 that the position of the
fundamental absorption edge or cutoff wavelength of C0 (pure sample) is noticed at
210 nm and it is abruptly red shifted to 264.5 nm with 0.05 mol% of Cr2O3 and then
decreased to 232.5 nm with 0.1 mol% of Cr2O3 dopant. With further addition of dopant
the cutoff wavelength is gradually redshifted to 250 nm beyond 0.1 mol% of Cr2O3.
From the absorption edges, the optical band gaps (Eg) are evaluated for all the glasses
by the extrapolation of the linear region, of the plots drawn between (áhõ)1/2 vs photon
energy (hõ), to (áhõ)1/2 = 0 as shown in Fig. 4.3. The values of the Eg thus obtained for
all the glass samples are given in Table 4.2.
Table 4.2.
Cutoff wavelength (ëc), 4A2→4T1(F) and 4A2→4T2 band positions, Optical band gap
(Eg) and Urbach energy (∆E) of LiF-SrO-B2O3:Cr2O3 glasses.
4
Sample
A2→4T1(F)
4
A2→4T2 band
ëc ( nm)
band positions of
positions of
Eg (eV)
∆E (eV)
(±0.1)
Cr3+ ions (nm)
Cr3+ ions (nm)
(±0.001)
(±0.0001)
(±0.1)
(±0.1)
C0
210.0
---
---
4.250
0.0801
C5
264.5
417
620
3.217
0.1171
C10
232.5
425
618
3.976
0.0858
C15
245.0
425
618
3.604
0.0935
C20
247.0
426
619
3.537
0.0966
C25
250.5
429
618
3.299
0.1024
122
15
C5 C25
C0
C15
C10
10
1/2
-1
(ah) (Cm eV)
1/2
C20
5
0
3.0
3.5
4.0
4.5
Photon energy h (eV)
5.0
5.5
Fig. 4.3. Optical band gap patterns of Cr2O3 doped LiF-SrO-B2O3 glasses.
Among all investigated samples the optical band gap of C0 is found to be maximum
(4.25 eV) and is gradually decreased to 3.217 eV with increase of dopant concentration
from 0.1 to 0.25 mol% of Cr2O3, But C5 (0.05 mol%) glass has least value.
The relation between á(õ) and Urbach energy (ÄE) is given by the well known Urbach
law equation [30]
ln á(õ) = (hõ/ÄE)-const
------- (4.1)
Usually ÄE is interpreted as the width of the tail of localized states in the band gap.
Urbach plots have been plotted between the natural logarithm of absorption coefficients
(lná), against photon energy (hõ). In the present study such an Urbach plot for all
123
glasses is shown in Fig. 4.4. The values of Urbach energy (ÄE) were calculated by
determining slopes of the linear regions of the curves and taking their reciprocals. The
values of ÄE thus determined are also included in Table 4.2 from which Urbach energy
is minimum (0.080 eV) for C0 sample and is maximum (0.117 eV) for C5 sample. Inset
of Fig. 4.4 shows the variation of Eg and ÄE with respect to concentration of Cr2O3 for
the present glass system.
5
4
C5 C25
C10
C20
C15
Ln()
3
C0
4.4
0.12
2
1
4.0
0.10
3.6
0.09
0.08
3.2
0.00
0
Eg(eV)
E(eV)
0.11
0.05
0.10
0.15
0.20
0.25
Conc of Cr2O3
2.5
3.0
3.5
4.0
4.5
5.0
Photon energy h (eV)
5.5
Fig. 4.4. Urbach energy (ÄE) of LiF-SrO-B2O3 glasses doped with Cr2O3
Inset shows the variation of optical band gap (Eg) and Urbach energy
(ÄE)with concentration of Cr2O3.
6.0
124
4.4.2. ESR spectra
No ESR signal was detected in the spectra of un doped LiF-SrO-B2O3 (C0) glass
indicating that the starting materials used in the preparation are free from paramagnetic
impurities. When chromium ions were introduced into C0 glass matrix, the EPR spectra
exhibit resonance signals due to Cr3+ ions entering the matrix as paramagnetic species.
g=5.21
g=4.26
g=2.43
First derivative of absorption
g=1.976
C25
C20
C15
C10
C5
0
100
200
300
400
500
Magnetic Field (mT)
Fig. 4.5. ESR spectra of Cr2O3 doped LiF-SrO-B2O3 glasses at room temperature.
600
125
Fig. 4.5 shows the EPR spectrum of LiF-SrO-B2O3 glasses doped with different
concentrations of Cr2O3: the spectra of all these glasses exhibit an intense absorption
line centered at g=1.976 and a combination of two absorption lines corresponding to
g=4.26 and 5.21; with increasing concentration of Cr2O3, no change in the value of g is
observed but the intensity of the signal is found to increase from 0.1 to 0.25 mol% of
Cr2O3 (glass C10 to C25) glasses. From ESR spectra of all the glasses, at the lower
concentration 0.05 mol% of Cr2O3 (C5 glass) the highest intensity of the signal is
observed. A shoulder resonance signal at g=2.43 also noticed to g=1.976.
4.4.3. FTIR spectra
The IR transmission spectra of Cr2O3 doped LiF-SrO-B2O3 glasses are shown in
Fig. 4.6 and the observed bands and their corresponding assignments are given in Table
4.3. The vibrational modes of the modified borate glasses are seen to mainly active in
three conventional broad bands originated from IR spectral regions at about 1383 cm−1
(due to BO3 units), 980 cm−1 (due to BO4 units) and at 690cm−1 due to bending
vibrations of B–O–B linkages. These broad bands are the result of convolution of
individual bands with each other. In the spectrum of pure C0 (Cr2O3 free) sample a
feeble band is appeared at 754 cm-1. However two new feeble bands are appeared at
431 cm-1 and around 1522, 1543 cm-1. With initial doping of Cr2O3 the two bands at
1018 and 900 cm-1 are get merged to form new band around 980 cm-1. Interestingly,
with increasing in the concentration of Cr2O3, the intensity of the bands at around 1383
cm-1 is increased and that of the bands around 980 cm-1 are observed to decrease.
126
C0
754
1280
690
1383
1018 900
C5
1271
C10
C15
980
%T(a.u.)
1543
1522
431
765
C20
C25
1600
1400
1200
1000
800
-1
Wavenumber cm
600
400
Fig. 4.6. IR spectra of Cr2O3 doped LiF-SrO-B2O3 glasses at room temperature.
Table 4.3 Assignment of absorption bands in the infrared spectra (with a probable error
of 0.1cm-1) of the glasses LiF-SrO-B2O3:Cr2O3
C0
C5 C10 C15 C20 C25
Assignment
1522 --------- Stretching vibrations of B-O- in BO2O- units
1543
from different borate groups
1383 1384 1384 1384 1384 1384 B-O sym stretch in BO3 units from varied
types of borate groups
1280 1271 1265 1269 1267 1267 B-O sym stretch in BO3 units from pyro and
ortho borate groups
1018
Stretching vibrations of B-O bonds in BO4
units from tri, tetra, penta borate groups
900 975 980 970 979
973
---
754
765
765
768
765
763
O4B-O-BO3 bending vibrations
690
691
692
691
689
689
B-O-B bend
---
431
430
429
431
430
õ4 - Cr2O3 structural units
127
4.4.4. Dielectric properties
The dielectric constant å' and loss tanä at room temperature (~ 30 °C) of the
sample C0 (Cr2O3 free LiF-SrO-B2O3 glass) at 500 kHz are measured to be 4.185 and
0.0078 respectively; the values of these parameters are increased considerably with
decrease in frequency for all the glasses. By adding Cr2O3 into the glass matrix the
value of å' measured at room temperature is found to increase with the concentration
from 0.1 to 0.25 mol% of Cr2O3 (C10-C25) at any frequency. But the lower
concentration (0.05 mol%) sample C5 has shown maximum value. The similar trend is
observed in the dielectric loss, tanä at room temperature with respect to the frequency.
12
0.040
10
0.036
'
tan
8
0.032
6
0.028
0.00
0.05
0.10
0.15
0.20
0.25
Cr2O3 Concentration(mol%)
Fig. 4.7. Variation of dielectric constant and loss with the concentration of Cr2O3 at
1kHz measured at room temperature for LiF-SrO-B2O3: Cr2O3 glasses.
128
The variation of dielectric loss with the concentration of Cr2O3 measured at room
temperature has exhibited a similar behavior as that of å'. The variation of these
parameters at room temperature with the concentration of Cr2O3 measured at 1 kHz is
shown in Fig. 4.7. The temperature dependence of å' for different concentrations of
Cr2O3 measured at 1 kHz is shown in Fig. 4.8 and for a particular glass C20 (containing
0.2mol% of Cr2O3) at different frequencies is shown as the inset of Fig. 4.8. The value
of å' is found to exhibit a considerable increase at higher temperatures especially at
lower frequencies.
45
40
C5
1kHz
40
10kHz
'
35
C25
30
100kHz
20
500kHz
1MHz
30
10
C20
C15
C10
C0
25
'
0
20
100
200
0
Temperature in C
300
15
10
5
0
100
200
300
0
Temperature in C
Fig. 4.8. Variation of dielectric constant å' with temperature at 1kHz for different
concentrations of Cr2O3 in LiF-SrO-B2O3 glasses. Inset (a) represents the
variation of å' with temperature at different frequencies of C20 glass.
129
The values of å' and tanä showed an increasing trend with the concentration
from 0.1 to 0.25 mol% of Cr2O3 (C10-C25). More interestingly the lower concentration
(0.05 mol%) glass sample C5 has maximum value. The temperature dependence of
tanä for different concentrations of Cr2O3 measured at 10 kHz is presented in Fig. 4.9.
From the figure it is clear that at any temperature the dielectric loss is observed to
decrease with increase in the frequency. Inset of the same figure represents the
temperature dependence of tanä at different frequencies for the glass C5.
0.20
0.20
tan
0.15
C5
10kHz
C25
100kHz
0.10
tan
1kHz
C20
500kHz
1MHz
C15
0.05
C10
0.15
0.00
0
100
200
0
Temperature in C
C0
300
0.10
0
100
200
0
Temperature in C
300
Fig. 4.9. Variation of tan ä with temperature at 10 kHz for different concentrations of
Cr2O3 in LiF-SrO-B2O3 glasses. Inset figure represents the variation of
dielectric loss tanä with temperature at different frequencies of the glass C5.
130
The curves of both pure and Cr2O3 doped glasses have exhibited distinct maxima, with
increasing frequency this maxima shifts towards higher temperature, indicating the
dielectric relaxation character of dielectric loss of all the glasses under investigation.
The observations on the variation of dielectric loss with temperature for different
concentrations of Cr2O3 further indicate a gradual increase in the broadness and
(tanä)max of relaxation curves with increase in the concentration from 0.1 to 0.25
mol% of Cr2O3 (glass C10-C25). More specifically the lower concentration (0.05
mol%) glass sample C5 has maximum value in dielectric loss with a shift of the
relaxation region towards lower temperature. The summary of the data on relaxation
effects of LiF-SrO-B2O3: Cr2O3 glass is presented in Table 4.4 along with other data,
pertinent to activation energy for dipoles and breakdown strength.
Using the relation
f = f0 exp (-Wd/KT)
--- (4.2)
the activation energy (AE) Wd, for the dipoles is evaluated. Wd is found to be the
maximum for glass C0 and minimum for C5 glass sample.
At different temperatures, the a.c. conductivity óac is calculated using the
equation
ó ac = ù å' åo tanä
--- (4.3)
where å0 is the vacuum dielectric constant. For different frequencies the variation of the
conductivity óac with 1/T is shown in Fig. 4.10 for the glass C25.
131
Table 4.4
Summary of the data on dielectric loss of LiF-SrO-B2O3 glasses at 1 kHz
Glass
(Tanä)max
x (10-2)
Temp. region of
relaxation (±1) oC
AE for dipoles
(±0.001) eV
Breakdown strength
(±0.01) kV/cm
C0
0.361
146-164
3.722
9.43
C5
0.577
81-115
2.912
8.97
C10
0.394
119-140
3.552
9.36
C15
0.438
110-135
3.386
9.29
C20
0.483
99-128
3.224
9.18
C25
0.515
90-121
3.066
9.07
1E-5
ac(-cm)
-1
1MHz
500kHz
1E-6
100kHz
10kHz
1E-7
1kHz
1E-8
1.6
2.0
2.4
2.8
3
-1
10 /T(K )
3.2
Fig. 4.10. Variation of óac with 1/T for C25 sample at different frequencies.
132
Using these plots, the activation energy for conduction, in the high temperature region
over which a near linear dependence of log óac with 1/T could be observed, is
calculated and presented in Table 4.5 and this activation energy is found to be the
lowest for the glass C5 and is observed to increase with further increase in the
concentration of Cr2O3. The variation of óac with 1/T for glasses doped with different
concentrations of Cr2O3 at 10 kHz is shown in Fig. 4.11. From this figure it can be seen
that the ac conductivity is found to increase with the concentration from 0.1 to 0.25
mol% of Cr2O3 (C10-C25). Where as the lower concentration (0.05 mol%) glass
sample C5 has the maximum value. Inset of the Fig. 4.11 represents the variation of ó ac
w.r.t A.E. for conduction at 70 0C. Such variation is found to be almost linearly.
Table 4.5
Summary of the data on ac. conductivity of LiF-SrO-B2O3: Cr2O3 glasses at 1 kHz
Glass
ó ac at 70 0C
(10-8)
(Ù-cm) -1
N(EF) (1020 eV-1/cm3)
C0
0.951
2.088
0.871
2.123
Activation
energy for
conduction
(eV)
1.153
C5
2.752
3.552
1.482
3.612
0.741
C10
1.249
2.393
0.999
2.433
1.038
C15
1.574
2.687
1.121
2.731
0.944
C20
1.848
2.911
1.215
2.960
0.865
C25
2.220
3.191
1.331
3.244
0.799
Austin
Butcher
Pollak
133
 ac( -cm)
-1
1.60E-007
1E-6
ac(-cm)
-1
8.00E-008
0.6
0.7
0.8
A.E.(eV)
0.9
C5
C25
C20
C15
C10
C0
1E-7
1.6
2.0
2.4
2.8
3
-1
10 /T(K)
3.2
Fig. 4.11. Variation of óac with 1/T at 10 kHz for different concentrations of Cr2O3 in
LiF-SrO-B2O3 glasses. Inset gives the variation of óac with activation energy
at 70 0C of LiF-SrO-B2O3 glasses at 10 kHz.
The dielectric breakdown strength for pure LiF-SrO-B2O3 glass at room
temperature is determined to be 9.43 kV/cm. The value of breakdown strength is found
to decrease substantially with increasing concentration from 0.1 to 0.25 mol% of Cr2O3
(glass C10-C25) and reached a least value 8.97 kV/cm at lower concentration for the
glass C5 (Table 4).
134
4.5. Discussion
Generally for amorphous materials density is a powerful tool for exploring
changes in the structure. It is affected by structural softening/compactness, change in
geometrical configuration, coordination number, crosslink density and dimension of
interstitial spaces of the glass [31]. The increasing trend of density with Cr2O3 doping
is expected due to replacement of a lighter cation (B3+) by heavier one Cr3+. Since the
density of a glass is found to be very sensitive to the ionic size and atomic weight [32].
However, the increase of C5 glass density and decrease of molar volume than that of
C10 glass sample indicates the higher compactness of C5 glass sample. It can be
explained as fallows: with initial doping of 0.05 mol% of Cr2O3 into LiF-SrO-B2O3
glass, the chromium ions fill interstices of the borate glass network by the formation of
B-O-Cr and Sr-O-Cr linkages, in sequence with conversion of BO4 tetrahedral to BO3
triangular structural units (the ionic radii of B3+IV(0.25 Å) is higher than B3+III(0.15
Å)). Such irregular behavior in density values of glasses was also observed by Culea et
al [33, 34].
Using Tanabe-Sugano diagrams for d3 ions, the observed two optical absorption
bands at 429 and 620 nm are assigned to 4A24T1(F) (peak 1), 4A24T2 (peak 2)
transitions and a weak band at 407 nm is assigned to 4A2(F) 2A1(G) in the increasing
order of energy. The observed bands are characteristic of Cr3+ ions in octahedral
symmetry [9]. With increasing concentration of Cr2O3, the intensity associated with
these peaks is found to increase from 0.1 to 0.25 mol% of Cr2O3 (C10-C25), where as
lower concentration glass sample C5 (0.05 mol%) exhibit lower intensity. Chromium
ions seem to exist in both Cr3+and Cr6+ states in LiF-SrO-B2O3 glass network; Cr3+ ion
135
enters the network as modifier where as Cr6+ ion enters as network former with CrO42structural units [35, 36]. Moreover, in the present case, these two transitions may be
considered to be superposed with that of Cr3+ ions resulting broad bands in the
absorption spectra. Additionally, a weak narrow feeble band at 688 nm due to
4
A22T1, have also been located beside 4T2 band. This band is weak because these are
spin and parity forbidden and narrow because their weak correlation with the
vibrational spectrum [37]. The increase of Cr2O3 content in the glass composition (from
0.1 to 0.25 mol%) a gradual decrease in the widths of 2T1 bands are observed. These
observations indicate a decrease of disorder in the glass network [37]. In the spectrum
of the glasses consists of a kink around 370 nm identified due to Cr6+ ions [38] has also
been observed. This observation indicates the presence of a part of chromium ions in
Cr6+ state in these glasses. The Cr5+ ions are expected to exhibit absorption band around
465 nm [39]; but such band is not observed in the spectra of the present glasses.
Therefore concentration of Cr5+ ions in the glass matrices seems to be insignificant.
The increase in the intensity of 4T1 and 4T2 absorption bands and a simultaneous
decrease in the intensity of the kink at 372 nm in the spectra of glasses C10 to C25
indicate an increase in the concentration of Cr3+ ions at the expense of Cr6+ ions in the
present glass system. These Cr6+ ions participate in the glass network with CrO42structural units and alternate with BO4 units, leading to a decrease in the disorder of the
glass network. The lower the concentration of octahedral Cr3+ ions, consequently the
lower is the concentration of non-bridging oxygens (NBO) in the glass matrix. The
presence of higher concentration of these donor centers decreases the optical band gap
and shifts the absorption edge towards higher wavelength side as observed from glass
136
C10 to C25. More specifically, for C5 glass sample at lower doping concentration of
Cr2O3 the bridging oxygens (BO) develop bonds with Cr3+ which in turn lead to the
gradual breakdown of the glass network. This breakdown seems to account for the
decrease in the Eg value, ie., edge shifts to longer wavelengths. Especially, higher the
concentration of octahedral Cr3+ ions, higher will be the concentration of non bridging
oxygen’s (NBOs) in the glass matrix. This causes to an increase in the degree of
localization of electrons there by increasing the donor centers in the glass matrix. The
presence of greater concentration of these donor centers decreases the optical band gap
and shifts the absorption edge towards higher wavelength as observed (Fig. 4.3) for
glass C5.
According to Hund’s rule the ground state of chromium ion is 4F which is
belongs to 3d3 electronic configurations. In an octahedral crystal field, this state splits
into an orbital singlet 4A2g and two more orbital triplets 4T1g and 4T2g [40]. In a
distorted octahedral site, the electronic levels can be described by a spin- Hamiltonian
[41]
H = gßBS + D [Sz2 - { S (S+1) / 3}] + E (Sx2-Sy2)
--- (4.4)
Here the first term represents the electronic zeeman term, second term characterization
represents the zero-field splitting of the quartet ground state. In the absence of an
external magnetic field B; the four fold spin degeneracy of the 4A2g state is removed by
a subsequent low symmetric field resulting in a zero field splitting of the Kramers
doublets |±3/2> and |±1/2> even at zero field, in the absence of an external magnetic
field B. In the presence of magnetic field, the degenerate doublets split further and
more transitions are possible. However, the number of resonance signals observed due
137
to transitions depend upon the magnitude of the zero field splitting for a given photon
energy [42].
In the present study, as mentioned in the optical absorption in an octahedral field
the ESR spectra of the investigated LiF-SrO-B2O3: Cr2O3 glass exhibit resonance
signals as shown in Fig. 4.5. The ESR spectra of the investigated glass samples exhibit
two resonance intense signals at g=1.976 and 4.26. The low field portion of the
spectrum is attributed to the isolated Cr3+ ions and the high field portion mainly
attributed to exchange coupled Cr3+ ion pairs and is related to strongly distorted sites
[43]. The resonance signal at g≈5 is typical for isolated Cr3+ ionic sites of rhombic
symmetry subjected to strong crystal field effects [44]. The resonance signal in the high
field region with 1.976 is due to exchange coupled Cr3+ - Cr3+ pairs [45]. If there is a
large separation between two Kramers doublets then the resonance signal at g=2.6 can
also be observed [46]. In the present investigation this resonance signals at g=2.43 arise
due to large separation between the two Kramers doublets [40].
Comparatively larger rate of increase in the intensity of high field portion peak
(ie., g=1.976 and 4.26) with the concentration of Cr2O3 for C10-C25 glass indicates
lower rate of conversion of octahedral Cr3+ ions into tetrahedral Cr6+ ions that
participate in the glass network with CrO42- structural units. More specifically, in the
present investigation ESR studies indicate that the concentration of isolated Cr3+ ions
increased for C5 glass. The ESR signals appear in the range of low magnetic field with
a resonant magnetic field of 130-180 mT indicates that the zero-field splitting, 2D of
the 4A2g simple orbital state is relatively large in comparison with the energy of the
microwave radiation used in the X-band ESR spectrometry.
138
FTIR spectrum of vitreous B2O3 contains only three coordinated boron atoms
[SP2 planar BO3 units], but by the addition of Li+ F- ions, some of these units are
transformed into four-coordinated tetrahedral (more stable SP3 tetrahedral BO4 units)
and also create non-bridging oxygens [47, 48]. Each BO4 unit is linked to two such
other units, and one oxygen from each unit with a metal ion and the structure leads to
the formation of long tetrahedral chains. The second group of bands at 980 cm-1 is
attributed to stretching vibrations of B-O bonds in BO4 units from tri, tetra, penta
borate groups. The first groups of bands are recognized as being due to the stretching
relaxation of the B-O bond of the triangular BO3 units and the third band at 690 cm-1 is
due to the bending of B-O-B linkages in the borate network [49]. In general SrO enters
the glass network, by transforming BO4 tetrahedral into BO3 units and thus a SrOn
polyhedron is formed when it is surrounded by several such tetrahedrons; this structure
behaves like a defect in the network of borate [50]. Addition of chromium ions into the
LiF-SrO–B2O3 glass network will twist or distort the interconnected chains of BO4 units
and increase the randomness of the glass matrix. From the literature [49-51] it is
observed that with increase in concentration of Cr2O3 the intensity of vibrational band
corresponds to BO3 increased at the expense of BO4 band. Similar observation is also
found in the present investigation. Hence with a gradual increase in the concentration
of Cr2O3 from 0.1 to 0.25 mol% (C10-C25) the increase in intensity of the band at
about 1383 cm-1 at the expense of the band at 980 cm-1 is due to conversion of [BO4]
units into [BO3] units. More precisely at the concentration 0.05mol% of Cr2O3 (for C5
sample) the increase in intensity reaches to maximum due to conversion of [BO4] units
into [BO3] units. The band at 766 cm-1 corresponds to O4B-O-BO3 bending vibrations
139
along with decrease in intensity and blue shift of B-O-B bending vibrations at 690 cm-1
to 693 cm-1 is due to formation of new kind of linkages in the network like B-O-Cr by
breaking B-O-B bonds and/or due to decrease of B2O3 concentration in the glass
composition. Additional band around 431 cm−1 is due to vibrational modes of CrO6
units [52, 53] and the bands at 1522, 1543 cm-1 is attributed to the stretching vibrations
of B-O- in BO2O- units from different borate groups [54, 55]. Addition of Cr2O3 to LiFSrO-B2O3 glass system causes a cross linking of asymmetrical BO3 units with
chromium ions to form B-O-Cr linkages. The increase in intensity of such vibrational
bands at lower concentration of Cr2O3 (for C5 glass) suggests the increase of CrO6
octahedra vibrational modes along with Cr-O-Cr. It causes the decrease in optical band
gap, increase in Urbach energy and EPR signal intensity by forming non bridging
oxygen in the C5 glass matrix. Therefore, at lower concentration, for C5 (0.05 mol%)
glass the structure of the glass is highly modified and the number of non-bridging
oxygens exists in these glass increases.
It is well known that the electronic, ionic, dipolar and space charge polarizations
are mainly responsible for the dielectric constant of a material. Among these, the space
charge polarization depends on the purity and perfection of the glasses and it
contributes more to the dielectric constant of the material. In the present investigation
on dielectric properties of LiF-SrO-B2O3 glasses, with the introduction of Cr2O3 (0.05
mol%), we observe a large increase in the dielectric parameters, the values of å', tanä
and óac (ac conductivity) are found to increase at any frequency with raise in
temperature, and the values of breakdown strength and activation energy for ac
conduction are observed to decrease for the glass C5 with respect to C10-C25. At this
140
particular concentration chromium ions mostly exist in Cr3+ state which act as
modifiers (evidenced from ESR and optical absorption measurements); similar to Sr2+
and create bonding defects by breaking B-O-B, B-O-Sr, etc bonds. Such defects thus
produced create easy path ways for the migration of charges that would build up space
charge polarization and lead to an increase in the dielectric parameters as observed
[56].
The path in which dielectric relaxation intensity varies with changing
concentration of Cr2O3 indicates that there is a spreading of relaxation which means the
observed dielectric relaxation effects in the network are due to presence of dipoles in
the glass matrix [57, 58]. Among the three constituents, viz, LiF, SrO and B2O3 of the
pure glasses, the divalent ions of strontium (Sr2+) with the association of cationic
vacancies may form dipoles and exhibit relaxation effects [59, 60]. The value of tanämax
is observed to increase with increase in the concentration 0.1 to 0.25 mol% of Cr2O3,
but at lower concentration 0.05mol% for glass C5 attains maximum value, while the
value of activation energy for dipoles is observed a least value. The spreading of
relaxation is due to the presence of Cr3+ ions that participate in relaxation effects in
addition to the Sr2+ ions. The values of å', tanä and ac conductivity are found to
increase at any frequency and temperature with increase in the concentrations from 0.1
to 0.25 mol% of Cr2O3. This may be due to the gradual conversion of Cr6+ ions into
Cr3+ ions, there by increasing the concentration of Cr3+ ions in the glass network that
act as modifiers. But the glass C5 (0.05 mol%) showed a maximum value. Therefore,
Chromium ions (Cr3+) occupy octahedral positions, act as modifiers and create bonding
defects at lower concentration 0.05 mol% of Cr2O3. The defects thus produced create
141
easy path ways for the migration of free ions that would build up space charge
polarization leading to an increase in the dielectric parameters as observed [61, 62].
Thus, weaker is the network more is the space charge polarization. The shifting of
relaxation region towards lower temperatures and decrease in the activation energy for
the dipoles (Table 4.4) at the lower concentration 0.05 mol% of Cr2O3, suggests an
increase in degree of freedom for dipoles to orient in the field direction in the glass
network. Indirectly, it leads to the conclusion that there is decrease in rigidity of the
glass network at lower concentration 0.05 mol% (C5 glass) of Cr2O3.
In present glasses in the high temperature region the conduction phenomenon
can be explained on the basis of defect model suggested by Ingram [63]. When a plot is
made between logó(ù) verses activation energy for conduction a near linear
relationship is observed (inset of Fig. 4.11); the near linearity between the conductivity
and the activation energy suggests the conductivity enhancement is directly related to
the increasing mobility of the charge carriers in the high temperature region. Since the
alkaline earth ions are much less mobile than the alkali ions; Sr2+ ions can be regarded
as virtually immobile within the time window of hopping processes of the (Li+)ions
[64]. Therefore, in the high temperature region, the monovalent lithium ions contribute
to the conduction in the present glasses. Hence the conductivity is found to increase
with increase in concentration 0.1 to 0.25 mol% of Cr2O3, but at lower concentration
0.05 mol% for glass C5 attains maximum value, due to increase in modifying action of
Cr3+ ions. The highest conductivity and the lowest activation energy observed for the
glass C5 is obviously due to the highest concentration of the charge carriers; which
may find easy paths for the migration in the disordered glass network. The
142
proportionate higher concentration of Cr3+ ions take network forming role which
restricts the mobility of Li+ ions and hence the conductivity is found to decrease for
lower concentration 0.1 mol% of Cr2O3 doped glass.
Various mechanisms of conduction in the amorphous materials (such as band
conduction, conduction in extended states, conduction in localized states near the band
edge and conduction in localized states near the Fermi level), the conduction in
localized states near the Fermi level occurs when ac conductivity is the independent of
temperature and varies linearly with frequency (Fig.4.10) up to 343 K, can be explained
on the basis of quantum mechanical tunneling (QMT) model [65, 66] as in the case of
many other glass systems reported recently [67, 68]. The value of N(EF) ie., the density
of the defect energy states near the Fermi level, is evaluated using the following
equation [69]
ó(ù)=çe2KT[N(EF)]2á-5ù[ln(õph/ù)]4,
---- (4.5)
where ç = /3[66] = 3.664/6 [70] = 4/96 [71], with the usual meaning of the symbols
reported in earlier papers [67-69] and furnished in Table 4.5. The value of N(EF) is
found to be the lowest for glass C0. Furthermore, the range of N(EF) values obtained
~1020 eV-1/cm3; such values of N(EF) suggests the localized states near the Fermi level
[66]. The value of N(EF) is found to increase at lower concentration 0.05mol% (C5) of
chromium ions, indicating an increasing disorder in the glass network. Hence, the
highest conductivity is observed for the glass C5.
The observations on breakdown strength of LiF-SrO-B2O3:Cr2O3 glasses, as
mentioned earlier, indicate the rate of increase of å'tanä with temperature is the highest
for glass C5. The heat liberated during the breakdown by means of applying the voltage
143
across the dielectric rises the å'tanä value. The dielectric breakdown strength is
inversely proportional to å'tanä [72]. Thus the dielectric breakdown strength is lowest
for the glass C5 when compared to the other glasses [Table 4.4]. Hence, the results on
dielectric breakdown strength of LiF-SrO-B2O3:Cr2O3 glasses revealed that there is a
maximum internal distortion in the glass C5.
144
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