ARTICLE IN PRESS
Physica B 393 (2007) 61–72
www.elsevier.com/locate/physb
The structural investigations of PbO–P2O5–Sb2O3 glasses with MoO3 as
additive by means of dielectric, spectroscopic and magnetic studies
G. Little Flower, G. Sahaya Baskaran, M. Srinivasa Reddy, N. Veeraiah
Department of Physics, Acharya Nagarjuna University P.G. Centre, Nuzvid 521 201, AP, India
Received 15 October 2006; received in revised form 8 December 2006; accepted 11 December 2006
Abstract
40PbO–40P2O5(16+x)Sb2O3:(4x)MoO3 glasses with nine values of x ranging from 4 to 0 were prepared. A number of studies viz.,
differential thermal analysis, infrared, optical absorption, Raman and ESR spectra, magnetic susceptibility and dielectric properties
(constant e0 , loss tan d, AC conductivity sAC over a range of frequency and temperature) of these glasses have been carried out. The
results have been analyzed in the light of different oxidation states of molybdenum ions. The analysis indicates that when the
concentration of Sb2O3 is less than 19.0 mol% in the glass composition, molybdenum ions are observed to exist mostly in Mo5+ state,
occupy network-modifying positions and decrease the rigidity of the glass network.
r 2007 Elsevier B.V. All rights reserved.
Keywords: PbO–P2O5–MoO3 glasses; Dielectric properties; Spectroscopic properties
1. Introduction
P2O5 glasses have several advantages over conventional
silicate and borate glasses due to their superior physical
properties such as high thermal expansion coefficients, low
melting and softening temperatures and high ultra-violet
and far infrared transmission [1] and are also the materials
of choice particularly for high power laser applications [2].
During the last two decades, phosphate glasses have been
investigated extensively, yet there is still a great interest in
developing new glasses suited to the demands of both
industry and technology. Many phosphate glasses are
prone to crystallization or devitrification either during
processing or while put into applications where they may
be held at high temperatures for longer periods. Further,
the poor chemical durability, high hygroscopic and
volatile nature of phosphate glasses prevented them from
replacing the conventional glasses in a wide range of
technological applications. In recent years, there has been
an enormous amount of research on improving the
physical properties and the chemical durability of phosCorresponding author. Tel.: +91 8656 23551; fax: +918656 235200.
E-mail address: [email protected] (N. Veeraiah).
0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2006.12.070
phate glasses by introducing a number of glass formers and
modifiers such as TiO2, V2O5, Al2O3, MoO3, Cr2O3, Ta2O3,
Sb2O3, As2O3 etc., into P2O5 glass network [3,4].
Among various phosphate glass systems, the alkali free
PbO–P2O5 glass systems are known to be more stable
against devitrification and moisture resistant. In contrast to
the conventional alkali/alkaline earth oxide modifiers, PbO
has the ability to form stable glasses due to its dual role;
one as the modifier [5] if Pb–O is ionic and the other as the
glass former [6,7], if Pb–O is covalent. When Pb2+ ions are
present in the glass as network formers, they impart a
three-dimensional character to the glass. This fact accounts
for the ability of PbO to form glasses up to 90 mol%.
Clearly this peculiar behavior, which distinguishes lead
from alkali and alkaline earth metals, depends on the
electronic structure of the Pb2+ ion. In fact the easily
polarizable valence shell of the Pb2+ ion strongly interacts
with highly polarizable O2 ion, giving rise to a rather
covalent Pb–O bond [8].
The addition of other heavy metal oxide like antimony
oxide to the phosphate glasses, makes them suitable for
potential applications in nonlinear optical devices (such as
ultra-fast optical switches and power limiters), broad band
optical amplifiers operating around 1.5 mm [9–11]. Further,
ARTICLE IN PRESS
62
G. Little Flower et al. / Physica B 393 (2007) 61–72
since the frequencies of some of the vibrational groups of
antimony oxide lie in the same regions of phosphate
groups, it is expected that Sb2O3 mixes easily with the P2O5
network and makes the glass more stable against moisture.
The antimony oxide participates in the glass network with
the oxygen at three corners and the lone pair of electrons of
antimony at the fourth corner in the glass network, the
Sb–O distances ranges from 2.0 to 2.6 Å with the
coordination number of Sb as 3.0 [12,13].
MoO3-containing glasses have been the subject of many
investigations due to their catalytic properties. The ions of
molybdenum inculcate high activity and selectivity in a
series of oxidation reactions of practical importance in the
glass matrices [14]. A considerable number of interesting
studies are available on the environment of molybdenum
ion in various inorganic glasses [15–18]. Mo–O bond in
molybdenum hexavalent oxide is identified as significantly
covalent. The molybdenum ion exists at least in two stable
valence states, viz., Mo (V) and Mo (VI) in the glass
network. Earlier ESR studies on the glasses containing
molybdenum ions have identified the presence of octahedrally coordinated Mo (V) ions along with distorted
octahedrons approaching tetragons [19,20]. The ions of
molybdenum act both as network formers with MoO2
4
structural units as well as network modifiers depending
upon their concentration and nature of the host network.
Quite a good number of studies on various physical
properties viz., spectroscopic, ionic conductivity, dielectric
properties etc., of some glass systems containing molybdenum ions are also available [21–24].
The objective of the present study is to investigate the
structural influence of molybdenum ions on PbO–P2O5–
Sb2O3 glass network with a gradual decrease in the
concentration of MoO3 through a detailed investigation
on spectroscopic (optical absorption, Raman, IR and ESR)
coupled with dielectric properties and magnetic studies.
2. Experimental
For the present study, a particular composition
40PbO–40P2O5(16+x)Sb2O3:(4x)MoO3 with nine
values of x ranging from 0 to 4 were prepared. The details
of the compositions chosen for the present study are
presented in Table 1.
The analytical grade reagents of ammonium dihydrogen
phosphate, PbO, Sb2O3 and MoO3 powders in appropriate
amounts (all in mol%) were thoroughly mixed in an agate
mortar and melted using a platinum crucible in the
temperature range of 950–1000 1C in a PID temperature
controlled furnace for about 2 h. The resultant bubble free
melt was then poured in a brass mould and subsequently
annealed at 300 1C. The X-ray diffraction pattern and
scanning electron microscope photographs taken on the
prepared samples confirmed the amorphous nature of the
glasses.
Differential thermal analysis was carried out using STA
409C, Model DTA-TG instrument with a programmed
heating rate of 10 1C min1, in the temperature range of
30–1000 1C, to determine the glass transition temperature,
crystallization temperature and melting temperature of the
glasses prepared.
The samples needed for optical, physical and dielectric
studies were prepared by suitable grinding and optical
polishing to the dimensions of 1 cm 1 cm 0.2 cm. The
density d of the glasses was determined to an accuracy of
0.001 by standard principle of Archimedes using o-xylene
(99.99% pure) as the buoyant liquid. A thin layer of silver
paint was applied on either side of the large-faces of the
samples, in order to serve as electrodes for dielectric
measurements. The dielectric measurements were made on
LCR Meter (Hewlett–Packard Model-4263 B) in the
frequency range 102–105 Hz. The accuracy in the measurement of dielectric constant is 0.001 and that of loss is
104. The IR spectra of the glasses were recorded by KBr
pellet method. Glass powders (2 mg) were mixed with
anhydrous potassium bromide powder (150 mg) and
pressed into pellets at 2000 kg cm2. The spectra were
recorded using a Digital Excalibur 3000 Spectrophotometer with a resolution of 0.1 cm1 in the range
400–2000 cm1. The optical absorption spectra of the
glasses were recorded at room temperature in the
wavelength range 300–1000 nm up to a resolution of
0.1 nm using CARY 5E UV–VIS–NIR spectrophotometer.
The ESR spectra of the fine powders of the samples were
recorded at room temperature on E11Z Varian X-band
(n ¼ 9.5 GHz) ESR spectrometer; the magnetic field was
scanned between 0 and 500 mT. The magnetic susceptibility
measurements were made by Guoy’s method using fine
Table 1
Various physical parameters (viz., density (d), concentration of Mo ions (Ni), Mo ion separation (Ri)), of PbO–P2O5–Sb2O3:MoO3 glasses
Composition of the glass
d (g cm3)
Ni ( 1022 cm3)
Ri (Å)
S16: 40PbO–40P2O5–16.0Sb2O3:4.0MoO3
S17: 40PbO–40P2O5–17.0Sb2O3:3.0MoO3
S18: 40PbO–40P2O5–18.0Sb2O3:2.0MoO3
S19: 40PbO–40P2O5–19.0Sb2O3:1.0MoO3
S19.2: 40PbO–40P2O5–19.2Sb2O3:0.8MoO3
S19.4: 40PbO–40P2O5–19.4Sb2O3:0.6MoO3
S19.6: 40PbO–40P2O5–19.6Sb2O3:0.4MoO3
S19.8: 40PbO–40P2O5–19.8Sb2O3:0.2MoO3
S20: 40PbO–40P2O5–20Sb2O3
5.175
5.18
5.185
5.190
5.191
5.192
5.193
5.194
5.196
6.28
4.68
3.10
1.54
1.23
0.92
0.61
0.31
—
2.52
2.77
3.18
4.02
4.33
4.77
5.46
6.88
—
ARTICLE IN PRESS
G. Little Flower et al. / Physica B 393 (2007) 61–72
powders of these glasses. The Raman spectra were
recorded on Raman Spectrometer (Bruker FRA 106/
RFS ) using 514 nm exciting light of argon laser.
3. Results
From the measured values of density d and calculated
average molecular weight M̄, various physical parameters
of PbO–P2O5–Sb2O3:MoO3 glasses such as molybdenum
ion concentration Ni, mean molybdenum ion separation Ri,
are evaluated and are presented in Table 1.
Fig. 1 shows typical traces of differential thermal
analysis of all the glasses under study. The curves exhibit
an endothermic effect due to glass transition temperature
Tg; the values of Tg are evaluated from the point of
inflection of this change. At still higher temperatures an
exothermic peak Tc due to the crystal growth followed by
an endothermic effect due to the melting effect symbolized
S16
Exo
S17
63
by Tm are also observed. The values of Tg, Tc and Tm
obtained for all the glasses are furnished in Table 2. The
appearance of single peak due to the glass transition
temperature in DTA pattern of all the glasses indicates the
high homogeneity of the glasses prepared. Further, the
parameters (Tg, Tc and Tm) show a considerable dependence on the concentration of MoO3. With a decrease in
the concentration of MoO3 in the glass matrix from 4 to
1.0 mol%, the quantity TcTg, which is proportional to
glass-forming ability, is found to increase, whereas the
quantity TmTc which is inversely proportional to glassforming ability is found to decrease (Table 2). From the
measured values of Tg, Tc and Tm, the parameters Tg/Tm,
(TcTg)/Tg, (TcTg)/Tm and glass forming ability parameter known as Hruby’s parameter Kgl ¼ (TcTg)/
(TmTc), are evaluated and presented in Table 2. The
variation of the parameter Kgl with the concentration of
MoO3 exhibits a maximum for glass S19 (Table 2),
indicating the highest glass forming ability of this glass
among all the glasses under investigation.
The dielectric constant e0 and loss tan d at room
temperature (E30 1C) of glass S16 at 100 kHz are
measured to be 18.5 and 0.00237, respectively; the values
of e0 and tan d of the glasses are found to increase
considerably with decrease in frequency. Fig. 2 represents
S18
0.004
Endo
30
S19.4
S19.6
10
0
S17
S18
S20
S19.8
S19.6
S19.4
S19.2
S19
4
0
0
0.8 0.4
0.004
Con. MoO3 (mol%)
0.002
S16
S17
S18
S20
S1.98
ε'
20
S19.8
2
10
S20
tan δ
S16
S19.2
20
0.002
ε'
tan δ
S19
0
S19.6
S19.4
S19.2
S19
0
1000
200
400
600
Temperature (°C)
800
1000
Fig. 1. Differential thermal analysis traces of PbO–P2O5–Sb2O3:MoO3
glasses.
10000
Frequency (Hz)
100000
Fig. 2. Variation of dielectric constant and dielectric loss with freqency at
room temperature for PbO–P2O5–Sb2O3:MoO3 glasses. Inset gives the
variation of e and tan d with concentration of MoO3 measured at 1 kHz.
Table 2
The values of glass transition temperature Tg, crystallization temperature Tc, melting temperature Tm and glass forming ability parameter Kgl of
PbO–P2O5–Sb2O3:MoO3 glasses
Glass
Tg (K)
Tc (K)
Tm (K)
Tg/Tm
(TcTg)/Tg
(TcTg)/Tm
Kgl ¼ (TcTg)/
(TmTc)
S16
S17
S18
S19
S19.2
S19.4
S19.6
S19.8
S20
728
737
748
759
754
748
740
725
711
999
1015
1033
1058
1045
1030
1012
992
969
1218
1208
1201
1192
1198
1205
1212
1218
1223
0.5977
0.6101
0.6228
0.6367
0.6294
0.6207
0.6106
0.5952
0.5814
0.3723
0.3772
0.3810
0.3939
0.3859
0.3770
0.3676
0.3683
0.3629
0.2225
0.2301
0.2373
0.2508
0.2429
0.2340
0.2244
0.2192
0.2110
1.237
1.440
1.696
2.231
1.902
1.611
1.360
1.181
1.016
ARTICLE IN PRESS
G. Little Flower et al. / Physica B 393 (2007) 61–72
the variation of dielectric constant and loss of PbO–
P2O5–Sb2O3 containing different concentrations of MoO3,
with frequency, measured at room temperature; inset of the
same figure shows the variation of dielectric constant and
loss with the concentration of MoO3 measured at 1 kHz.
The parameters, e0 and tan d are observed to decrease with
a decrease in the concentration of MoO3 from 4 to
1.0 mol% and for further decrease, the parameters are
observed to increase gradually.
The temperature dependence of e0 at different frequencies of glass S17 and that of the glasses containing different
concentrations of MoO3 at 10 kHz are shown in Figs. 3
and 4, respectively. The value of e0 is found to exhibit a
considerable increase at higher temperatures especially at
lower frequencies; however the rate of increase of e0 with
temperature is found to be the highest for the glass
containing 16 mol% of Sb2O3 (or 4 mol% of MoO3) and
the lowest for the glass containing 19 mol% of Sb2O3 (or
1 mol% of MoO3).
0.024
1 kHz
0.016
tan δ
64
10 kHz
0.008
100 kHz
0
0
50
100
150
200
Temperature (°C )
250
300
Fig. 5. Variation of dielectric loss with temperature at different
frequencies for PbO–P2O5–Sb2O3 glasses containing 4 mol% of MoO3.
S16
1 kHz
10 kHz
30
0.016
S17
100 kHz
S18
S19.8
tan δ
ε'
S20
20
S19.6
S19.4
0.008
S19.2
10
0
50
100
150
200
Temperature (°C)
250
300
S19
Fig. 3. Variation of dielectric constant with temperature at different
frequencies for PbO–P2O5–Sb2O3 glass containing 3 mol% of MoO3.
0
40
0
50
100
150
200
250
Temperature (°C )
300
350
S16
S17
S18
30
Fig. 6. A comparison plot of variation of dielectric loss at 1 kHz, with
temperature for PbO–P2O5–Sb2O3:MoO3 glasses.
S20
ε'
S19.8
20
S19.6
S19.4
S19.2
S19
10
0
0
50
100
150
200
Temperature (°C )
250
300
350
Fig. 4. A comparison plot of variation of dielectric constant at 10 kHz,
with temperature for PbO–P2O5–Sb2O3:MoO3 glasses.
The temperature dependence of tan d of glass S16 at
different frequencies is shown in Fig. 5 and a comparison
plot of variation of tan d with temperature, measured at a
frequency of 1 kHz is presented in Fig. 6. The curves of all
the glasses have exhibited distinct maxima; with increasing
frequency the temperature maximum shifts towards higher
temperature and with increasing temperature the frequency maximum shifts towards higher frequency, indicating the dielectric relaxation character of dielectric losses of
these glasses. Further, the observations on dielectric loss
ARTICLE IN PRESS
G. Little Flower et al. / Physica B 393 (2007) 61–72
variation with temperature for different concentrations of
MoO3 clearly show a decrease in the broadness and
(tan d)max of relaxation curves with decrease in the
concentration of MoO3 up to 1 mol%.
Using the relation
(1)
the effective activation energy, Wd, for the dipoles is
calculated for different concentrations of MoO3 and
presented in Table 3; the activation energy is found to
increase from glass S16 to S19; and for further decrease in
MoO3 (below 1.0 mol%) a slight decrease in activation
energy has been observed.
-5
(Ω-cm)
-1
10
10
-6
a
σ
ac
10-5
10
-7
4.0
3.0
2.0
1.0
0.0
Conc. MoO (mol%)
10-6
σac (Ω-cm)-1
f ¼ f o expðW d =KTÞ,
65
3
S16
S17
S18
S20
S19.8
10-7
10-5
S19.6
S19.4
S19.2
10-8
10-7
0.4
b
S19
0.5
0.6
A.E. (eV )
10-9
1.5
Table 3
Data on dielectric loss and activation energy for dipoles in PbO–P2O5–
Sb2O3:MoO3 glasses
0.7
2.0
2.5
3.0
3.5
1/T (10-3, K-1)
Glass
(tan dmax)avg
Temp. region of
relaxation (1C)
Activation energy
for dipoles (eV)
Fig. 8. A comparison plot of variation of AC conductivity at 100 kHz
with 1/T for PbO–P2O5–Sb2O3:MoO3 glasses. The inset (a) shows the
variation of AC conductivity with concentration of MoO3 and (b)
represents the variation of AC conductivity with the activation energy.
S16
S17
S18
S19
S19.2
S19.4
S19.6
S19.8
S20
0.0075
0.0066
0.0058
0.0007
0.0012
0.0016
0.0023
0.0027
0.0031
90–120
100–125
108–130
170–188
160–184
155–182
150–179
141–172
125–160
2.42
2.60
2.89
3.25
3.19
2.92
2.86
2.70
2.55
Table 4
Concentration of defect energy states N(EF) and activation energy for
conduction in PbO–P2O5–Sb2O3:MoO3 glasses
Glass
S16
S17
S18
S19
S19.2
S19.4
S19.6
S19.8
10-6
N(EF) in (1020 eV1 cm3)
AE for
conduction (eV)
Austin–Mott
Butcher–Hyden
Pollak
4.09
3.56
3.11
0.527
0.996
1.54
1.93
2.33
1.71
1.49
1.30
0.22
0.42
0.64
0.81
0.97
4.16
3.62
3.15
0.54
1.01
1.57
1.96
2.37
0.44
0.48
0.56
0.65
0.61
0.58
0.54
0.52
100 kHz
The AC conductivity sAC is calculated at different
temperatures using the equation
10-7
σac (Ω-cm)-1
sAC ¼ oo tan d,
10 kHz
10-8
1 kHz
10-9
1.5
1.9
2.3
2.7
3.1
3.5
1/T (10-3, K-1 )
Fig. 7. Variation of AC conductivity with 1/T at different frequencies for
PbO–P2O5–Sb2O3 glass containing 4 mol% of MoO3.
(2)
where eo is the vacuum dielectric constant) for different
frequencies and the plot of log sAC against 1/T for glass
S16 is shown in Fig. 7 and for all the glasses at 100 kHz in
Fig. 8; the conductivity is found to decrease considerably
with decrease in the concentration of MoO3 at any given
frequency and temperature up to 1.0 mol% (inset (a) of
Fig. 8). From these plots, the activation energy for the
conduction in the high-temperature region over which a
near linear dependence of log sAC with 1/T could be
observed, is evaluated and presented in Table 4. The
activation energy is found to increase with a decrease of
MoO3 content in the glass matrix up to 1.0 mol% and for
further decrease it is found to decrease.
The optical absorption spectra of PbO–P2O5–Sb2O3:
MoO3 glasses were recorded in the wavelength region,
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G. Little Flower et al. / Physica B 393 (2007) 61–72
66
asymmetrical stretching vibrations of PO
2 groups, this
region may also consists of bands due to PQO stretching
vibration), 1062 cm1 (arising out of symmetrical stretch1
ing vibrations of PO
due to P–O–P
2 ), at 720 cm
symmetrical stretching vibration [25]. This region may also
consist of bands due to pyrophosphate groups (P2O7)4
[26]. Another band at about 920 cm1 presumably due to
P–O–P asymmetrical stretching vibrations is also observed.
A band due to bending vibrations of PO
2 group is also
located at about 534 cm1. IR spectrum of crystalline
Sb2O3 exhibited 4 fundamental absorption bands designated by n1 (925 cm1)—due to symmetric stretching
vibrations, n2 (600 cm1)—due to symmetric bending
vibrations, n3 (710 cm1)—due to doubly degenerate
stretching vibrations and n4 (485 cm1)—due to doubly
degenerate bending vibrations (n4) of SbO3 structural units
as reported by Bishay and Magravi and also Dubois et al.
[27]. In the infrared spectrum of glass S16, the band due to
n1 vibrations of Sb2O3 structural groups is observed at
about 964 cm1 and the band related to n2 vibrations of
these units is observed at 622 cm1. Additionally a band at
300–1000 nm. The absorption edge observed at 448.6 nm
for glass S16 (Fig. 9) is found to be shifted to 370.9 nm
when the concentration of MoO3 is decreased from 4 to
1 mol%; below this concentration, the edge is shifted
towards higher wavelength. From the observed absorption
edges, we have evaluated the optical band gaps Eo of these
glasses by drawing Urbach plot (Fig. 10). The value of
optical band gap is found to increase with decrease in
MoO3 concentration up to 1.0 mol% and for further
decrease in the content of MoO3, the value of Eo is
observed to decrease gradually. Additionally, the spectra of
the glasses show a broad absorption band in the region
650–950 nm; with the successive replacement of MoO3 by
Sb2O3, the band fades out. The pertinent data related to
optical absorption spectra of these glasses are presented in
Table 5.
The infrared transmission spectrum of PbO–P2O5–Sb2O3
glass containing 4.0 mol% of MoO3 (Fig. 11) exhibit
vibrational bands around 1266 cm1 (identified due to
Absorbance (arb.units)
6
4
Table 5
Summary of data on optical band gap and band position of molybdenum
ions in PbO–P2O5–Sb2O3:MoO3 glasses
S20
S18
S17
S16
S19.8
S19.6
S19.4
S19.2
S19
2
0
300
400
500
600
700
Wavelength (nm)
800
900
1000
Fig. 9. Optical absorption spectra of PbO–P2O5–Sb2O3:MoO3 glasses.
Glass
Optical band
gap (eV)
Cutoff Wavelength
(nm)
Band position
(nm)
S16
S17
S18
S19
S19.2
S19.4
S19.6
S19.8
2.42
2.46
2.5
2.74
2.7
2.66
2.61
2.6
448.6
436.0
422.6
370.9
377.0
386.0
394.8
403.6
790
790
790
—
—
—
790
790
10.0
S19.2
S19
S19.4
(αhν)1/2 (cm-1 eV)1/2
8.0
S19.6
S19.8
S20
S18
6.0
S17
S16
4.0
2.0
0.0
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
3.1
3.2
hν (eV)
Fig. 10. Urbach plots of PbO–P2O5–Sb2O3:MoO3 glasses.
3.3 3.4
3.5
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G. Little Flower et al. / Physica B 393 (2007) 61–72
Transmittance %
808 cm1 which is the characteristics of isolated MoO4
groups [28,29], has also been located in the spectrum of this
glass. A band due to PbO4 structural units is also observed
at about 469 cm1. With a decrease in the concentration of
MoO3 up to 1.0 mol%, the following changes have been
observed in the spectra of these glasses: (i) phosphate
groups—the intensity of the bands due to PO
2 vibrations
and P–O–P symmetric stretching vibrations, is observed to
increase with a shift in the peak positions towards lower
wavenumber, (ii) antinomy groups—the intensity of the
1400
bands due to symmetrical stretching (n1) vibrations of SbO3
structural groups is observed to increase with a shift
towards lower wave number, whereas the intensity of the
band due to bending vibrations (n2) is observed to decrease
with a shift towards higher wave number. A reversal trend
in the intensity and the peak positions of these bands has
been observed when the concentration of MoO3 is
decreased below 1.0 mol% in the glass matrix. The
pertinent data related to the infrared transmission spectra
of these glasses are presented in Table 6.
Fig. 12 shows the Raman spectra of PbO–P2O5–Sb2O3:
MoO3 glasses. The spectra of all the glasses have exhibited
three conventional bands due to phosphate groups:
(i) in the region 1050–1060 cm1 due to (ns PO2 mode),
(ii) between 710 and 730 cm1 due to (ns POP mode) and
(iii) at about 400 cm1 due to bending and torsional
vibrations of PO4 structural units [30]; in this region a band
due to the symmetric stretching vibrations of SbO3
pyramidal units [31] is also expected. In view of this there
is a possibility for cross-linking between PO4 and SbO3
units.
Raman intensity (arb.units)
MoO4
1200
1000
800
600
67
PO2
symmetric
deformed
MoO6
bending mode
Sb-O
of PO4
Mo-O-Mo antisymmetric
asymmetric
Mo-O-Mo
ν2 -MoO42symmetric
symmetric stretching
of P-O-P
S16
S17
S18
S19
S19.2
S19.4
S19.6
S19.8
S20
400
1250
1050
Wavenumber (cm-1)
850
650
450
250
50
Wavenumber (cm-1)
Fig. 11. IR Spectra of PbO–P2O5–Sb2O3:MoO3 glasses.
Fig. 12. Raman spectra of PbO–P2O5–Sb2O3:MoO3 glasses.
Table 6
Various band positions (cm1) in IR spectra of PbO–P2O5–Sb2O3:MoO3 glasses
Glass
S16
S17
S18
S19
S19.2
S19.4
S19.6
S19.8
S20
c-Sb2O3
Sb2O3 groups
Phosphate groups
PO
2
asymmetrical
stretching
PO
2
symmetrical
stretching
P–O–P
symmetrical
stretching
P–O–P
asymmetrical
stretching
PO
2
bending
n1
n2
1266
1260
1258
1251
1253
1256
1261
1268
1273
—
1062
1056
1051
1049
1051
1053
1055
1058
1062
—
720
715
713
709
713
715
718
720
722
710
923
920
917
890
900
905
910
915
—
—
534
537
540
545
543
542
539
536
530
—
964
950
943
—
925
927
930
932
935
924
622
624
628
629
621
616
615
612
607
600
PbO4
units
Isolated
MoO4 units
469
469
469
469
469
469
469
469
470
—
808
811
815
819
815
810
808
808
—
—
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G. Little Flower et al. / Physica B 393 (2007) 61–72
68
Table 7
Various band positions (in cm1)in the Raman spectra of PbO–P2O5–Sb2O3:MoO3 glasses
S16
S17
S18
S19
S19.2
S19.4
S19.6
S19.8
S20
ðMoO2
4 Þ units
ðMo2 O7 Þ2 units
n1
n2
Mo–O–Mo
asymetric
Mo–O–Mo
symmetric
823
821
816
814
819
821
827
834
—
315
311
309
307
310
311
313
315
—
617
622
624
626
624
622
620
619
—
457
455
453
451
454
455
458
462
—
Deformed
MoO6
units
With a gradual decrease of MoO3 from 4 to 1 mol% in the
glass matrix, the bands due to ns (PO2 mode) and (POP mode)
vibrations have been shifted towards lower wavenumber with
increasing intensity whereas the band due to bending and
torsional vibrations of PO4 structural units has been observed
to shift towards higher wavenumber with a considerable
decrease in the intensity. Further decrease in the content of
MoO3 causes a reverse trend of all the three bands.
The n1 and n2 vibrational bands of monomeric MoO2
4
tetrahedral units have also been located at about 823 and
315 cm1, respectively, in the spectrum of glass S16. With a
decrease in the concentration of MoO3 up to 1 mol%, a
clear increase in the intensity of these two bands has been
observed; for further decrease, a noticeable decrease in the
intensity with a considerable shift towards a higher
wavenumber of these bands has been noticed. A band
identified due to MoO6 structural units (whose intensity
decreases considerably with a decrease in the concentration
of MoO3 from 4.0 to 1.0 mol%) has also been located at
about 910 cm1. Additionally, two clearly resolved bands
at 617 and 457 cm1 identified due to asymmetric and
symmetric vibrations of Mo–O–Mo linkages in (Mo2O7)2
units have also been observed [32]; with a decrease in the
concentration of MoO3 up to 1 mol%, the intensity of the
first of these bands, is observed to decrease whereas that of
the second band is observed to increase. For further
decrease of MoO3, an increase in the intensity of band due
to asymmetric vibrations of Mo–O–Mo linkages could
clearly be detected. A significant band at about 596 cm1
identified due to the anti-symmetric stretching vibrations of
Sb–O–Sb bridges is also observed [33] in the spectrum of
each glass. The intensity of this band is found to be the
highest for the glass S16 and the lowest for S19. The
pertinent data related to the Raman spectra of these glasses
is presented in Table 7.
Fig. 13 shows the ESR spectra of PbO–P2O5–Sb2O3:
MoO3 glasses recorded at room temperature. The spectra
exhibiting a signal consisting of a central line surrounded
by smaller satellites (with g?1.933 and gJ1.883) is
910
912
916
919
916
912
910
908
—
Phosphate groups
Sb–O–Sb
asymmetric
groups
PO2 mode
POP mode
PO4 units
1062
1056
1050
1048
1051
1054
1055
1056
1058
720
717
713
709
712
716
720
726
728
400
404
408
410
408
404
402
398
415
Intensity (arb.units)
Glass
596
598
600
610
608
606
604
602
600
S16
S17
S18
S19
S19.2
S19.4
S19.6
S19.8
3300
3900
B (gauss)
Fig. 13. ESR spectra of PbO–P2O5–Sb2O3:MoO3 glasses recorded at
room temperature.
detected. The intensity of the signal is observed to decrease
with a gradual decrease in the concentration of MoO3; the
intensity and half-width DB1/2 are observed to be the lowest
for the glass S19 (Table 8).
The magnetic susceptibility measurements were also
undertaken for these glasses at room temperature and the
values of w obtained are presented in Table 8; the value of
w is found to decrease gradually with a decrease in the
concentration of MoO3. From the measured values of w the
concentration of Mo5+ ions (N0 ) is estimated by taking
the value of magnetic moment as 1.7mB. The ratio (C), N0 /
total molybdenum ion concentration (Ni), is estimated
from the values of N0 and is furnished in Table 8; the value
of C is observed to decrease significantly with a decrease in
the concentration of MoO3 up to 1 mol% and below this
concentration it is observed to increase.
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G. Little Flower et al. / Physica B 393 (2007) 61–72
69
Table 8
Data on g values, half-width of the ESR signal and magnetic susceptibility of PbO–P2O5–Sb2O3:MoO3 glasses
Glass
gJ
g?
DB1/2 (mT)
w (106 emu)
C ¼ N 0i =N i
Parameter d (nm)
S16
S17
S18
S19
S19.2
S19.4
S19.6
S19.8
1.871
1.881
1.889
1.892
1.891
1.886
1.885
1.883
1.933
1.937
1.941
1.946
1.938
1.936
1.934
1.933
7.2
6.7
6.5
6.4
6.6
6.8
6.9
7.0
5.17
3.50
1.95
0.88
0.73
0.58
0.42
0.23
0.42
0.38
0.32
0.29
0.30
0.32
0.35
0.37
0.306
0.29
0.271
0.261
0.266
0.275
0.284
0.291
4. Discussion
P2O5 is a well known network former with PO4
structural units with one of the four oxygen atoms in
PO4 tetrahedron is doubly bonded to the phosphorus atom
with the substantial p-bond character to account for
pentavalency of phosphorous [34]. The PO4 tetrahedrons
link together with covalent bonding in chains or rings by
bridging oxygens. Neighboring phosphate chains are
connected together by cross-bonding between the metal
cation and two nonbridging oxygen (NBO) atoms of each
PO4 tetrahedron. In general the P–O–P bond between PO4
tetrahedra is much stronger than the cross-bond between
chains via the metal cations [35].
PbO usually is a glass modifier; as modifier it enters the
glass network by transforming two Q3 tetrahedra (viz., PO4
tetrahedra with three bridging oxygens and one terminal
de-bonded oxygen) into two Q2 tetrahedra (viz., PO4
tetrahedra with two bridging oxygens and two terminal debonded oxygen) and thus a PbO polyhedron is formed
when it is surrounded by such two Q2 and several Q3
tetrahedraons. This structure behaves like a defect in the
network of P2O5 [36]. In this case the lead ions are
octahedrally positioned. To form octahedral units, Pb
should be sp3d2 hybridized (6s, 6p and 6d orbitals) [37].
However, PbO may also participate in the glass network
with PbO4 structural units when lead ion is linked to four
oxygens in a covalency bond configuration. In such a case
the network structure is considered to build up from PbO4
units [37] and alternate with PO4 structural units and may
form the linkages of the type Pb–O–P. The presence of such
PbO4 units is evident from the band in the IR spectra at
above 480 cm1.
Molybdenum ions are expected to exist mainly in Mo6+
state in the present PbO–P2O5 glass network. These Mo6+
ions are expected to participate in the glass network with
tetrahedral MoO2
4 structural units. However, regardless of
the oxidation state of the molybdenum ion in the starting
glass batch, the final glass contains both Mo6+ and Mo5+
ions. When the concentration of MoO3 in the glass
network is about 4.0 mol%, the color of glasses appears
to be thick brown, indicating the reduction of a fraction of
molybdenum ions from Mo6+ state into Mo5+ state. These
Mo5+ ions are quite stable and occupy octahedral position
with distortion due to Jahn–Teller effect [38,39].
It is well known that the compound Sb2O3 as such does
not form glass; however, in the presence of the modifiers
like PbO, it forms glass with triangular SbO3 pyramids
with oxygen at three corners and the lone pair of electrons
at the fourth corner. In the glass network, the Sb–O
distance lies in between 2.0 and 2.6 Å with the coordination
number of Sb as 3.0. The coordination polyhedra are
joined by sharing corners to form double infinite chains
with the lone pairs pointing out from the chains. These
chains are held together by weak secondary Sb–O bonds
with lengths greater than 2.6 Å. The third oxygen in each
SbO3 units must take part with the linkage of Sb–O–P [40].
With an increase in the concentration of Sb2O3 from 16.0
to 19.0 mol% (or decrease in the concentration of MoO3
from 4.0 to 1.0 mol%), the value of the glass transition
temperature Tg and glass forming ability parameter Kgl,
have been observed to increase. Normally, an increase in
bond length, cross-link density and closeness of packing,
are responsible for increase of these parameters. Such an
observation indicates that there is an increasing presence of
molybdenum ions in Mo6+ state that take part in networkforming positions with MoO2
4 tetrahedral structural units
(with increase in the concentration of Sb2O3) and cause an
increase in the rigidity of glass network. We have observed
a gradual decrease of the values, Tg, Kgl and other related
parameters with an increase in the concentration of Sb2O3
from 19.0 to 20.0 mol% or (or decrease in the concentration of MoO3 below 1.0 mol%); this observation correlates
with a reduction of cross-link density and weakening of the
mean bond strength in the glass network probably due to
the increasing concentration of octahedrally sited molybdenum and also Pb ions that take part modifying positions.
The pragmatic increase in the intensity of the band due
to PO
2 and P–O–P symmetric stretching vibrations in the
IR spectra with a decrease in the concentration of MoO3
up to 1.0 mol% clearly suggests the decreasing modifying
action of MoO3. As mentioned earlier in this concentration
range (4–1.0 mol%) of MoO3, there is a growing concentration of molybdenum ions that take part in networkforming positions with MoO2
tetrahedral units. With a
4
decrease in the concentration of MoO3 from 4.0 to
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G. Little Flower et al. / Physica B 393 (2007) 61–72
1.0 mol%, there is a decrease in the intensity of the band
located around 920 cm1) due to P–O–P asymmetric units
with a shift towards the region of the band due to
symmetric vibrations of Sb2O3 units and exhibited a
common band. In view of this observation we may expect
the maximum concentration of Sb–O–P linkages in the
network of glass S19.
In the Raman spectra, the steady increase in the intensity
of the bands due to the symmetric vibrations of PO2 and
POP modes with a simultaneous decrease of the band due
to torsional vibrations of PO4 units (in the spectra of the
glasses S16–S19) indicates a gradual reduction in the degree
of disorder in the glass network. We have also observed an
increase in the intensity of the bands due to n1 and n2
vibrations of MoO4 structural units when the concentration of MoO3 is decreased from 4.0 to 1.0 mol% in the glass
matrix; this observation clearly predicts that there is a
growing concentration of molybdenum ions that would
participate in the glass network. The gradual increase
in the intensity of the band due to symmetric vibrations at
the expense of band due to asymmetric vibrations of
Mo–O–Mo linkages in (Mo2O7)2 units (in this composition range, 4.0–1.0 mol%) of MoO3 also supports this
argument. The reverse trend observed in the intensity
of all these bands in the spectra of the glasses S19–S19.8
specifies a growing disorder in the glass network, likely due
to the transformation of molybdenum ions from the
network-forming sites into modifying sites and also
increase in the concentration of Pb ions that take part
modifying positions.
Broad absorption band observed in the region
650–950 nm in the optical absorption spectra of these
glasses (S16–S18) is attributed to the excitation of Mo5+
(4d1) ion. In fact, for this ion, two optical excitations were
predicted starting from b2(dxy) ground state to (dxzyz) and
ðdx2 y2 Þ with d ¼ 15000 cm1 and D ¼ 23000 cm1 [41].
Perhaps, due to inter-charge transition transfer
(Mo5+3Mo6+) in the glass network, the resolution of
these transitions could not be observed. The highest
intensity of this band observed in the spectrum of glass
S16 reveals the presence of Mo5+ ions in higher
concentration in these glasses. Such Mo5+ ions may form
Mo5+O
3 molecular orbital states and are expected to
participate in the depolymerization of the glass network
[42] that create more bonding defects and NBOs. The lower
the concentration of such modifiers, the lower is the
concentration of NBOs in the glass matrix. This leads to
decrease in the degree of localization of electrons there by
decreasing the donor centers in the glass matrix. The
presence of smaller concentration of these donor centers
increases the optical band gap and shifts the absorption
edge towards lower wavelength side as observed (for the
glasses S16–S19). Nevertheless, when the concentration of
Sb2O3 is increased from 19.0 to 20.0 mol% (or MoO3 is
decreased lower than 1.0 mol%) in the glass matrix, the
concentration of Sb–O–P, Pb–O–P etc., linkages seems to
be decreased due to the presence of high concentration of
modifiers in the glass network, leading the absorption edge
to shift towards higher wavelength side.
The magnetic properties for these glasses arise due to
Mo5+ (4d1) paramagnetic ions. The decrease in the value
of redox ratio C (obtained from magnetic susceptibility
measurements), with a decrease in the concentration of
MoO3 (from 4.0. to 1.0 mol%), indicates a gradual
decrease in the reduction of molybdenum ions from
Mo6+ state to Mo5+ state in the glass matrix.
The existence of molybdenum ions in Mo5+ state in
these glasses is further confirmed by ESR spectral studies.
The ESR spectrum of these glasses consists of a main
central line surrounded by less-intense satellites. The
central line arises from even molybdenum isotopes (I ¼ 0)
whereas satellite lines correspond to the hyper-fine
structure from odd 95Mo and 97Mo (I ¼ 5/2) isotopes
[43]. The highest intensity of the signal observed in the
spectrum of the glass, S16, suggests the presence of the
highest concentration of Mo5+O
3 complexes. The values
of g? and gJ from this spectrum have been evaluated as
1.93 and 1.88; the structural disorder arising from the
site-to-site fluctuations of the local surroundings of the
paramagnetic Mo5+ ions can be accounted for the two
components of the g values. The variation of MoO3
content has considerably affected the intensity of the
signal; in fact the signal is observed to be feeble for the
glass S19. The g values obtained for these glasses are found
to be consistent with the reported values for many other
glass systems containing molybdenum ions like P2O5–
Li2WO4–Li2O, Li2O–P2O5, lead borate, lead arsenate etc.,
[44–46].
The intensity and the half-width DB1/2 of the signal is
found to decrease with a decrease in the concentration of
MoO3 from 4.0 to 1.0 mol%. For dipolar interactions
between paramagnetic ions, the half-width (in Tesla) of the
signal is given by [47]
2DB1=2 ¼ 3:62
mo ðg2? þ 2g2k Þ b
C.
g
4p
d3
(3)
Here g is the mean g-value. From this equation,
we have calculated the minimal distance d between two
molybdenum ions, (with C being the value of reduction
factor) and presented in Table 8; these distances are
observed to be far greater than the Mo–O–Mo distance in
several compounds [48]. From these data, we conclude that
the molybdenum containing structural units in these glass
matrices can only share edges or faces and not common
corners [47].
Summing up the discussion, there is a maximum
concentration of molybdenum ions that are present largely
in Mo5+ state, adopt modifying positions and create
bonding defects when the concentration of MoO3 is around
4.0 mol% in the glass network. The increase of Sb2O3 at the
expense of MoO3 caused an enhancement in the concentration of Mo6+ ions that take part network forming
positions with tetrahedral MoO2
4 structural units.
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G. Little Flower et al. / Physica B 393 (2007) 61–72
Among various polarizations (electronic, ionic, dipolar
and space charge polarizations) that contribute to the
dielectric constant, the space charge polarization depends
on the perfection of the glasses. With a gradual increase in
the concentration of Sb2O3 in the glass network from 16.0
to 19.0 mol%, the values of dielectric constant e0 , loss tan d
and AC conductivity sAC are found to decrease at any fixed
frequency and temperature and the activation energy for
AC conduction is observed to increase. Obviously, this is
because of decreasing concentration of Mo5+ ions
(evidenced from ESR, optical absorption and magnetic
susceptibility measurements); these ions act as modifiers
and generate bonding defects. The defects thus produced
create easy pathways for the migration of charges that
would build up space charge polarization leading to an
increase in the dielectric parameters [49,50]. The lowest
values of the dielectric parameters observed for glass
S19 is obviously owing to the presence of Mo5+ in low
concentrations that create disorder in the glass network.
The observed dielectric relaxation effects may be
attributed to association of Pb2+ ions with a pair of PO
2
groups (which exhibit the vibrational bands in the highfrequency side of PQO vibrational region) in analogy with
the mechanism association of divalent positive ion with a
pair of cationic vacancies—in conventional glasses, glass
ceramics and crystals [50,51]. The way the dielectric
relaxation intensity varies with changing concentration of
MoO3 indicates that there is a spreading of relaxation with
a set of relaxation times t, which means the relaxation
effects are due to several types of dipoles [52,53]. In
addition to the above type of dipoles there is a possibility
for Mo5+O
3 complexes to act as dipoles as reported in
other glass systems [28]. The shifting of relaxation region
towards higher temperatures and increase in the activation
energy for the dipoles with a decrease in the concentration
of MoO3 from 4.0 to 1.0 mol% (Table 3) suggests a
decreasing degree of freedom for dipoles to orient in the
field direction in the glass network.
When a plot is made between log s(o) vs. activation
energy for conduction (in the high-temperature region) a
near-linear relationship is observed (inset (b) of Fig. 8); this
observation suggests the conductivity enhancement is
directly related to the increasing mobility of the charge
carriers in the high-temperature region [54]. Further,
the variation of high-temperature conductivity with a
decrease in the concentration of MoO3 (from 4.0 to
1.0 mol%) shows a decreasing trend (inset (a) of Fig. 8)
and increases in the region, 1pMoO3p0 mol%. The
decreasing trend of conductivity up to 1.0 mol% suggests
the conductivity is related to ionic motion, whereas in the
other region, the conductivity seems to be related with
electronic motion [55,56].
The low temperature part of the conductivity (a near
temperature independent part, as in the case of present
glasses up to nearly 100 1C) can be explained on the basis of
quantum mechanical model [57] similar to many other
glass systems reported recently from our laboratory
71
[58–60]. The value of N(EF) i.e. the density of the defect
energy states near the Fermi level, evaluated using the
equation [57]
sðoÞ ¼ Ze2 KT ½NðE F Þ2 a5 o½lnðnph =oÞ4 ,
(4)
2
where Z ¼ p/3 (Austin and Mott [57]), ¼ 3.66p /6 (Butcher
and Hyden [61]), ¼ p4/96 (Pollak [62]), with the usual
meaning of the symbols reported in earlier papers and
furnished in Table 4. The value of N(EF) is found to
decrease from glass S16 to S19 indicating a decreasing
disorder in the glass network. Further more, the range of
N(EF) values obtained is of the order of 1020 eV1 cm3;
such values of N(EF) suggests localized states near the
Fermi level.
5. Conclusion
In conclusion, the analysis of the results of various
studies, viz., optical absorption, ESR, IR and Raman
spectra and dielectric properties of PbO–P2O5–Sb2O3
glasses added with different concentrations of MoO3,
indicates that there is a possibility of conversion of
a part of Mo6+ ions into Mo5+ ions, especially when the
concentration of MoO3 is less than 1.0 mol%. Such ions
(Mo5+ ions) mostly act as modifiers similar to Pb2+ ions
and weaken the lead antimony phosphate glass network.
Acknowledgement
One of the authors, Mrs. G. Little Flower is grateful to
the University Grants Commission, New Delhi for
providing financial assistance in the form of a Research
Project. She also wishes to thank the management of Maris
Stella College, Vijayawada for their precious support and
to Rev. Sr. Dr Theresiamma, the Principal of the same
college for her appreciation and kindness.
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