# Structural and Physical Properties ofFe2O3-B2O3-V2O5 Glasses

Authors: Virender Kundu; R. L. Dhiman; A. S. Maan; D. R. Goyal
Journal: Advances in Condensed Matter Physics
 (2008)

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## Abstract

The structural and physical properties ofxFe2O3-(40-x) B2O3-60V2O5(0≤x≤20) glass system have been investigated. The samples were prepared by normal melt-quench technique. The structural changes were inferred by means of FTIR by monitoring the infrared (IR) spectra in the spectral range 600–4000 cm-1. The absence of boroxol ring (806 cm-1) in the present glass system suggested that these glasses consist of randomly connected BO3 and BO4 units. The conversion of BO3 to BO4 and VO5 to VO4 tetrahedra along with the formation of non-bridging oxygen's (NBOs) attached to boron and vanadium takes place in the glasses under investigation. The density and molar volume of the present glass system were found to depend on Fe2O3 content. DC conductivity of the glass system has been determined in the temperature range 310–500 K. It was found that the general behavior of electrical conductivity was similar for  all glass compositions and found to increase with increasing iron content. The parameters such as activation energy, average separation between transition metal ions (TMIs), polaron radius, and so forth have been calculated in adiabatic region and are found consistent with Mott's model of phonon-assisted polaronic hopping.


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## Body

## 1. Introduction

Borate
glasses are  generally insulating in nature, and the addition of transition
metal oxide such  as Fe2O3 and V2O5
makes these glasses
semiconducting [1, 2]. These semiconducting
glasses have been extensively studied owing to their potential applications as
optical and electrical memory switchings, cathode materials for making solid state devices, and optical fiber
[3–5].Structure
of the borate glasses has
been studied by various physical and chemical methods including Raman and
Infrared spectroscopy, Brillouin experiments, NMR, and neutron scattering
investigations [6–9]. Infrared (IR)
spectroscopy is one of the important techniques which are used to study the local arrangement in inorganic glasses. In
oxide glasses, B2O3 is a basic glass former because of
its higher bond strength, lower cation size, smaller heat of fusion, and
trivalancy of boron. In these glasses, boron (B3+) ions are
triangularly coordinated by oxygen to form glasses easily. The main structural
units of vitreous B2O3 glasses are BO3 triangles
forming three member (boroxol) rings connected by B–O–B linkage [10]. Boroxol ring is a high-planar
ring with a bond  length 1.36±0.005 Å, whereas the B–O bond length for BO4 tetrahedra was observed to be 1.47±0.01 Å
[11]. In IR spectra of borate glasses,
the boroxol  ring has its characteristic absorption at
806 cm−1
[12]. It has been observed that the structure of these
glasses depends on the nature of the network formers as well as the network
modifier. It has been reported that addition of a network modifier in borate
glasses could produce the conversion of the triangular BO3 structural units to BO4 tetrahedra with coordination number of 4,
which are
incorporated in more complex cyclic groups such as diborate, triborate, tetra
or pentaborate, and the formation of NBO atoms [8, 9].Transition-metal
oxides (TMOs) glasses exhibit semiconducting properties due to the existence of
TMIs in more than one valence state [1]. The electron-phonon interaction in
these glasses is strong enough to form small polaron, and the electrical
conduction process occurs by the hopping of small polarons between different
valence states as proposed by Austin and Mott [2]. Hoping conduction in these
glasses was generally known to be adiabatic for V2O5 content above 50 mol%
[13], and for V2O5<50 mol%,
conduction becomes non-adiabatic [14].Sufficient
work regarding the structural and physical properties of different oxide
glasses having  TeO2 and P2O5 as glass former has been reported
[15–18], but
relatively few work has been done on semiconducting oxide glasses (having Fe and V as TMIs) with B2O3 as a network
former. The objective of present paper is to study the structural and physical
properties of iron-boro-vandate glasses to shed some light on the role of the Fe2O3 in this glass system.


## 2. Experimental Details

### 2.1. Sample Preparation

Iron-doped
vanadium borate glasses were prepared from analytical reagent grade powder ofFe2O3,V2O5, and H3BO3 of high purity which are thoroughly mixed, in appropriate proportions. The
batch materials were dry mixed and melted in porcelain crucibles placed in an
electrically heated muffle furnace at 1473 K for about two hours, until a
bubble-free liquid was formed. The melts were quickly cooled at room
temperature by pouring and pressing between two stainless plates. As obtained
glass samples were polished and finally cut into desired size (≈10mm×10mm×1mm).


### 2.2. FTIR Measurements

The
vibration spectra of the glass system were obtained at room temperature using
FTIR spectrophotometer model ABB Bomen (MB-Series) in the range 600–4000cm−1.
The measurements were performed directly on glass pallets obtained as above.


### 2.3. Density Measurements

The density “d” of the glasses was determined at room
temperature using Archimedes principle with Xylene as an inert immersion
liquid.The molar volumeVM of each glass sample
was calculated using formula [19] (1)VM=∑xiMid, where xi is the molar fraction, and Mi is the molecular weight
of the ith component.


### 2.4. D.C. Conductivity Measurements

The
conductivity measurements were carried out by using Keithely electrometer
(Model 617) in the temperature range of 310–500 K. Silver
paste electrodes were deposited on both faces of the polished samples. The
absence of  the
barrier layers at the contacts was confirmed by linearI−V characteristics.


## 2.1. Sample Preparation

Iron-doped
vanadium borate glasses were prepared from analytical reagent grade powder ofFe2O3,V2O5, and H3BO3 of high purity which are thoroughly mixed, in appropriate proportions. The
batch materials were dry mixed and melted in porcelain crucibles placed in an
electrically heated muffle furnace at 1473 K for about two hours, until a
bubble-free liquid was formed. The melts were quickly cooled at room
temperature by pouring and pressing between two stainless plates. As obtained
glass samples were polished and finally cut into desired size (≈10mm×10mm×1mm).


## 2.2. FTIR Measurements

The
vibration spectra of the glass system were obtained at room temperature using
FTIR spectrophotometer model ABB Bomen (MB-Series) in the range 600–4000cm−1.
The measurements were performed directly on glass pallets obtained as above.


## 2.3. Density Measurements

The density “d” of the glasses was determined at room
temperature using Archimedes principle with Xylene as an inert immersion
liquid.The molar volumeVM of each glass sample
was calculated using formula [19] (1)VM=∑xiMid, where xi is the molar fraction, and Mi is the molecular weight
of the ith component.


## 2.4. D.C. Conductivity Measurements

The
conductivity measurements were carried out by using Keithely electrometer
(Model 617) in the temperature range of 310–500 K. Silver
paste electrodes were deposited on both faces of the polished samples. The
absence of  the
barrier layers at the contacts was confirmed by linearI−V characteristics.


## 3. Results and Discussion

### 3.1. FTIR Analysis

The infrared
spectra  ofxFe2O3-(40-x) B2O3-60V2O5 glasses, with
x=0,5,10,15,
and 20 mol%, are shown
in Figure 1. The  vibrational modes of the borate network are seen
to be mainly
active in three infrared regions which are similar to those reported earlier
[20, 21]. The group of
bands that occur at 1200–1600 cm−1  is due
to the asymmetric stretching relaxation of the B–O band of trigonal BO3 units. The second group of band lies between 800–1200 cm−1  and is due
to the B–O bond stretching of the tetrahedral BO4 units.
The third group of absorption bands is observed around 700 cm−1 and
is due to bending of B–O–B linkages in the borate network. Similar results have
been reported for V2O5-B2O3 glasses
[22, 23].Figure 1
IR transmission spectra ofxFe2O3-(40-x) B2O3-60V2O5 glasses.The absorption
peaks assigned in IR spectra of glasses under study are listed in Table1.
The
absorption peak observed in all glass samples at 3204–3217 cm−1 is attributed
to hydroxol or water groups
[24] and it is due to hygroscopic
nature of glass samples [25]. The peaks around 2270–2286 cm−1 and 2342–2345 cm−1  are attributed
to –OH group
[26]. The
absorption peak around 1722–1730 is due to
H–O–H bending [22]. In the present glass system, the absence of peak at 806 cm−1 indicates the absence of boroxol ring formation
[12], which suggests that the glass
system under investigation consists
of randomly connected BO3 and BO4  groups. The absorption
peaks at 1192, 1445, 1537, and 1549 cm−1
[27, 28] are related
with the fundamental asymmetrical stretching vibration of the B–O bond of the
trigonal BO3 units. The absorption bands observed at 814, 883, and
1023 cm−1
[20, 29] are assigned to the stretching vibrations of B–O
bond of tetrahedral BO4 units and are shifted toward lower wave
numbers with increasing iron content. It is also observed
(Figure 1) that on
increasing Fe2O3 content, the absorption band at around 1445 cm−1 shifts toward lower wave number (1445–1440 cm−1)
with noticeable decrease in intensity. In the present glass system, the shift
of the vibrational band from higher to lower wave number is ascribed to the
increase in the bond length of B–O groups and the formation of BO4 units. The decrease in intensity of vibrational bands (for x≤10 mol%) at  1445 and 1192 cm−1 and clear
appearance of band in the lower region at 814 cm−1
suggest the formation of NBOs. A sharp absorption
band observed at around 1192 cm−1 may be attributed to triangular
B–O stretching vibrations of BO3 units
[27]. On increasing the Fe2O3 contents, the frequency of this band remains almost the same, revealing the strong appearance of
the triangular borate units (BO3). The absorption band observed at
621 cm−1 is due to the bending of O–B–O linkage
[30]. The shifting of
this band toward higher wave number (621–646 cm−1)
with increasing Fe2O3 content indicates the formation of
FeO4  group
[30]. This possibility is more in the borate glasses in
which boroxol rings are absent. The absorption peak observed at 704 cm−1 is assigned to B–O–B bending vibration in borate network
[31]. At low
concentration of Fe2O3, a weak absorption band observed
at 814 cm−1 originates from stretching vibration of V–O–V bridges
[32]
and becomes slight intense as the concentration of Fe2O3 increases. The weak absorption band appearing in all the samples at
≈883 cm−1 is due to V2O5 content. The shoulder at 1023 cm−1 can be referred to stretching
vibrations of BO4 tetrahedra and is also due to higher content of V2O5 in the glass system. It has been reported that the V2O5 structure is built up by deformed VO5 trigonal bonded zigzag chains.
Each VO5 group contains a short V=O bond (Vanadyl group), which
shows its characteristic vibration band at around 1023 cm−1
[33].
The appearance of this frequency band at 1020 cm−1 at x=10 (mol%) and the clear separation
of the peak at 814 cm−1 on increasing Fe2O3 contents suggest
that this absorption is not only due to the presence of non-bridging V–O−1 and V=O bond, but also due to the presence of BO4 tetrahedra.
Similar results have also been observed in potasium-boro-vanadate-iron glasses
[34]. From Figure 1, it can be clearly inferred that a weak absorption band corresponding
to 814 cm−1 appears for low concentration of Fe2O3. This
peak, however, disappears
as the concentration of Fe2O3 increases (x=5 mol%) and with further increase
in Fe2O3 content (x=10 mol%), the increase in intensity of this peak indicates that the Fe2O3 acts as a glass modifier. The intensity of BO3 structural units
decreases on further increases of Fe2O3 contents, and clear
appearance of peaks (≈814 cm−1)
in the lower region suggests
that some BO3 structural units are converted into some BO4 tetrahedral units, which results in the formation of NBOs. On further addition
of Fe2O3 contents (15–20 mol%), the absorption
bands observed in the lower region, that is, at 1190, 881, 814, 706, and 646 cm−1
remain almost
unaffected, however, their intensities changes which indicate that  Fe2O3 also acts as a
glass former. These observations suggest that at low concentration (x≤10 mol%), Fe2O3 acts as a glass modifier and at high concentration (x=15–20 mol%), Fe2O3 acts as a glass former. Therefore, in the present glass system, Fe2O3 acts both, as a glass modifier as well as a glass former. The results
obtained are well consistent with already reported result [26].Table 1
Peak frequencies from IR spectra forxFe2O3-(40-x) B2O3-60V2O5 glass
system.
xPeak position(cm−1)(mol%)O–B–OB–O–BBO4BO3OH−1 group06217048148831023119214451537154917272270234232045635705—882102111911442——173022732345321410643706814881102011911440—1548172222702344321715646706814881—11901441——172722822346320520645705815880—11911440——1728228623453208Table 2
Electrical conductivity at
different temperatures and the prefactor of Mott-Austin formula
ofxFe2O3-(40-x) B2O3-60V2O5 glasses.
x (mol%)σ320K(Ωm)−1σ400K(Ωm)−1σ500K(Ωm)−1logσ000.313×10−45.435×10−47.945×10−32.67351.600×10−41.783×10−31.987×10−22.793105.817×10−47.705×10−34.339×10−21.750152.680×10−31.794×10−28.853×10−21.581208.220×10−34.194×10−214.73×10−21.450


### 3.2. Density and Molar Volume

The determined
values of density “d” and molar
volume “VM” of the glass
samples are presented in Table 3. In general, the density of glass system is
explained in terms of a competition between the masses and sizes of the various
structural units present in glass. In other words, the density is related to
how tightly the ions and ionic groups are packed together in the structure. The
variation of density as a function of glass composition (x) is shown in
Figure 2. It is observed that the density increases
gradually with the increase in Fe2O3 content in the
present glass system. The relationship between density and glass composition (x) can be explained in terms of an
apparent volume VM occupied by 1 gm atom of oxygen. The value of VM has been calculated from the density and composition
using the formula reported earlier [19], and its composition dependence is
shown in Figure 2. It is observed that molar volume decreases monotonically
with the increase of Fe2O3 content which indicates that
the topology of the network is significantly changed with composition. On the
other hand, these trends can be explained rather simply due to the replacement
of a lighter cation (B) by a heavier one (Fe). As observed in IR spectra, the
addition of Fe2O3 causes increase in NBOs, which in turn
randomizes the structure
and, therefore, glass structure becomes relatively more open. These results are
found consistent with the results reported earlier [35, 36].Table 3
Chemical composition and physical properties ofxFe2O3-(40-x)
B2O3-60V2O5 glasses.
x (mol%)Molar volumeVM(cm3/mol)Densityd(g/cm3)W (eV)048.042.850.427547.462.980.3611045.473.210.3291544.643.370.2662044.283.500.220Figure 2
Variation of density and
molar volume with Fe2O3 content.


### 3.3. DC Conductivity

The temperature
dependence  of dc conductivity “σ” for the different glass compositions
is shown in Figure 3. It
is  observed that conductivity increases smoothly with increasing temperature,
indicating temperature dependence activation energy “W” which is characteristic of small polaron hopping (SPH) conduction
mechanism in TMO glasses [37]. As shown in
Figure 3, the logarithmic conductivity
in the temperature range (310–500 K) exhibits a
linear dependence on reciprocal temperature. The composition dependence of dc
conductivity at particular temperature (Figure 4) indicates that the
conductivity increases with increasing Fe2O3 content. The
activation energy calculated from the slop of the graphs
(Figure 3) is listed
in Table 3. It is clear from
Figure 5 that the activation energy decreases with
increasing Fe2O3 content. The composition dependence of
dc conductivity at 400 K (Figure 4) and activation energy
(Figure 5) indicate that the
variation of “σ” as well as “W" with
composition is much faster for the present glass system than those for the
traditional vandate glasses. It is observed that as the concentration of Fe2O3 increases, the activation of electrical conduction decreases and electrical
conductivity increases. The low value of activation energy and high value of
electrical conductivity are similar to those of
V2O5-BaO-B2O3 glasses
[38]. The change in conductivity and activation energy may help to
detect the structural changes as a consequence of increasing of Fe2O3 content and decreasing of boron oxide content. Generally, it is known that
addition of Fe2O3 in borate glasses increases the
conductivity as result of increasing of NBO ions [39]. It is clear from
Figures
4 and 5 that the magnitude of conductivity is higher for those compositions which
have lower activation energy. This result is in consistent with the small
polaron hopping theory [40]. According to this theory, the conduction process
at higher temperature is considered in terms of optical phonon assisted hopping
of small polaron between localized states. The dc conductivity in adiabatic region
is given by (2)σ=σ0exp(−WkT), where σ0 is
a pre-exponentional factor, W is the
activation energy, k is Boltzmann constant, and T is temperature in Kelvin. The
values of log σ0 were determined from the intercept of the
conductivity versus temperature curve (Figure 3). The observed values of log σ0 are found to be independent of Fe2O3 content
(Figure
5) which confirms the adiabatic SPH for the present glass system. The present
glass system consists
of two types of TMIs, that is, Fe and V. The density “n” of respective ions was calculated (in order to confirm the
relation between the activation energy W and mean distance R) using the formula
[14]: (3)n=2[(dWtMw)NA], where d,NA,Wt,  and Mw are the measured density
of the sample, Avogadro’s number, weight fraction, and molecular weight of the
respective ions (Fe & V), respectively. The calculated values of iron and vanadium
ions are listed in Table 4. The average distance “R” between the transition ions (assuming homogenous distribution of
transition ion in glass volume) was calculated as follows: (4)R=(1N)1/3, where “N” is the concentration of total TMIs (nFe2O3+nV2O5).Table 4
Hopping parameters ofxFe2O3-(40-x)
B2O3-60V2O5 glasses.
x (mol%)nFe2O3(cm−3)nV2O5(cm−3)N(cm−3)R(nm)rp(nm)00×10230.1131×10230.1131×10230.32120.17950.0112×10230.1183×10230.129×10230.30740.171100.024×10230.1274×10230.151×10230.29170.163150.038×10230.1337×10230.171×10230.27990.156200.052×10230.1389×10230.191×10230.26970.150Figure 3
Variation of logσ versus
1000/T(K−1) for studied glasses.Figure 4
Effect of Fe2O3 content
on dc conductivity at different temperatures.Figure 5
Effect of Fe2O3 content on activation energy
W and
pre-exponential factor log σ0.The small
polaron radii “rp” for  the
iron and vanadium ions have been calculated using [41] (5)rp=12(π6N)1/3.  The calculated values of
N,R, and rp are tabulated in
Table 4. The variation of activation
energy W  with average distance R, as shown in
Figure 6, suggests that
there is a prominent positive correlation between activation energy and average
site separation. It is evident from Figure 7 that the small polaron (SP) radius
decreases with the increase in Fe-ion concentration. From the above
observations, it is concluded that on addition of Fe2O3,
conductivity increases, while the activation energy decreases due to the decrease
in  respective  ion separation. These results are in good agreement with those reported
by El-Desoky  [36]. However, in the present investigation for sample x=10 mol% of Fe2O3,
there are some
minor changes observed in the density and electrical parameters as compared to
our earlier investigation [42]. These changes may be attributed to difference
in sample preparation conditions [11].Figure 6
Variation of activation
energyW with average ion separation R.Figure 7
Variation of small
polaron radius with iron content.


## 3.1. FTIR Analysis

The infrared
spectra  ofxFe2O3-(40-x) B2O3-60V2O5 glasses, with
x=0,5,10,15,
and 20 mol%, are shown
in Figure 1. The  vibrational modes of the borate network are seen
to be mainly
active in three infrared regions which are similar to those reported earlier
[20, 21]. The group of
bands that occur at 1200–1600 cm−1  is due
to the asymmetric stretching relaxation of the B–O band of trigonal BO3 units. The second group of band lies between 800–1200 cm−1  and is due
to the B–O bond stretching of the tetrahedral BO4 units.
The third group of absorption bands is observed around 700 cm−1 and
is due to bending of B–O–B linkages in the borate network. Similar results have
been reported for V2O5-B2O3 glasses
[22, 23].Figure 1
IR transmission spectra ofxFe2O3-(40-x) B2O3-60V2O5 glasses.The absorption
peaks assigned in IR spectra of glasses under study are listed in Table1.
The
absorption peak observed in all glass samples at 3204–3217 cm−1 is attributed
to hydroxol or water groups
[24] and it is due to hygroscopic
nature of glass samples [25]. The peaks around 2270–2286 cm−1 and 2342–2345 cm−1  are attributed
to –OH group
[26]. The
absorption peak around 1722–1730 is due to
H–O–H bending [22]. In the present glass system, the absence of peak at 806 cm−1 indicates the absence of boroxol ring formation
[12], which suggests that the glass
system under investigation consists
of randomly connected BO3 and BO4  groups. The absorption
peaks at 1192, 1445, 1537, and 1549 cm−1
[27, 28] are related
with the fundamental asymmetrical stretching vibration of the B–O bond of the
trigonal BO3 units. The absorption bands observed at 814, 883, and
1023 cm−1
[20, 29] are assigned to the stretching vibrations of B–O
bond of tetrahedral BO4 units and are shifted toward lower wave
numbers with increasing iron content. It is also observed
(Figure 1) that on
increasing Fe2O3 content, the absorption band at around 1445 cm−1 shifts toward lower wave number (1445–1440 cm−1)
with noticeable decrease in intensity. In the present glass system, the shift
of the vibrational band from higher to lower wave number is ascribed to the
increase in the bond length of B–O groups and the formation of BO4 units. The decrease in intensity of vibrational bands (for x≤10 mol%) at  1445 and 1192 cm−1 and clear
appearance of band in the lower region at 814 cm−1
suggest the formation of NBOs. A sharp absorption
band observed at around 1192 cm−1 may be attributed to triangular
B–O stretching vibrations of BO3 units
[27]. On increasing the Fe2O3 contents, the frequency of this band remains almost the same, revealing the strong appearance of
the triangular borate units (BO3). The absorption band observed at
621 cm−1 is due to the bending of O–B–O linkage
[30]. The shifting of
this band toward higher wave number (621–646 cm−1)
with increasing Fe2O3 content indicates the formation of
FeO4  group
[30]. This possibility is more in the borate glasses in
which boroxol rings are absent. The absorption peak observed at 704 cm−1 is assigned to B–O–B bending vibration in borate network
[31]. At low
concentration of Fe2O3, a weak absorption band observed
at 814 cm−1 originates from stretching vibration of V–O–V bridges
[32]
and becomes slight intense as the concentration of Fe2O3 increases. The weak absorption band appearing in all the samples at
≈883 cm−1 is due to V2O5 content. The shoulder at 1023 cm−1 can be referred to stretching
vibrations of BO4 tetrahedra and is also due to higher content of V2O5 in the glass system. It has been reported that the V2O5 structure is built up by deformed VO5 trigonal bonded zigzag chains.
Each VO5 group contains a short V=O bond (Vanadyl group), which
shows its characteristic vibration band at around 1023 cm−1
[33].
The appearance of this frequency band at 1020 cm−1 at x=10 (mol%) and the clear separation
of the peak at 814 cm−1 on increasing Fe2O3 contents suggest
that this absorption is not only due to the presence of non-bridging V–O−1 and V=O bond, but also due to the presence of BO4 tetrahedra.
Similar results have also been observed in potasium-boro-vanadate-iron glasses
[34]. From Figure 1, it can be clearly inferred that a weak absorption band corresponding
to 814 cm−1 appears for low concentration of Fe2O3. This
peak, however, disappears
as the concentration of Fe2O3 increases (x=5 mol%) and with further increase
in Fe2O3 content (x=10 mol%), the increase in intensity of this peak indicates that the Fe2O3 acts as a glass modifier. The intensity of BO3 structural units
decreases on further increases of Fe2O3 contents, and clear
appearance of peaks (≈814 cm−1)
in the lower region suggests
that some BO3 structural units are converted into some BO4 tetrahedral units, which results in the formation of NBOs. On further addition
of Fe2O3 contents (15–20 mol%), the absorption
bands observed in the lower region, that is, at 1190, 881, 814, 706, and 646 cm−1
remain almost
unaffected, however, their intensities changes which indicate that  Fe2O3 also acts as a
glass former. These observations suggest that at low concentration (x≤10 mol%), Fe2O3 acts as a glass modifier and at high concentration (x=15–20 mol%), Fe2O3 acts as a glass former. Therefore, in the present glass system, Fe2O3 acts both, as a glass modifier as well as a glass former. The results
obtained are well consistent with already reported result [26].Table 1
Peak frequencies from IR spectra forxFe2O3-(40-x) B2O3-60V2O5 glass
system.
xPeak position(cm−1)(mol%)O–B–OB–O–BBO4BO3OH−1 group06217048148831023119214451537154917272270234232045635705—882102111911442——173022732345321410643706814881102011911440—1548172222702344321715646706814881—11901441——172722822346320520645705815880—11911440——1728228623453208Table 2
Electrical conductivity at
different temperatures and the prefactor of Mott-Austin formula
ofxFe2O3-(40-x) B2O3-60V2O5 glasses.
x (mol%)σ320K(Ωm)−1σ400K(Ωm)−1σ500K(Ωm)−1logσ000.313×10−45.435×10−47.945×10−32.67351.600×10−41.783×10−31.987×10−22.793105.817×10−47.705×10−34.339×10−21.750152.680×10−31.794×10−28.853×10−21.581208.220×10−34.194×10−214.73×10−21.450


## 3.2. Density and Molar Volume

The determined
values of density “d” and molar
volume “VM” of the glass
samples are presented in Table 3. In general, the density of glass system is
explained in terms of a competition between the masses and sizes of the various
structural units present in glass. In other words, the density is related to
how tightly the ions and ionic groups are packed together in the structure. The
variation of density as a function of glass composition (x) is shown in
Figure 2. It is observed that the density increases
gradually with the increase in Fe2O3 content in the
present glass system. The relationship between density and glass composition (x) can be explained in terms of an
apparent volume VM occupied by 1 gm atom of oxygen. The value of VM has been calculated from the density and composition
using the formula reported earlier [19], and its composition dependence is
shown in Figure 2. It is observed that molar volume decreases monotonically
with the increase of Fe2O3 content which indicates that
the topology of the network is significantly changed with composition. On the
other hand, these trends can be explained rather simply due to the replacement
of a lighter cation (B) by a heavier one (Fe). As observed in IR spectra, the
addition of Fe2O3 causes increase in NBOs, which in turn
randomizes the structure
and, therefore, glass structure becomes relatively more open. These results are
found consistent with the results reported earlier [35, 36].Table 3
Chemical composition and physical properties ofxFe2O3-(40-x)
B2O3-60V2O5 glasses.
x (mol%)Molar volumeVM(cm3/mol)Densityd(g/cm3)W (eV)048.042.850.427547.462.980.3611045.473.210.3291544.643.370.2662044.283.500.220Figure 2
Variation of density and
molar volume with Fe2O3 content.


## 3.3. DC Conductivity

The temperature
dependence  of dc conductivity “σ” for the different glass compositions
is shown in Figure 3. It
is  observed that conductivity increases smoothly with increasing temperature,
indicating temperature dependence activation energy “W” which is characteristic of small polaron hopping (SPH) conduction
mechanism in TMO glasses [37]. As shown in
Figure 3, the logarithmic conductivity
in the temperature range (310–500 K) exhibits a
linear dependence on reciprocal temperature. The composition dependence of dc
conductivity at particular temperature (Figure 4) indicates that the
conductivity increases with increasing Fe2O3 content. The
activation energy calculated from the slop of the graphs
(Figure 3) is listed
in Table 3. It is clear from
Figure 5 that the activation energy decreases with
increasing Fe2O3 content. The composition dependence of
dc conductivity at 400 K (Figure 4) and activation energy
(Figure 5) indicate that the
variation of “σ” as well as “W" with
composition is much faster for the present glass system than those for the
traditional vandate glasses. It is observed that as the concentration of Fe2O3 increases, the activation of electrical conduction decreases and electrical
conductivity increases. The low value of activation energy and high value of
electrical conductivity are similar to those of
V2O5-BaO-B2O3 glasses
[38]. The change in conductivity and activation energy may help to
detect the structural changes as a consequence of increasing of Fe2O3 content and decreasing of boron oxide content. Generally, it is known that
addition of Fe2O3 in borate glasses increases the
conductivity as result of increasing of NBO ions [39]. It is clear from
Figures
4 and 5 that the magnitude of conductivity is higher for those compositions which
have lower activation energy. This result is in consistent with the small
polaron hopping theory [40]. According to this theory, the conduction process
at higher temperature is considered in terms of optical phonon assisted hopping
of small polaron between localized states. The dc conductivity in adiabatic region
is given by (2)σ=σ0exp(−WkT), where σ0 is
a pre-exponentional factor, W is the
activation energy, k is Boltzmann constant, and T is temperature in Kelvin. The
values of log σ0 were determined from the intercept of the
conductivity versus temperature curve (Figure 3). The observed values of log σ0 are found to be independent of Fe2O3 content
(Figure
5) which confirms the adiabatic SPH for the present glass system. The present
glass system consists
of two types of TMIs, that is, Fe and V. The density “n” of respective ions was calculated (in order to confirm the
relation between the activation energy W and mean distance R) using the formula
[14]: (3)n=2[(dWtMw)NA], where d,NA,Wt,  and Mw are the measured density
of the sample, Avogadro’s number, weight fraction, and molecular weight of the
respective ions (Fe & V), respectively. The calculated values of iron and vanadium
ions are listed in Table 4. The average distance “R” between the transition ions (assuming homogenous distribution of
transition ion in glass volume) was calculated as follows: (4)R=(1N)1/3, where “N” is the concentration of total TMIs (nFe2O3+nV2O5).Table 4
Hopping parameters ofxFe2O3-(40-x)
B2O3-60V2O5 glasses.
x (mol%)nFe2O3(cm−3)nV2O5(cm−3)N(cm−3)R(nm)rp(nm)00×10230.1131×10230.1131×10230.32120.17950.0112×10230.1183×10230.129×10230.30740.171100.024×10230.1274×10230.151×10230.29170.163150.038×10230.1337×10230.171×10230.27990.156200.052×10230.1389×10230.191×10230.26970.150Figure 3
Variation of logσ versus
1000/T(K−1) for studied glasses.Figure 4
Effect of Fe2O3 content
on dc conductivity at different temperatures.Figure 5
Effect of Fe2O3 content on activation energy
W and
pre-exponential factor log σ0.The small
polaron radii “rp” for  the
iron and vanadium ions have been calculated using [41] (5)rp=12(π6N)1/3.  The calculated values of
N,R, and rp are tabulated in
Table 4. The variation of activation
energy W  with average distance R, as shown in
Figure 6, suggests that
there is a prominent positive correlation between activation energy and average
site separation. It is evident from Figure 7 that the small polaron (SP) radius
decreases with the increase in Fe-ion concentration. From the above
observations, it is concluded that on addition of Fe2O3,
conductivity increases, while the activation energy decreases due to the decrease
in  respective  ion separation. These results are in good agreement with those reported
by El-Desoky  [36]. However, in the present investigation for sample x=10 mol% of Fe2O3,
there are some
minor changes observed in the density and electrical parameters as compared to
our earlier investigation [42]. These changes may be attributed to difference
in sample preparation conditions [11].Figure 6
Variation of activation
energyW with average ion separation R.Figure 7
Variation of small
polaron radius with iron content.


## 4. Conclusion

The structural
and physical properties ofxFe2O3-(40-x) B2O3-60V2O5 glass system, with 0≤x≤20 (mol%), have been studied. No boroxol ring formation
was observed in the structure of these glasses which suggest that glasses under
study consist of randomly connected BO3 and BO4 groups. Addition
of Fe2O3 produces NBOs in borate as well as in Vandate
glass network along with the formation of VO4 groups. The IR spectra
of the present glass system indicate
that Fe2O3 acts both, as a glass modifier
(x≤10 mol%) as well as a glass former (x=15–20 mol%). The
density of all the glass samples increases with Fe2O3 content as a result of the increase in NBOs due to the conversion of trigonal
BO3 structural units into BO4 tetrahedral unit. In the
temperature range 310–500 K, the
variation of log σ with T−1  is approximately linear. It was observed
that the dc conductivity increases with increasing iron content,
and ranges from
5.435×10−4 to  4.194×10−2(Ωm)−1 at 400 K. The
conduction in the present glass system was confirmed to be a result of primarily adiabatic
hopping of small polarons between TMIs.



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*Source: 937054-2008-12-23.xml*