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Impact of manganese ions on spectroscopic and electrical
properties of lithium lead borophosphate glasses
6.1 Introduction.
B2O3 is one of the most significant glass formers incorporated in various kinds
of glass systems as a flux material in order to achieve the substances with specific
physical and chemical properties suitable for high technological applications. Borate
glasses doped with transition metal ions find potential applications in different
domains of modern science and technology [1–3]. When P2O5 is mixed in borate glass
system, it reduces the melting and softening temperatures and shifts the optical edge
towards lower wavelength. The phosphate ions may influence the physical properties
of glasses very strongly because these ions participate in the glass network with
different structural groups, viz. PO4, PO43–, P–O– and P=O, etc. In recent years much
research is going on, in improving the physical properties and chemical durability of
various glass networks containing P2O5. Due to these reasons, much attention is given
to research on borophosphate glasses doped with different transition metal ions,
because of their remarkable technological importance in the development of tunable
solid state lasers, luminescence materials, solar energy converters, fiber optic
communication devices, cathode materials in batteries and in number of electronic
appliances [4–9]. The conductivity of borophosphate glasses can be enhanced by
incorporating sufficient amount of Li2O in the glass network. Further, when these
glasses are alloyed with multivalent transition metal ions like manganese, the mixed
electronic and ionic conduction is expected depending on the glass composition.
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Electronic conduction in this type of materials is predicted due to polaron hopping
between different valence states of manganese ions [10], whereas the ionic conduction
is due to the diffusion of lithium ions [11–13].
PbO is a conditional glass former which induces lower rates of crystallization
and raises the resistance against moisture by creating the stable glass due to its dual
role; one as modifier with PbO6 structural units, when Pb–O bond is ionic and the
other as glass former with PbO4 structural units when Pb–O bond is covalent [14,15].
It is well known that manganese ions very strongly influence the electrical,
optical and magnetic properties of the glasses. A large number of fascinating studies
are available on the environment of manganese ions in various inorganic glass
networks and their tremendous industrial and scientific applications [3]. It is also
quite likely for manganese ions to have link with borate groups; strengthen its
structure and may enhance the chemical resistance of the glass. These ions exist in
different valence states in different glass matrices. For example, Mn3+ ions exists in
borate glasses with octahedral coordinations whereas Mn2+ ions with both octahedral
and tetrahedral coordinations in silicate and germinate glasses. Further, among
different manganese ions Mn2+ and Mn3+ are well known paramagnetic ions.
Interestingly Mn2+ and Mn4+ ions are noticed as luminescence activators [16]. The
content of manganese in different forms in different valence states existing in the
glass depends on the quantitative properties of modifiers and glass formers, size of the
ions in the glass structure, their field strength, mobility of the modifier cation etc
[5,7,14].
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The Mn2+ ion has the electronic configuration of 3d5, which leads to a half
filled d shell. Most of the Mn2+ ions occupy the octahedral sites and have a high spin
arrangement with five unpaired electrons [17]. The d5 electronic configuration has a
6
S5/2 ground state in the free atom and possesses zero orbital angular momentum and
hence an electron spin resonance (EPR) signal is anticipated with ‘g’ value very close
to the free electron value of 2.00232. The Mn2+ ions are widely used as acceptable
structural probes in the glasses because their EPR spectrum can be easily identified at
ambient temperature.
Due to these extraordinary characteristics of the valency of manganese ions
they are expected to influence the physical properties particularly the optical
properties of Li2O-PbO-B2O3-P2O5 glass network and have strong bearing on
dielectric properties of the glasses.
Literature survey on borophosphate glasses clearly shows that most of the
investigations on these glasses are limited to structural analysis by means of DSC, ion
transport and spectroscopic studies etc. Especially, no considerable researches on
dielectric properties (dielectric constant ε′, loss tanδ, a.c. conductivity ζac and
dielectric breakdown strength) of manganese ions doped lithium lead borophosphate
glasses are available. These studies are very much crucial in estimating the
conductivity and topology of the glass network. The main aim of the present
investigation is to make a comprehensive study on the influence of manganese ions on
the structural aspects of lithium lead borophosphate glasses from a systematic study
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on dielectric properties and spectroscopic studies such as optical absorption, FTIR and
EPR spectra.
Seven glasses in the particular glass system 20Li2O-20PbO-45B2O3-(15-x)
P2O5: xMnO (x ranging from 0 to 3 mol %) are synthesized. The glass samples are
labeled as follows:
M0: 20Li2O–20PbO–45B2O3–15.0P2O5
M1: 20Li2O–20PbO–45B2O3–14.8P2O5: 0.2MnO
M2: 20Li2O–20PbO–45B2O3–14.6P2O5: 0.4MnO
M3: 20Li2O–20PbO–45B2O3–14.4P2O5: 0.6MnO
M4: 20Li2O–20PbO–45B2O3–14.0P2O5: 1.0MnO
M5: 20Li2O–20PbO–45B2O3–13.0P2O5: 2.0MnO
M6: 20Li2O–20PbO–45B2O3–12.0.0P2O5: 3.0MnO
(all in mol%)
The methods of fabrication of the samples and the techniques adopted for
recording X-ray diffraction (XRD) pattern, optical absorption, FTIR, EPR and for
measuring dielectric properties are same as that reported in the earlier chapters.
6.2 Brief review of the previous work on MnO doped glasses
Chakradhar et al. [18] have recently delineated the micro structure,
mechanical, EPR and optical properties of lithium disilicate glasses and glassceramics combined with Mn2+ ions developed by melt quenching and controlled
crystallization. Terczynska-Madej et al. [19] have investigated the valence state and
coordination of transition metal ions in alkali-borate glasses. The optical absorption
spectra of these glasses in UV-VIS-NIR range were recorded. Kiran et al. [20] have
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analyzed the spectral studies on Mn2+ ions added in sodium-lead borophosphate
glasses and the EPR spectra demonstrate resonance signal with effective g value at geff
~2.02 with six line hyperfine structure. Shiv Prakash Singh et al. [21] have explored
the EPR, optical absorption and photoluminescence properties of MnO doped
23B2O3-5ZnO-72 Bi2O3 glasses. The mixed alkali borophosphate xLi2O-(30-x)K2O35B2O3-34.5P2O5 (5≤x≤25) glasses mixed with 0.5 mol% of manganese ions have
been investigated using EPR and photoluminescence processes by Kesavulu et al.
[22]. The EPR spectra at different temperatures lying in the range 123 to 295 K have
also been studied. Krishna Mohan et al. [23] probed the PbO-Nb2O5-P2O5 glasses
containing different concentrations of MnO ranging from 0 to 2.5 mol% were
prepared. A number of studies viz., differential thermal analysis, infrared, optical
absorption, luminescence, Raman and ESR spectra, magnetic susceptibility and
dielectric properties (constant ε', loss tanδ, ac conductivity ζac over a range of
frequency and temperature) of these glasses have been carried out. Sreekanth
Chakradhar et al. [24] examined the EPR and optical studies of Mn2+ ions in Li2ONa2O-B2O3 glasses an evidence of mixed alkali effect and the theoretical values of
optical basicity have also been evaluated. Srinivasa Reddy et al. [25] have interpreted
the spectroscopic and magnetic studies of manganese ions in ZnO-Sb2O3-B2O3 glass
system and a number of studies, viz., optical absorption, infrared and ESR spectra and
magnetic susceptibility were carried out as a function of manganese ion concentration.
The analysis of the results indicates that manganese ions mostly subsist in Mn2+ state
in these glasses when the concentration of MnO ≤ 0.6 mol% and above this
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concentration, these ions seem to exist in Mn3+ state in the glass network.
Thulasiramudu and Buddhudu have reported [26] the development and optical
characterization of heavy metal oxide (HMO)-based transparent glasses in the
chemical composition of 15PbO-40B2O3-(45-x)ZnO-xTM2+ (Mn2+) (where x=0.2, 0.5
mol%). Sreekanth Chakradhar et al. [7] have inspected the electron paramagnetic
resonance (EPR) and optical absorption spectra of manganese ions in different alkali
lead tetraborate glasses. The paramagnetic susceptibilities have been calculated using
EPR data for manganese ions in these glasses at different temperatures. Venkat Reddy
et al. [5] have studied the alkali fluoroborate glass systems containing manganese
ions. They thoroughly investigated the samples in order to obtain information about
the structural role of manganese in the R2O-RF-B2O3: MnO (with R = Li and Na)
glass network. They concluded that the intensity of optical absorption bands and the
ESR signal due to Mn2+ ions decrease with increasing MnO concentration indicating
the conversion of Mn2+ ions into Mn3+ ions in the glass network. Sreekanth
Chakradhar and his group [27] have investigated the mixed alkali borate xNa2O-(30x)K2O-70B2O3 (5≤x≤25) glasses doped with 1mol% of manganese ions using EPR
and optical absorption techniques as a function of alkali content to look for ‘mixed
alkali effect’ (MAE) on the spectral properties of the glasses. Venkateswara Rao et al.
[28] have investigated thermoluminescece (TL) characteristics of X-ray irradiated
calcium fluoro borate glasses mixed with three different alkali oxide modifiers viz.,
Li2O, Na2O and K2O have been studied. The results were analyzed the probable
mechanism responsible for quenching of TL emission by manganese ions in these
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glasses has also been supported by the aid of optical absorption, IR spectra and
differential scanning calorimetric studies. Krishna et al. [29] have studied the
influence of Mn2+ ions in alkali barium borophosphate glasses by EPR and optical
absorption techniques.
6.3 Characterization
6.3.1 X- Ray diffraction
The X–ray diffraction pattern of M0, M1 and M6 glasses are depicted in Fig.
6.1. The absence of sharp Bragg peaks in the pattern in reveals the amorphous nature
of the glasses. However, a small hump in the diffraction pattern specifies the presence
of short range order in the present glasses.
6.3.2 Physical parameters
The prepared glasses are free from visible in homogeneities such as bubbles,
cracks or inclusions. Using the measured values of density ρ and the average
molecular weight M of the samples, various physical parameters such as manganese
ion concentration Ni, mean manganese ion separation Ri, polaron radius Rp and molar
volume Vm are calculated and presented in Table 6.1 along with refractive index ‘n’ of
the samples. The measured density of M0 glass is 3.5642 g/cm3 and noticed to
increase gradually with the increase in concentration of MnO. A similar trend in
refractive index and a reverse trend in molar volume are observed; such changes are
due to variations in the structural compactness/softening i.e. the change in geometrical
configurations, co-ordination number and cross-link density, etc. in the glass sample
with increase in the concentration of MnO [14].
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Intensity (arb.units)
M6
M1
Mo
10
20
30
40
50
60
70
80
2
Fig. 6.1. X–ray diffraction pattern of Li2O–PbO–B2O3–P2O5: MnO glasses.
6.4 Results
6.4.1 Optical absorption
One of the important tools for structural characterization of glass materials is
UV–visible spectroscopy. Optical absorption spectra of Li2O–PbO–B2O3–P2O5: MnO
glasses are recorded in the wavelength region 200–1000 nm but for the sake of clarity,
the absorption peaks of studied glasses are depicted in Fig. 6.2 in the wavelength
range 250–800 nm.
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Table 6.1 The density ρ, average molecular weight M, molar volume V m, manganese
ion concentration Ni, mean manganese ion separation Ri, polaron radius Rp and
refractive index n of Li2O–PbO–B2O3–P2O5: MnO glasses.
Sample
M0
M1
M2
M3
M4
M5
M6
M
Vm
Ni x1021
Ri
3
3
3
(g/cm ) ( g/mol) (cm /mol) (ions/cm ) ( Å )
±0.01
±0.01
±0.01
±0.0001 ±0.01
3.5642
3.5861
3.6083
3.6132
3.6253
3.6496
3.6525
103.23
103.09
102.95
102.81
102.52
101.81
101.10
28.96
28.75
28.53
28.45
28.28
27.90
27.68
–
4.19
8.44
12.70
21.30
43.18
65.28
–
6.20
4.91
4.29
3.61
2.85
2.48
Rp
(Å)
±0.01
n
±0.001
–
2.50
1.98
1.73
1.45
1.15
1.00
1.682
1.695
1.702
1.715
1.724
1.738
1.742
The spectrum of MnO free glass (M0) does not exhibit any absorption peak. In the
spectra of the samples containing lower concentrations (viz., 0.2, 0.4 and 0.6 mol %)
of MnO, no detectable bands are observed in the spectral region 370–600 nm. In the
spectrum of glass doped with 1.0 mol% of MnO a broad absorption band in the region
450–550 nm with a meta center at about 480 nm and a weak band at around 410 nm
could clearly be visualized. The broad band observed at about 480 nm exhibited
spectrally red shift with increasing intensity with raise in the concentration of MnO
from 1.0 to 3.0 mol%, whereas the intensity of the band observed at about 410 nm
exhibited decreasing trend in this concentration range of MnO. The absorption edge
observed at 299 nm for M0 glass is red shift with increase in the concentration of
MnO (Table 6.2).
Optical density (arb.units)
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M0
M1
M2
489 M6
M3
415
M5
M4
300
400
500
600
Wavelength ( ) nm
700
800
Fig. 6.2 Optical absorption spectra of MnO doped Li2O–PbO–B2O3–P2O5 glasses at
room temperature.
Optical band gap (Eo) of these glasses were evaluated by drawing Tauc plots
between (αhυ)1/2 and hυ through extrapolating the linear part of such plots to (αhυ)1/2
= 0 as shown in Fig. 6.3. It is observed that optical band gap (E o) decreases gradually
from 4.33 eV to a value of 3.74 eV with increasing MnO content in the glass matrix
from 0 to 3.0 mol%. Urbach energy (ΔE) which determines the defect states is
obtained from absorption coefficient α(υ) values in Urbach’s exponential tail region
lying between 102 and 104 cm–1 is given by Eq. (6.1)
α(υ) = Cexp(hυ/ΔE),
(6.1)
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where C is a constant and ΔE is the Urbach’s energy interpreted as the energy gap
between localized tail states in the forbidden region. ΔE values are obtained by taking
the reciprocals of the slopes of the linear portion of the ln α(υ) versus hυ plots as
shown in Fig. 6.4. Calculated ΔE values are noted in Table 6.2. ΔE value is minimum
(0.25 eV) for M0 glass and maximum (0.50 eV) for M6 glass. Inset of the Fig. 6.4
shows the variation of optical band gap and Urbach energy with concentration of
dopant. Theoretical optical basicity values of all samples are calculated using Eq.
(6.2) [30] and presented in Table 6.2.
n
th
i 1
Zi ri
Z0 i
(6.2)
where ‘n’ is the number of cations present, Zi is the oxidation number of ith cation, ri is
the ratio of the number of ith cation to the number of oxides present, Z0 is oxidation
number of cation and γi is the basicity, moderating parameter of the ith cation. γi values
are calculated using Eq. 6.3.
γi = 1.36(xi–0.26)
(6.3)
where ‘xi’ is Pauling electro negativity of cation. The value of optical basicity (
th)
for the glass M0 is evaluated to be 0.4475; this value is found to increase with
increase in concentration of MnO and reached a maximum value (0.4543) for the
glass doped with 3.0 mol%.
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M5
M0
M4
( h ) (cm eV)
1/2
M6
1/2
-1
M1
M2
M3
3.2
3.6
4.0
Photon energy (h ) eV
4.4
Fig. 6.3 Tauc plots of MnO doped Li2O–PbO–B2O3–P2O5 glasses.
Table 6.2 Cut off wavelength ( λc ), absorption band positions, optical band gap
( Eo ), Urbach energy ( ΔE ) and theoretical optical basicity ( Λth ) of the Li2O–PbO–
B2O3–P2O5: MnO glasses.
Sample
λc
( nm )
M0
M1
M2
M3
M4
M5
M6
299
310
315
324
342
356
375
Band
position
(nm) ±0.1
5
Eg → 5T2g
–
–
–
–
477
486
489
Bandposition
(nm) ±0.1
6
A1g(S)→4T2g(G)
Eo (eV )
±0.01
ΔE (eV)
±0.01
Λth
±0.0001
–
–
–
–
410
412
415
4.33
4.30
4.23
4.15
4.07
3.82
3.74
0.25
0.29
0.32
0.36
0.47
0.48
0.50
0.4475
0.4479
0.4484
0.4489
0.4498
0.4520
0.4543
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M5
M4
M3
ln( )
M6
M1 M0
M2
0.50
4.2
0.45
4.0
0.40
3.8
0.35
0.30
3.6
3.4
0.25
0
1
2
Urbach energy ( E) eV
Optical band gap (E g) eV
4.4
3
Concentration (x) mol%
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
Photon energy (h ) eV
Fig. 6.4 ln(α) versus photon energy (hν) of MnO doped Li2O–PbO–B2O3–P2O5
glasses. Inset shows the variation of optical band gap and Urbach energy with
concentration of MnO.
6.4.2 Electron paramagnetic resonance
Fig. 6.5 shows the EPR spectra of Li2O–PbO–B2O3–P2O5 glasses doped with
different concentrations of MnO recorded at room temperature; the spectra exhibited
three resonance signals centered at g ≈ 1.9 (in the literature such signal is observed at
g ≈ 2.0 with sextet of hyperfine lines where as in the present study we found a
decrement in g by a value 0.1 [31]), g ≈ 2.6 and g ≈ 4.05. Among these, the signal at g
≈ 1.9, displayed a predominant broad resonance, which consists of a sextet of
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hyperfine lines. The sextet of hyperfine lines slowly disappeared at higher
concentration of MnO. With the increase of MnO concentration, the intensity of
resonance signals at g ≈ 1.9 is observed to increase, whereas that of the signals at g ≈
2.6 and g ≈ 4.05 is observed to decrease.
6.4.3 FTIR
The IR transmission spectra of glass system Li2O–PbO–B2O3–P2O5: MnO
recorded in the wavenumber range 400–1250 cm–1 are shown in Fig. 6.6. The
absorption bands and their corresponding assignments are presented in Table 6.3.
Three main conventional broad bands due to the vibrations of borate groups are
observed at about (i) 1193 cm–1 due to BO3 units which act as glass modifiers, (ii)
1029 cm–1 due to BO4 units which play the role of glass formers and (iii) at 694 cm–1
due to bending vibrations of B–O–B linkages [32]. A feeble band at 779 cm–1 and a
shoulder at 1108 cm–1 and a moderate intensity band at about 926 cm-1 due to P–O–P
and PO2– symmetric vibrations and P–O–P asymmetric vibrations, respectively, are
also observed in these spectra [32]. With increase of MnO content, all the
asymmetrical bands are observed to grow at the expense of symmetrical bands. A new
band predicted due to Mn–O specific vibrations [33,34] is also located at about 549
cm–1 in the spectra of all the glasses. Interestingly another conventional band due to
the vibrations of PbO4 structural units is also detected at about 451 cm–1 in the spectra
of these samples [35].
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g=2.6
g=4.05
g=1.9
First derivative of
Absorption (arb.units)
M6
M5
M4
M3
M2
M1
0
100
200
300
400
500
600
700
800
Magnetic field (mT)
Fig. 6.5 ESR spectra of MnO doped Li2O–PbO–B2O3–P2O5 glasses at room
temperature.
6.4.4 Dielectric studies
The dielectric constant (ε′) and loss (tanδ) at room temperature (30 oC) of
Li2O–PbO–B2O3–P2O5 glass at 10 kHz are measured to be 6.12 and 0.011
respectively; these values are found to increase with decrease in frequency. The
variation of ε′ with temperature for the glasses doped with different concentrations of
MnO measured at 1 kHz is shown in Fig. 6.7 and inset shows the variation of
dielectric constant with temperature at different frequencies of M5 glass system. The
value of dielectric constant is found to increase considerably at higher temperatures,
particularly at lower frequencies; the rate of increase of ε′ with temperature is found
to be the highest for M6 sample and minimum for M0 glass.
210
M6
1030
M5
M4
% T (a.u.)
M3
M2
549
884
1193
1108 1029
779
926
1200
M1
M0
694
800
451
400
-1
Wavenumber (cm )
Fig. 6.6 IR spectra of MnO doped Li2O–PbO–B2O3–P2O5 glasses at room
temperature.
Table 6.3 Assignment of vibrational modes in the FTIR spectra (with a probable
resolution of ±0.5 cm–1) of the glasses Li2O–PbO–B2O3–P2O5: MnO glasses.
M0
451
M1
444
M2
436
M3
425
M4
424
M5
418
M6
412
Assignment
Vibrations due to PbO4 structural
units
–
549 547 549 541 554 551 Mn–O specific vibrations of
different lengths in distorted
octahedrons
694 695 695 701 692 695 694 B–O–B bending vibrations
779 784 790 784 779 777
–
P–O–P symmetric vibrations
–
884 886 884
–
–
–
B–O bonds stretching vibrations
in BO4 units from tri, tetra and
penta borate groups
926 926 926 924 926 931 1016 P–O–P asymmetric vibrations
1029 1024 1037 1029 1034 1036
–
Stretching vibrations of B–O
bonds in BO4 units
1108 1102 1108 1101 1103 1104
–
PO2– symmetrical stretching
1193 1194 1193 1194 1184 1181
–
B–O stretch in BO3 units from
pyro and ortho borate groups
211
1kHZ
40
10kHZ
'
40
100kHZ
20
1MHZ
M6
500kHZ
200
'
100
o
300
M5
M4
Temperature ( C)
M3
M2
20
M1
M0
100
200
o
Temperature ( C)
300
Fig. 6.7 Variation of dielectric constant ε' with temperature at 1 kHz frequency for
different concentration of MnO in Li2O–PbO–B2O3–P2O5 glasses. Inset shows the
variation of dielectric constant with temperature at different frequencies of M5 glass
system.
The temperature dependence of dielectric loss (tan ) of all the glasses gauged
at a frequency of 10 kHz is presented in Fig. 6.8. The inset of the figure shows the
variation of tanδ with temperature for M1 glass at different frequencies. These graphs
have showed distinct maxima; with increase in frequency the peak maximum shifts
towards higher temperature indicating the dielectric relaxation character of dielectric
loss of these glass samples. The relaxation intensity is found to be the maximum for
M6 glass and minimum for M0 glass.
212
1kHz
M6
0.30
10kHz
tan
0.8
M5
100kHz
500kHz
1MHz
0.15
M4
0.00
100
200
300
M3
o
Temperatue ( C)
M2
tan
0.4
M1
M0
0.0
100
200
o
Temperature ( C)
300
Fig. 6.8 Variation of dielectric loss tanδ with temperature at 10 kHz frequency for
different concentrations of MnO in Li2O–PbO–B2O3–P2O5 glasses. Inset shows the
variation of tanδ with temperature at different frequencies of the M1 glass.
The modification of dielectric parameters (ε and tanδ) at room temperature with
different concentrations of manganese ions at 10 kHz is shown in Fig. 6.9. The
effective activation energy for dipoles (Wd) of all the samples is evaluated using the
equation given below and presented in Table 6.4.
f = f0exp (–Wd/KT)
(6.4)
The activation energy is found to be the minimum (2.34 eV) for M6 glass and the
maximum (3.15 eV) for the M0 glass matrix.
213
12
0.12
'
0.08
tan
8
0.04
4
0
1
2
Concentration (mol%)
3
Fig. 6.9 Response of ε' and tanδ with different concentrations of MnO at 10 kHz at
room temperature (30 oC).
The a. c. conductivity (ζac) at different temperatures is calculated using the relation
ζac = ωε εo tanδ
(6.5)
where εo is the dielectric constant of free space and ω is the frequency. The plots of
variation of ζac with 1/T at different frequencies for M2 glass is represented in Fig.
6.10. From these plots it is concluded that the conductivity of the glasses escalates
with the hike in frequency. Inset shows the alteration of conductivity for various
concentrations MnO. It clearly indicates the progressive response of conductivity with
increase in the concentration of manganese ions. The variation of ζac as a function of
1/T for pure and doped glasses at 500 kHz is displayed in Fig. 6.11. From these plots,
214
the activation energy for conduction, in the high temperature region over which an
approximate linear dependence of log ζac with 1/T could be noticed, is computed and
noted in Table 6.5. The activation energy is found to be high for M0 glass (0.25 eV)
and low for M6 glass (0.14 eV). The inset (a) reflects the fact that the activation
energy for conduction decreases with increase in the impurity concentration. Inset (b)
represents the modification of ζac as a function of activation energy for conduction at
250 oC. An approximate linearity is found between ζac and activation energy for
conduction.
1MHz
0.00010
ac
( -cm)
-1
0.00015
500kHz
100kHz
0
1
2
3
Conc. (mol%)
10kHz
ac
( -cm)
-1
0.00010
0.00008
1kHz
0.00005
2.0 1/T (X10-3 K-1) 2.5
3.0
Fig. 6.10 The variation of ζac with 1/T at different frequencies for M2 glass. Inset
demonstrates the magnification of conductivity for different concentrations of MnO.
215
The dielectric breakdown strength of all the prepared glasses were measured at room
temperature and presented in Table 6.5. The sample with higher concentration of
(a)
0.00010
A c tiv a tio n e n e rg y e V
MnO possesses lower breakdown strength (13.62 kV/cm).
0 .2 4
0 .1 6
0
1
2
3
M6
(b)
0.00005
M5
0.000100
M4
( -cm)
-1
ac
( -cm)
-1
C o n c . (m o l% )
ac
M3
M2
M1
M0
0.000075
0.16
0.00000
0.24
Activation energy eV
2.0
2.5
-3
-1
1/T (X10 K )
3.0
3.5
Fig. 6.11 Variation of a. c. conductivity ζac with 1/T for different glasses at frequency
500 kHz. Inset (a) shows the variation of activation energy with the concentration of
MnO. Inset (b) shows the relation between activation energy and conductivity at 250
o
C.
Table 6.4 Synopsis of the data on dielectric loss of Li2O–PbO–B2O3–P2O5: MnO
glasses.
Glass
(tanδ)maxavg
M0
M1
M2
M3
M4
M5
M6
0.060
0.049
0.070
0.094
0.118
0.142
0.166
Temp. region of
relaxation ( 0C )
100–148
90–140
80–133
70–125
60–118
50–110
40–103
Activation energy
for dipoles Wd ( eV )
3.15
3.00
2.90
2.76
2.61
2.47
2.34
216
Table 6.5 Epitome of data on a.c. conductivity ζac for Li2O–PbO–B2O3–P2O5: MnO
glasses.
Glass
ζac (x10–6)
(Ω–cm)–1
M0
M1
M2
M3
M4
M5
M6
8.6
15.6
25.6
35.1
44.7
56.3
66.7
N(EF) in (x1021eV–1/cm3)
Austin Butcher Pollak
and
and
Mott
Hyden
1.49
0.62
1.51
2.01
0.84
2.04
2.57
1.07
2.61
3.01
1.26
3.06
3.40
1.42
3.45
3.81
1.59
3.87
4.15
1.73
4.22
Activation
energy for
conduction
(eV)
0.25
0.20
0.19
0.17
0.16
0.15
0.14
Breakdown
strength
kV/cm
15.42
14.98
14.64
14.48
14.15
13.88
13.62
6.5 Discussion
6.5.1 Physical parameters
Generally density of a material is an excellent source for exploring the
structural changes. Density is very much influenced by the dimensions of interstitial
vacancies of the glass matrix [14]. The gain in density due to mixing of MnO is
predicted due to the replacement of lighter cations P5+ by heavier manganese ions
showing that the density of a system is highly sensitive to the atomic weight and ionic
size. The minimum molar volume and maximum value of density of M6 sample
reveal the compact nature of glass network. When the basic glass is mixed with MnO,
the manganese ions occupy the interstices of the glass, in sequence with conversion of
BO4 tetrahedral to BO3 triangular structural units. The ionic radii of B3+ in BO4 units
(0.25 Å) are larger than B3+ in BO3 units (0.15 Å).
217
6.5.2 Optical absorption
The optical absorption spectra of the present glasses are analyzed using
Tanabe–Sugano diagrams of manganese ions. Generally manganese ions exist in Mn2+
(3d5) and Mn3+ (3d4) states in oxide glasses, though the higher states are possible [20].
In a cubic crystal field of low to moderate strength, the d5 electrons of Mn2+ ions are
distributed in t2g and eg orbitals, with the ground state configuration (t2g)3 and (eg)2.
According to Hund’s rule, this type of configuration gives rise to the electronic states,
6
A1g, 4A1g, 4Eg, 4T1g, 4T2g, 4A2g and to a number of doublet states of which 6A1g lies at
the deepest. The Mn2+ ion is rarely found in regular octahedral sites with spherically
non–degenerate 6A1g state. All the transitions of Mn2+ ions are spin forbidden and low
intensity absorption bands are observed [30,36]. The feeble band observed around 410
nm is assigned to spin forbidden 6A1g(S) → 4T2g(G) transitions of Mn2+ ions in
tetrahedral symmetry (Td). In the spectral region 450–550 nm two bands due to spin
allowed 5Eg → 5T2g transition of Mn3+ ions and 6A1g(S) → 4T1g(G) transition of Mn2+
ions are predicted; both are octahedral bands [37-40].
With an increase in the concentration of MnO, the intensity of octahedral band
increases at the expense of the tetrahedral band of Mn2+ ions at 410 nm. This
observation indicates that an increasing proportion of octahedral manganese ions in
the glass network with increase in the concentration of MnO
The electronic structure in amorphous materials strongly depends upon the
optical absorption near the edge. In the present analysis, the red shift of the cutoff
wavelength and the decrease of optical band gap energy E o with doping of MnO are
218
due to formation of octahedral Mn2+ and Mn3+ ions which in turn causes the increase
in non–bridging oxygen (NBO) atoms and decrease of optical band gap along with
increase in Urbach energies. As modifiers these ions break B–O–B and P–O–P
linkages and increase the degree of disorder in the glass network and lead to decrease
of optical band gap (Eo) as observed. The decrease in optical band gap with increasing
MnO content can also be explained in terms of the structural changes that are
happening in the glasses. The value of E o decreases as the concentration of MnO
increases from 0 to 3 mol % in the glass system. The addition of MnO as a network
modifier brings about the formation of NBOs simply bounded by O2– ions. In metal
oxides, the valence band maximum (VBM) chiefly consists of O(2p) orbital and the
conduction band minimum (CBM) mainly consists of M(ns) orbital. The NBO ions
contribute to the VBM. When a metal-oxygen bond is broken, the bond energy is
liberated. The non bridging orbitals have huge amount of energies than bonding
orbitals. Therefore, an increase in the concentration of NBO ions results in a shift of
the VBM and a reduction of the band gap energy. The increase in Urbach energy (ΔE)
with increase in the content of MnO is due to formation of defects like dangling bonds
of NBOs and fluctuations in bond angle distortions [41]. The other factor contributing
to edge broadening is static disorder which increases the density of localized states
N(EF) of these defects. The observed minimum Urbach energy for M0 glass indicates
the least width of the tails among investigated glasses.
Increase in the values of basicity (Λth) with dopant concentration reveals an
increase of localized donor pressure on cations of glass matrix. In other words, the
219
covalence of the glass network decreases. The network formers interact covalently
with oxygen, while the modifiers are the elements that interact ionically. Moreover,
Duffy et al. [42] concluded that the polarizability of oxygen ions is directly
proportional to the optical basicity. The increase in polarizability of oxygen ions
indicates the increase in the concentration of NBOs [43] and the same is also
supported by the observed decrease in Eo and increase in ΔE values.
6.5.3 Electron paramagnetic resonance
The EPR spectra of MnO doped glasses exhibit resonance signals due to Mn2+
(3d5) ions entering the glass network as paramagnetic agents. The six line hyperfine
structure (hfs) of the resonance signal at g = 1.9 is due to the interaction of electron
spin of manganese ions with its own nuclear spin I = 5/2 [44]. The ability to observe
the hfs has two tangible benefits: (i) it generally allows unambiguous assignments of
positions of complex resonance lines to manganese ions and (ii) the magnitude of
hyperfine splitting provides a measure of the covalent bonding between Mn2+ ions and
its surrounding ligands [22]. Their relative intensities vary with glass structure and
composition. The ground state of octahedral Mn2+ ions is 6S5/2 [25] and the g value is
expected to lie very near the free–ion value of 2.0023. However, sometimes the g
value is more than 2.0. These large g values are expected due to the presence of some
symmetric elements in the glass matrix. The theory of these large g values is generally
expressed by the spin–Hamiltonian [45].
H = gβBS+D[Sz2 –{S(S+1)/3}]+E(Sx2–Sy2)
(6.6)
220
where S = 5/2. Here D and E are the axial and rhombic structure parameters, λ = E/D
lies within the limits 0<λ<1/3. Literature survey reveals that axial distortion of
octahedral symmetry of d5 transition metal ions gives rise to three Kramer’s doublets
±5/2>, ±3/2> and ±1/2> even at zero field [36]. The spin degeneracy of Kramer’s
doublet is lifted when Zeeman field is applied. As the crystal field splitting is
normally much greater than the Zeeman field, the resonances observed are due to
transitions within the Zeeman field split Kramer’s doublet. The resonances at g ≈ 4.05
and 2.6 can be attributed to the rhombic surroundings of the Mn2+ ions which act as
glass formers [46]. The resonance at g ≈ 1.9 is due to the Mn2+ ions which behave like
glass modifiers in an environment close to octahedral symmetry and is known to arise
from the transition between the energy levels of the lower doublet ±1/2> [45,47]. The
resonances at g ≈ 4.05 and g ≈ 2.6 are due to the transition between the energy levels
of middle Kramer’s doublet ±3/2>. The highest intensity of the signal strength at g ≈
1.9 than at g ≈ 4.05 and g ≈ 2.6 indicates that the concentration of Mn2+ ions with
octahedral environment is more than that rhombic symmetry. The increase in the
intensity of the signal at g ≈ 1.9 with the increase in the concentration of MnO,
indicates the increase in the concentration of octahedral Mn2+ ions in the glass
network. The observed decrement in g–value, with respect to 2.0023 indicates that the
Mn2+ ions are in an ionic environment [48].
6.5.4 FTIR
The FTIR spectra of the system are analyzed in the wavenumber region 400–
1250 cm–1 to investigate the structural changes in the present glass network. It is
221
widely accepted that incorporation of Li2O into B2O3 leads to conversion of sp2 planar
BO3 units into more stable sp3 tetrahedral BO4 units [5, 25]. Each BO4 unit is linked
to two other similar units and the structure leads to the formation of long tetrahedron
chains. However, addition of PbO into glass system changes the BO4 tetrahedral units
into triangular BO3 units, by forming a PbOn polyhedron when it is surrounded by
several BO4 tetrahedrons. This structure behaves like a defect in the glass network
[49]. In the present analysis the band at around 1193 and 1029 cm–1 is assigned to the
stretching vibrations of the B–O bond of triangular BO3 and tetrahedral BO4 units,
respectively. The band at nearly 694 cm–1 is ascribed to B–O–B bending vibrations
and a band at 549cm–1 is assigned to Mn–O specific vibrations of different lengths in
distorted octahedrons. When MnO is added to Li2O–PbO–B2O3–P2O5 glass network, it
will twist or distort the interconnected chains of BO4 units and increase the
randomness of the glass system. It is clear that with increase in concentration of MnO
the intensity of vibrational band corresponding to BO3 units is increased at the
expense of band due to BO4 units. Similarly, a significant decrease in the intensity of
the bands due to P–O–P and PO2– symmetric stretching vibrations at 779 and 1108
cm–1 is observed, whereas that of P–O–P asymmetric stretching band at 926 cm–1
increases with increase in concentration of MnO. The intensity of the band at 451 cm–
1
due to PbO4 structural units which act as glass formers decreases with increase in the
concentration of MnO [49].These observations support the view point that with
increase in the concentration of MnO there is an increasing proportion of octahedral
manganese ions that act as modifiers in the glass network [50].
222
6.5.5 Dielectric properties
The dielectric nature of a glass is due to electronic, ionic, dipolar and space
charge polarizations. The space charge polarization, which can be ignored at low
temperatures but noticeable in low frequency region, depends on the purity and
perfection of the glasses. The dipolar effects can be observed in the glasses even up to
106 Hz. It is found from the literature that the slight increase in the dielectric constant
and loss at room temperature, particularly at low frequencies, may be attributed to the
defects produced in the glass network. These defects create easy path ways for the
migration of charges that would build up space charge polarization leading to increase
in dielectric parameters such as ε′ and tanδ [51–54]. In the present investigation, we
have observed increase in the values of dielectric parameters of Li2O–PbO–B2O3–
P2O5: MnO glasses with increase in the concentration of MnO. The breakdown
strength and activation energy for ac conduction are observed to decrease with
increase in concentration of MnO. Manganese ions mostly exist in Mn2+ and Mn3+
states, occupy octahedral positions (as observed from optical absorption and EPR
measurements) act as modifiers and create bonding defects by breaking B–O–B and
P–O–P bonds [52] and facilitate for the increase of dielectric parameters.
The observed dielectric relaxation effects are mainly due to divalent metal ions
such as Pb2+ and Mn2+ ions in the glass network [30]. Among different constituents of
these glasses, the divalent manganese ions together with a pair of cationic vacancies
may form dipoles and such types of dipoles are responsible for the observed dielectric
relaxation effects. The value of (tanδ) maxavg is observed to increase with an increase in
223
the concentration of MnO, whereas the value of activation energy for dipoles is
observed to decrease. Such decrease in activation energy for dipoles, suggests an
increase in the degree of freedom for dipoles to orient in the field direction in the
glass network.
In the present investigation of the glass systems, in the high temperature
region the conduction phenomenon can be explained on the basis of defect model
given by Ingram [55]. When a graph is drawn between ζac and activation energy for
conduction a near linear relationship is noticed (inset b of Fig. 6.11). This
approximate linearity between the conductivity and the activation energy reflects the
fact that conductivity enhancement is directly proportional to the increasing mobility
of the charge carriers in the high temperature region. Since the Pb2+ ions are much
less mobile than the Li+ ions; Pb2+ ions can be regarded as virtually immobile within
the time span of hopping process of Li+ ions [56]. Therefore, in the high temperature
region, the Li+ ions contribute to the conduction in the present glasses. The gain in the
conductivity with hike in the concentration of MnO is due to increasing modifying
nature of manganese ions in the glass network. The lowest activation energy and
highest conductivity observed for the M6 glass is due to the increasing mobility of
free charge carriers in the more disordered glass network. The proportionate higher
concentration of manganese ions taking network forming sites, prevent the mobility of
Li+ ions and hence the conductivity is observed to decrease for lower concentration of
MnO in the present glasses.
224
Among various types of conduction mechanisms in the glasses (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 Fermi level takes place when a.c. conductivity is nearly
temperature independent and varies linearly with frequency. The conduction in
present Li2O–PbO–B2O3–P2O5: MnO glasses in the low temperature region (up to
approximately 350 K) can safely be ascribed to take place by the mechanism. The
value of density of energy states N(E F) near Fermi level, for 500 kHz frequency and at
323 K is estimated on the basis of quantum mechanical tunneling (QMT) model [57,
58] as in the case of many other systems reported recently [59, 60]. The density of the
defect energy states near the Fermi level is evaluated using the equation [61]
ζ(ω) = ηe2KT{N(EF)2α–5 ω[Ln(νph/ ω)]}4
(6.7)
Where η = π/3 [57] = 3.66 π 2/6 [62] = π 4/96 [63], with the usual meaning of the
symbols reported in earlier papers [60,61] and presented in Table 6.5. The value of
N(EF) is observed to increase with increasing concentration of manganese ions,
indicating once again an increase in the disorder of the present glass network and is
supported by the increase of Urbach energy (ΔE) values at higher concentrations of
impurity in the glass network. The value of N(EF) is observed to be the highest for M6
glass.
A gradual decline in the dielectric breakdown strength (from that of the
undoped glass) of glasses has been observed when the concentration of MnO is
increased. When an electric field is applied to the dielectric, the heat of dielectric loss
225
is liberated. If the applied electric field (E) is an alternating field, the specific
dielectric loss, i.e. the loss per unit volume of the dielectric is given by the following
equation [64].
ρ1(W/m3) = E2ωε′εotanδ
(6.8)
This equation indicates that higher the values of ε′ tanδ of the glass at a given
frequency, higher are the values of ρ1. When a voltage is applied across a dielectric,
heat is liberated, then the temperature of the dielectric increases and loss increases
further more. The dielectric breakdown strength is, in fact, inversely proportional to
the specific dielectric loss given by Eq. (6.8).
The observations on dielectric breakdown strength of Li2O-PbO-B2O3-P2O5:
MnO glasses, as mentioned earlier, indicate the rate of increase of ε′ tanδ with
temperature is the highest for M6 glass. The heat liberated during the breakdown
achieved by the application of voltage across the dielectric raises the ε′ tanδ value.
The dielectric breakdown strength is inversely proportional to ε′ tanδ [64]. Thus the
dielectric breakdown strength is lowest for the M6 glass when compared to the other
glasses [Table 6.5]. Hence, the results on dielectric breakdown strength of Li2O-PbOB2O3-P2O5: MnO glasses revealed that there is a maximum internal disorder in the M6
glass.
6.6 Conclusions
The following conclusions are drawn from the analysis of the results on
spectroscopic studies and dielectric properties of Li2O–PbO–B2O3–P2O5: MnO
glasses:
226
i.
Glasses with composition 20Li2O–20PbO–45B2O3–(15–x)P2O5–xMnO,
with 0 ≤ x ≤ 3 mol% were synthesized by melt quenching technique.
ii.
The optical absorption spectra indicated the presence of Mn2+ and Mn3+
ions are in octahedral positions at higher concentrations of dopant.
iii.
The gradual increase in the values of theoretical optical basicity was
attributed to increase in the ionic nature of glasses.
iv.
IR studies indicated the conversion of some of BO4 units into BO3 units due
to the increasing concentration of modifying manganese ions with increase
in the concentration of MnO in the glass network.
v.
The increase in the values of ε′, tanδ and ζac with the increase in
concentration of MnO is attributed to space charge polarization.
vi.
The lower value of dielectric breakdown strength of the glass doped with 3
mol% of MnO indicates its maximum value of conductivity among all the
glasses.
227
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