Structural impact of cobalt ions on BaO-Bi2O3-B2O3 glass system by means

of spectroscopic and dielectric studies

6.1. Introduction

Cobalt ions are expected to have profound influence on the physical properties

of barium bismuth borate glasses, as they exist in different valance states viz., Co2+

,

Co3+

and Co4+

states and participate in the glass network with different structural units

and sizes as given in the following table 6.1 [1].

The glasses and glass ceramics containing cobalt ions are good candidates for nonlinear

optical absorbers, needed for passive modulations of laser beams [2]. These materials

are also considered as useful materials for Q-switching devices, in view of the fact that

they exhibit strong luminescence in the visible and near-infrared regions. In general

CoO, an anti-ferromagnetic material, is more stable in the natural environment. When it

is incorporated in a heavy metal oxide system, like barium bismuth borate (BaBiBO4)

glasses it improves the sintering behavior of the system which could have wide range

ION Charge Coordination Spin State Ionic Radius(Å)

IV High Spin 0.58

V 0.67

High Spin 0.745 VI

Low Spin 0.65

2

VIII 0.9

Low Spin 0.545 3 VI

High Spin 0.61

IV 0.4

Co

4 VI High Spin 0.53

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of applications in various industrial sectors. The glass network containing interstices of

varying diameters such as various ions of different sizes are more easily accommodated

in the sites [2]. The concentration of different structural groups of cobalt ions with

different oxidation states present in the glass matrix at a given temperature depends on

the quantitative properties of modifiers, glass formers, size of ions in the glass

structure, mobility of the modifier cation etc. The structural groups of cobalt oxide are

expected to alternate with BiO3, BiO6, BO3 and BO4 structural groups and are likely to

influence the physical properties of BaBiBO4 glasses to a large extent. The presence of

alkaline earth oxide, like BaO between the long chain molecules in the vicinity of

cobalt ions in the bismuth borate network, the symmetry and/or covalency of the glass

at the Co2+

ions will be varied. Additionally, the variations in the oxidation states of

bismuth ions are also expected to modify the crystal field around cobalt ions in the host

glass network. As a result the structural impact of cobalt ions on BaBiBO4 glass

network is expected to be highly interesting and useful for the practical applications of

these glasses.

The objective of the present study is to investigate the structural changes that

take place with the varied oxidation states of cobalt ions in BaO–Bi2O3–B2O3 glass

network by a systematic study on spectroscopic properties (optical absorption, IR and

luminescence spectral studies) and dielectric properties (viz., dielectric constant, loss

and ac conductivity over a wide range of frequency and temperature) after

characterizing the samples XRD studies. Five glasses in the quaternary system 10BaO-

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20Bi2O3-(70-x)B2O3 : x CoO with the values of x ranging from 0 to 2 wt% (in steps of

0.5 wt %) are synthesized. The details of the composition are as follows.

C0: 10BaO-20Bi2O3-70B2O3

C5: 10BaO-20Bi2O3-69.5B2O3-0.5CoO

C10:10BaO-20Bi2O3-69.0B2O3-1.0CoO

C15:10BaO-20Bi2O3-68.5B2O3-1.5CoO

C20:10BaO-20Bi2O3-68.0B2O3-2.0CoO

The methods of preparation of the samples and the techniques adapted for recording X-

ray diffraction (XRD) pattern, optical absorption, FT-IR, luminescence spectra etc., and

for measuring dielectric properties are same as that reported in our earlier papers [3].��

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Fig 6.1 Physical appearance of the BaBiO4:CoO glass samples of 0.5 mm thickness.

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6.2. Brief review of the previous work on the glasses containing cobalt ions

Das et al [4] systematically explored the compositional dependence of the

electrical conductivity and the structure of silver phosphate glass upon doping with

different proportions in the composition (50-x)Ag2O-50P2O5-xCoCl2 where x=1, 5, 10

and 15 mol%. They observed that these glasses are essentially ionic conductors and the

electrical conductivity of silver phosphate glasses doped with 5 mol% of CoCl2 is

found to be the maximum. Besides they also found that with increasing amount of

CoCl2, cross link density is enhanced with the formation of P-O-Co bonds. Nagaraja et

al [5] studied the electrical conductivity properties of two sets of alkali doped cobalt-

borate glasses of composition (Li2O)x-(CoO)0.2-(B2O3)0.8-x and (Li2O)0.2-(K2O)x-

(CoO)0.2-(B2O3)0.6-x. They found that in single alkali doped glasses the conduction

mechanism predominantly changed from electronic to ionic at 0.4 mol% of Li2O and in

mixed alkali glasses mixed alkali effect is found at 0.2 mol% of Li2O. Moreover, they

analyzed the data in the light of Motts small polaron Hopping (SPH) model. Volk et al

[6] analyzed the simulated emission (SE) measurements of cobalt doped alumnosilicate

glass-ceramics in the spectral range of 600-700 nm and they compared SE properties of

Co2+

ions in these spinel-based glass-ceramics and in spinel single crystals. Oliva et al

[7] prepared and compared two samples of composition La0.9M0.1CoO3 (M=Sr, Ce) as

catalysts for the flameless of spin glass in La0.9Sr0.1CoO3 glass samples. Satyanarayana

et al [2] studied the principal influence of cobalt ions and their environment

[tetrahedral, octahedral] on PbO-Sb2O3-B2O3:CoO glass ceramics by means of

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spectroscopic and dielectric studies. The analysis of results suggests that with increase

in the concentration of CoO, cobalt ions transformed from tetragonal to octahedral

coordination. Colak [8] et al reported the optical and thermal properties of V2O5 and

CuO doped P2O5-Na2O-CaO-Al2O3:CoO glasses so as to investigate their possible use

in solar collection applications and found that the obtained glasses are promising

materials for solar collector applications. Bae et al [9] objectively studied the effect of

CoO addition on phase separation and crystallization of lithium aluminosilicate glasses

containing titania via Raman scattering spectram, optical absorption spectra and X-ray

diffraction patterns. Thulasiramudu et al [10] brings out the optical characterization of

Co2+

ions doped 15PbO-40B2O3-(45-x)ZnO-xCoO when x=0.2, 0.5 mol% via

absorption and emission studies. Both optical and physical properties found to be more

encouraging towards their use as novel luminescence optical materials. Nagarjuna et al

[11] studied the concentration effect of cobalt ions as structural probe in PbO-As2O3

glass ceramics via optical absorption, IR and photoluminescence. However, very little

literature is available on the spectroscopic and dielectric properties of BaBiBO4 glasses

doped with cobalt ions. The structural and electrical conductivity studies are important

studies for deciding the nature and use of solid glassy electrolytes for better ionic

device applications and fabrication of solid state batteries [12].

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6.3. Results and discussion:

6.3.1. XRD and Physical Parameters:

As shown in the Fig 6.1 the prepared samples were homogeneous and transparent in

nature. Therefore, XRD is used only to identify or confirm the glass formation or

noncrystalinety of the samples. Fig 6.2 shows the XRD patterns of pure and 0.5 wt%

CoO doped glass samples. The X-ray diffraction patterns of the present samples

exhibited broad and diffused humps which confirmed the amorphous character of the

prepared samples and lack of long range order.

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Fig 6.2 XRD patterns of (a) BaBiBO4 and (b) 0.5 wt% CoO doped BaBiBO4 glasses

10 20 30 40 50

(b) C5

(a) C0

Inte

nsi

ty (

arb

.un

its)

2θθθθ

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Table 6.2 The density, molar volume, Co2+ ion concentration Ni, mean Co2+

ion

separation Ri polaron radius Rp and average boron-boron separation and theoretical

optical basicity (�th) of the 10BaO.20Bi2O3.(70-x)B2O3.x CoO glasses.

The measured density value of C0 glass is found to be 4.5331 g/cm3 and is gradually

increased up to 1.5 wt% doping of CoO and then decreased at 2.0 wt% of CoO. A

reversed trend in molar volume and similar trend is found in the calculated Co2+

ion

concentration Ni as presented in Table 6.2.�The observed increase in density by the

replacement of B3+

by Co2+

in the composition is due to replacement of a lighter cation

by heavier one since the density of a glass is very sensitive to the ionic size and atomic

weight [13]. From the Table 6.2 the increase in density followed by a decrease in the

molar volume values up to C15 sample suggests that the glass network become more

Sample

� (g/cm3)

(±0.001)

Vm

(cm3/mol)

(±0.001)

Ni X1021

(ions/cm3)

(±0.01)

Ri (�)

(±0.001)

Rp (�)

(±0.001)

B Bd −

(nm)

(±0.0001)

�th

C0 4.533 30.560 -- -- -- 0.4578 0.4540

C5 4.559 34.500 8.73 4.86 1.96 0.4545 0.4541

C10 4.644 33.874 8.89 4.83 1.95 0.4493 0.4543

C15 4.685 33.583 8.97 4.81 1.94 0.4456 0.4544

C20 4.490 35.048 8.59 4.88 1.97 0.4497 0.4545

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compact. Using density and molar volume values some important physical parameters

like Co2+

ion concentration Ni, mean Co2+

ion separation Ri, polaron radius Rp are

calculated and are given in Table 6.2. From which though the Co2+

ion concentration Ni

is increased upto 1.5 Wt% of CoO and decreased at 2.0 Wt% (it will be explained later

in optical absorption and emission studies). Besides the average boron-boron separation

B Bd − is also evaluated [14], to get more insight into the glass network, using the

following Eq:

1/3B

mB B

A

Vd

N−

� �= � �� �

----- (6.1)

where NA is Avogadro’s number, 6.0228X1023

mol-1

and BmV is the volume containing

one mole of boron atoms in the given glass network. Since boron is central atom of

BO3/2 and 4/2BO− units, B

mV is given by

2(1 )

B mm

B

VV

x=

− ----- (6.2)

Where xB refers to the molar fraction of B2O3. The calculated values of the average

boron-boron separation are given in Table 6.2. Generally BmV depends on the cations

present in the glass network and hence depends on the radius of the modifier ions

present in the system. By the gradual doping of CoO in the glass network at the

expense of B2O3 leads to a substantial decrease in boron-boron separation from 0.4578

nm (C0) to 0.4494 nm (C20).

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6.3.2. Optical Absorption:

The absorption spectra of 0.5 mm thick samples are shown in Fig 6.3. In visible region

an absorption band is noticed at 577 nm along with two shoulders around 500 and

631.5 nm. In addition to this an absorption band in IR region around 1420 nm is also

observed. The peak positions and their corresponding assignments are given in Table

6.3. The peak area, FWHM and the band widths are calculated, by deconvoluting the

absorption bands separately, and presented in Table 6.4. The observed two main bands

and their shoulders are attributed to Co2+

and Co3+

ions separately as follows.

Fig 6.3 Optical absorption spectra of 0.5 mm thick 10BaO.20Bi2O3.(70-x)B2O3.xCoO glasses. Inset shows the absorption in NIR region.

400 600 800

0

1

2

1000 1200 1400 1600 1800 2000

0.10

0.15

0.20

0.4

0.5

0.6

4A

2(4F)

4T

1(4F)

(Td)

C20

C15

C10

C5

C0

Wavelength (nm)

2T

2

2E

4A

2(

4F)

4T

1(

4P)

(Td)

4T

1g(F)

2T

1g(H)

(Oh)

C20

C15

C10

C5

C0

Abso

rbac

ne

(arb

. unit

s)

Wavelength (nm)

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In amorphous and ceramic materials the electronic spectra of Co2+

ions are well

understood. In general the free ion ground state (4F) of Co

2+ (d

7) ions in the presence of

tetrahedral crystal field splits into 4A2,

4T2 and

4T1 levels with

4A2 as the ground state.

Since d3 configuration is conjugate to the (d

7) ion, using Tanabe Sugano diagrams, the

intense signal around 577 nm and its shoulder around 500 nm is attributed to 4A2(

4F)�

4T1(

4P) and

4T1g�

2T1g(H) transitions of tetrahedral and octahedral Co

2+ ions

respectively. In the literature these bands are found around 600 and 523 nm [2, 10, 11]

respectively. As discussed in the previous chapter, section 5.3.4, the observed blue shift

from the literature is due to either decrease of average bond length of Co-O and/or

increase in the radial position of electron cloud [15].

Table 6.3 Cutoff wavelength (�c), Band positions, Optical band gap (Eg), Urbach

energy (�E), and g values of the glasses 10BaO.20Bi2O3.(70-x)B2O3.xCoO.

Band Position (nm)

Co2+

ions Co

3+

ions Sample

�c (nm)

(±0.1)

4T1g(F)�2T2g(H) 4A2(4F)�4T1(4P) 4A2(4F)�4T1(4F) 5T2 �5E

Eg (eV)

(±0.001)

�E (eV)

(±0.0001)

C0 347.5 --- --- --- --- 3.531 0.064

C5 345.5 500.5 577.5 1419.5 631.5 3.545 0.061

C10 353.0 501.5 578.5 1423.5 633.0 3.484 0.113

C15 355.0 501.5 578.5 1422.0 633.0 3.457 0.156

C20 357.0 500.5 577.0 1420.5 630.0 3.427 0.217

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Table 6.4 Some parameters evaluated from optical absorption spectra of BaBiBO4 glasses doped with CoO.

Sample

Name

Peak position

(nm)

Area Peak height

C5 500.5

577.5

631.5

1419.5

20.39

9.74

3.35

14.81

0.36

0.22

0.08

0.02

C10 501.5

578.5

633.0

1423.5

29.55

17.09

9.36

28.85

0.58

0.44

0.23

0.04

C15 501.5

578.5

633.0

1422.0

39.83

24.92

11.12

43.15

0.82

0.63

0.31

0.06

C20 500.5

577.0

630.0

1420.5

44.32

28.96

13.82

29.23

0.94

0.75

0.37

0.04

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In turn it might be due to the presence of high polarizing nature of bismuth ions in the

present glass system. Besides the band in the NIR region around 1420 nm is assigned to

the 4A2(

4F)� 4T1(

4F) transition of Co

2+(IV) ions. In addition, a small shoulder at about

663 nm is found in all CoO doped glasses and it is identified due to the 5T2�

2E

transition of Co3+

ions [2, 10, 11]. From Table 6.4 and inset of Fig 6.3 the area under

the absorption band corresponds to 4A2(

4F)� 4T1(

4F) transition of Co

2+(IV) is gradually

increased upto 1.5 wt% of CoO then it is observe to decrease at 2.0 Wt% of CoO. It is

also supported by decrease in Co2+

ion concentration Ni at 2.0 Wt% of CoO in Table

6.2, calculated from density values.

6.3.3. Optical band gap and Urbach energy:

For cobalt free C0 glass sample the fundamental absorption edge observed at

347.5 nm is found to blue shifted to 345.5 nm with 0.5 wt% doping of CoO to the glass

matrix. With further addition of dopant the absorption edge is gradually red shifted to

357 nm. Figure 6.4 presents the Tauc’s plots correspond to (�h�)1/2 vs h� for the

present glasses, to calculate the significant data of optical band gap Eg [16].The

obtained Eg values, as given in Table 6.3 are measured by extrapolating the linear

region of the graph which will touch the X-axis i.e, (�h�)1/2 =0. The Eg value of C0

glass, 3.531 eV, is increased to 3.545 eV with 0.5 wt% doping of CoO i.e, for C5 glass

sample. With further addition of CoO the band gap is gradually decreased to 3.427 eV.

The increase in bang gap or decrease in cut off wavelength (fundamental absorption

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edge) with initial (0.5 wt%) doping of CoO is due to the formation of some tetrahedral

Co2+

ions in

�

Fig 6.4 (�h�)1/2

vs h� plots to determine optical band gaps of the BaBiBO4:Co glasses

the glass matrix, which strengthens the network by decreasing non bridging oxygens

(NBOs). However, the decrease of optical band gap with further addition of dopant is

an indication of the formation of NBOs in the glass matrix.

As given Table 6.3 the Urbach energy (E) values of the present samples are

determined by taking the reciprocals of the slopes of the linear portion of the ln(�) vs

3.25 3.30 3.35 3.40 3.45 3.50 3.55 3.60

0

2

4

6

8

10

12

14

16

18

20

22

24

C20

C15

C10

C5

C0

(αh

ν)1

/2 (

cm-1eV

)1/2

Photon energy (eV)

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h� plots as shown in Fig 6.5, moreover the inset of which shows the variation optical

band gap and Urbach energy with respect to dopant concentration in the glass matrix.

Fig 6.5 Urbach plots between ln(�) vs photon energy for BaBiBO4:Co glasses. Inset

shows the variation of Eg and �E with respect to dopant concentration.

In the present investigation, in reminiscent with optical band gap, the E value

is decreased with initial doping of CoO than the pure value and then it is increased with

increase of dopant concentration. Among the present investigated samples E is

minimum (0.061 eV) for C5 and maximum (0.217 eV) for C20 glass sample. It

3.25 3.30 3.35 3.40 3.45 3.50 3.55 3.60

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0.0 0.5 1.0 1.5 2.03.40

3.42

3.44

3.46

3.48

3.50

3.52

3.54

3.56

wt% of CoO

Eg (e

V)

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

∆E (eV

)

C20

C15

C10 C0 C5

Photon energy (eV)

ln (

α)

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indicates the C5 has least width of tails among present investigated samples. The

formation of defects, like wrong bonds and fluctuations in bond angle distortions are

the reasons for the increase of Urbach energy from C10 to C20 [17]. Besides the

additional factor contributing to edge broadening is static disorder which increases the

density of localized states N(EF) of these states.

6.3.4. Theoretical Optical basicity (�th) :

From Table 6.2, the value of theoretical optical basicity (�th) is increased

with gradual doping of CoO in the glass matrix. As the �th of a chemically complex

system is the mean polarization of the ligands [anions, O2-

] and their mean ability to

transfer fractional charges to the central cation [18], the increase in �th suggests that an

increase of localized donor pressure on cations of glass matrix. In other words, the

covalence of the glass network decreases. Besides depending on the role of network

formers and modifiers, optical basicity will change. The network formers interact

covalently with oxygen, while the modifiers are the elements that interact ionically.

Moreover, Duffy et al [19] concluded that the polarizability of oxygen ions is directly

proportional to the optical basicity. The increase in the polarizability of oxygen ions is

another indication that the concentration of NBOs increases [20].

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6.3.5. Photoluminescence:

The photoluminescence spectra of the present investigated samples are given in Fig 6.6.

From the figure C0 sample showed a red luminescence with intense centered at 622 nm

along with a small kink at 706 nm, when excited at 532 nm CW laser of 1 W power in

the range 550-900 nm. The intensity of emission peak at 622 nm is gradually decreased

upto 1.5 wt% doping of CoO and then surprisingly it is increased at 2.0 wt% of dopant.

As discussed in earlier chapters III and IV the observed red luminescence is ascribed to

the Bi2+

ions because such similar observation is reported by Qui et al [21].

Fig 6.6 Visible Luminescence spectra of BaBiBO4:CoO glasses, excited at 532 nm

550 600 650 700 750 800 850 900

0

2000

4000

6000

8000

10000

706

622

C15C10

C5

C20

C0λ

exc = 532 nm

PL

Inte

nsi

ty (

arb. unit

s)

Wavelength (nm)

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Bi2+

activated SrB4O7, M2+

SO4 and M2+

PO5 (M= Ba2+

, Sr2+

and Ca2+

) glasses exhibited

luminescence maximum in the wavelength region 585 to 638 nm when such glasses

were excited at wavelength range 413 and 622 nm [22-24]. The detailed mechanism for

such red luminescence was investigated [22-24] and is assigned to electronic transition

between the first excited level 2P3/2 and the ground level

2P1/2 of Bi

2+ [25]. The

observed red luminescence in CoO free sample, therefore, is clearly due to 2P3/2�

2P1/2

transition of Bi2+

ions. However, the mechanism for such red emission in doped glasses

has not clear specified in the earlier reported [2, 10, 11] in which such red

luminescence at 660 nm was assigned to 4T1(

4P) �4A2(

4F) transition of Co

2+ ions. But

the presence of similar red emission band in pure glass and decrease of luminescence

intensity with doping of CoO is clearly suggested that such emission in the present

glasses is most likely originated from 2P3/2 �

2P1/2 transition of Bi

2+ ions. The

quenching of luminescence intensity up to 1.5 Wt% of CoO is either due to decrease of

Bi2+

ions in the glass matrix and/or may due to non-radiative relaxation processes.

At higher concentration of CoO, by correlating decrease in Co2+

ion

concentration Ni calculated from density value and decrease of optical absorption area

corresponds to 4A2(

4F)� 4T1(

4F) transition of Co

2+(IV) ions with increase in intensity

of emission spectrum corresponds to 2P3/2 �

2P1/2 transition of Bi

2+ ions: one could

conclude that, as Bi2O3 concentration is fixed in all glasses, such increase in Bi2+

luminescent centers is due to reduction of Bi3+

ions with Co2+

ions as redox partners

(Bi3+

+Co2+�Bi2++Co3+). As ionic size of Bi2+ is higher than Bi3+, at higher

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1600 1400 1200 1000 800 600 400

560643

758

15011384

12251070

943

689495

C20

C15

C10

C5

C0

% o

f T

ransm

itta

nce

(ar

b. unit

s)

Wavenumber cm-1

concentration of CoO molar volume is increased with decrease in density and Co2+

ions

concentration.

Fig 6.7 Reduction of Bi3+

ions with Co2+

ions as redox partners in C20 glass matrix.

6.3.5. FTIR:

Fig 6.8 Presents the FTIR transmission spectra of all the present samples.

Fig 6.8 FTIR spectra of BaBiBO4:Co glass samples

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The band positions and their corresponding assignments are given in Table 6.5. Due to

convolution of individual bands with one another, broad bands are observed in the

above Fig 6.8.

Table 6.5 Assignment of absorption bands in the infrared spectra (with a probable

error of ±0.1cm-1

) of the glasses 10BaO.20Bi2O3.(70-x)B2O3.xCoO.

C0 C5 C10 C15 C20 Assignment

495 489 480 478 476 A doubly degenerate vibrations of Bi-O

bonds BiO6 octahedral units

-- 560 552 550 550

-- 643 657 -- -- Specific vibrations of Co-O bond.

684 687 683 683 681 B-O-B bend

-- 758 -- -- -- O3B-O-BO4 bend

943 909 930 932 959 B-O stretch in BO4 units from diborate

groups and/or stretching vibrations of Bi-

O bonds in BiO6 units

1070 1051 1029 1027 1023 Stretching vibrations of B-O bonds in BO4

units from tri, tetra and penta borate

groups.

1225 1274 1232 1224 1230 B-O sym stretch in BO3 units from pyro and

ortho borate groups

1384 1371 1348 1373 1380 B-O sym stretch in BO3 units from varied

types of borate groups

1501 -- 1464 1456 1490 B-O- stretch in BO2O

- units from varied

types of borate groups

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BaBiBO4 glass showed three principle bands at 1384, 943 and 684 cm-1

with three

shoulders around 1501, 1225 and 1070 cm-1

along with a weak band at about 495 cm-1

.

With 0.5 Wt% doping of CoO three new bands are found at 758, 643 and 560 cm-1

, in

which former band at 758 cm-1

is not observed for higher concentration of CoO doped

glasses samples. Interestingly the band at 643 cm-1

is blue shifted and merged with the

band at around 684 cm-1

while the band at 560 cm-1

is red shifted to 550 cm-1

. In

addition the band at 687 cm-1

is red shifted to 681 cm-1

with simultaneous blue shift of

the band at 909 cm-1

to 959 cm-1

form C5 to C20.

As discussed in previous chapters the above observations are interpreted in the

light of bismuth borate glasses. The observed broad bands in the present investigation

in the regions from 1500 – 1100 cm-1

and from 1100 – 800 cm-1

are assigned to the

stretching vibrations of both triangular BO3 and tetrahedral BO4 units respectively. The

band at 684 cm-1

is attributed to B-O-B bending vibration and a feeble band around 495

cm-1

is ascribed to degenerate vibrational bands of Bi-O bonds from BiO6 units. Among

the observed three new bands at about 758, 643 and 560 cm-1

, with the initial 0.5 wt%

doping of CoO, the former one is assigned to bending vibrations of O3B-O-BO4 bonds

and the later two are due to specific vibrations of Co-O bonds [26, 27]. In compliance

with increase in optical band gap the observed disappearance of the band around 1501

cm-1

corresponds to B-O- stretch in BO2O

- units from varied types of borate groups

indicates the decrease of NBO’s in C5 glass matrix. It might be due to formation of

CoO4 in the glass network at initial concentration of dopant. Interestingly, with further

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addition of CoO at the expense of B2O3, the bridging band corresponds to O3B-O-BO4

is not found simultaneously the band intensity corresponds to BO3 and BO2O- units is

further increased. Meanwhile the intensity of the band around 938 cm-1

is decreased,

which may due to conversion of BO4 into BO3 at higher concentration of dopant by

forming and/or increasing NBO’s in the glass matrix. The blue shift and merge of the

band at 643 cm-1

with the band corresponds to B-O-B bend can be explained by

considering the blue shift of the band at 909 cm-1

to 959 cm-1

form (C5 to C20). This

type of band shifting and merging is due to the formation of new bridging bonds, like

Bi-O-B and Co-O-B [28, 29]. This new bonds might have been formed due to the

inducement of the electric field of the high polarizing nature of Bi3+

ions. As stretching

force constant of Bi-O and Co-O bonds is differed from that of B-O bond, the effective

force constant of Bi-O-B and Co-O-B bonds is changed. The increase in band intensity,

at higher concentration of dopant, at 495 cm-1

could be reasonably attributed to the

formation or gradual increase of BiO6 units. In compliance with the structural changes

inferred from IR studies, the observed decrease in band gap and increase in E [Table

6.3] induced by the doping of CoO at higher concentrations in BaBiBO4 glass matrix

might be explained by the increase of BiO6 and BO3 structural units along with the

increase of NBOs (with increase of B-O- bonds).

6.3.6. Dielectric studies:

The temperature dependence of dielectric constant ε� in the temperature range 30-400

0C at 500 KHz is shown in Fig 6.9.

�

�

��������

0 100 200 300 400

0

10

20

30

40

500 KHz

0 100 200 300 4000

10

20

30

40

50 C5

500kHz

100kHz

10kHz

ε'

Temperature (oC)

C20

C15

C10

C5

C0

ε'

Temperature (oC)

Fig 6.9 Variation of dielectric constant ε' with temperature at 500 KHz for BaBiBO4

glasses doped with different concentrations of CoO. Inset shows the variation of ε' with

temperature for different frequencies of C5 sample.

Though ε� is increased considerably in the entire temperature range but its rate of

increase is very small in low temperature region. Inset of Fig 6.9 shows the variation of

ε� with temperature at different frequencies for C5 sample. From which it is observed

that the dielectric constant is decreased with increase in frequency. C0 sample has

shown the dielectric constant ε�, 4.5, at 500 KHz and 30 oC. By doping 0.5 wt% of CoO

to C0 glass matrix its value is decreased except in relaxation temperature region. A

�

�

��������

maximum ε� 42.8, is observed for C20 glass at 400 oC and 10 KHz. All investigated

samples have been exhibited very broad dielectric dispersion. Cobalt free C0 sample

showed a small dielectric dispersion at 220 oC at 10 KHz and is shifted to lower

temperature region by the doping of CoO. From the inset of Fig 6.9, the dispersion is

shifted to higher temperature region with increase in frequency. Fig 6.10, shows the

variation of dielectric loss (tanδ) with temperature for BaBiBO4:CoO glasses at 100

KHz. And its inset represents the loss (tanδ) of C15 sample, with temperature at

different frequencies. Moreover the loss maxima is shifted to lower temperature side

and broadened with the increase of CoO doping. Using the following equation 6.3 the

effective activation energy of dipoles (Wd) of the present glass samples have been

calculated [30] from the following relation and presented in Table 6.6.

� � � 0 expdWf f

KT

−� �=

� �������������������������������������----- (6.3)�

Where f is the frequency and f0 is a constant. At 10 kHz, the Wd (4.831 eV) for C0

sample is increased with 0.5 Wt% doping of CoO and thereafter it is decreased to 4.727

eV at higher concentrations. The a.c electrical conductivity (σac) of as prepared glasses

has been evaluated [30] by following Eq.

0' tanacσ ωε ε δ= ----- (6.4)

where all the terms have their usual meaning. The variation of log(σac) with respect to

1000/T at different frequencies 10, 100 and 500 KHz for C20 glass is shown in Fig

�

�

��������

0 100 200 300 400

0.0

0.1

0.2

0.3

0.4

0.5

0 100 200 300 4000.0

0.1

0.2

0.3

0.4

0.5C15

500kHz

100kHz

10kHz

tan

δ

Temperature (oC)

100 KHz

C20

C15

C10

C5

C0

tan

δ

Temperature (oC)

Table 6.6 Summary of the data on dielectric loss of BaO-Bi2O3-B2O3 : Fe2O3 glasses at10 kHz

�

�

�

�

�

�

�

�

Fig 6.10 Variation of dielectric loss Tan� with temperature at 100 KHz for BaBiBO4:Co glasses. Inset shows the variation of Tan�

with temperature at different

frequencies of C15 sample.

Glass (Tan)max Temp. region of relaxation (±1)

oC

AE for dipoles

(±0.01) eV

C0 0.1032 200-245 4.831

C5 0.0842 170-285 5.326

C10 0.1195 155-280 5.122

C15 0.1355 145-275 4.922

C20 0.1485 130-270 4.727

�

�

��������

1.5 2.0 2.5 3.0 3.5

1E-6

1E-5

1E-4

1.5 2.0 2.5 3.0 3.51E-7

1E-6

1E-5

10 kHz

C0

C20C15C10

C5

σac

(Ω-c

m)-

1

103/T(K)

-1

C20

500kHz

100kHz

10kHz

σac

(Ω-c

m)-

1

103/T(K)

-1

6.11 and its inset is shown the log(σac) dependence on 1000/T at 10 KHz of all the

present glass samples.

Fig 6.11 Variation of �ac with 1000/T for different frequencies of C20 sample. Inset

shows the variation of �ac with 1000/T at 10 KHz for BaBiBO4:Co glasses.

In the higher temperature region (the linier region) of log(σac) vs 1000/T plots the

values of activation energy for conduction are evaluated, as reported in the literature

[30, 31]. The values of activation energy for conduction at 100 KHz are given in Table

6.7 and is highest 0.610 eV for C5 sample. As given in Table 6.7 the activation energy

�

�

��������

for conduction is increased with 0.5 Wt% of doping of CoO and decreased beyond that

concentration.

Table 6.7 Summary of the data on ac. conductivity of BaO-Bi2O3-B2O3 : CoO glasses at 100 kHz

N(EF) (1021

eV-1

/cm3) Glass � ac at 70

0C

(10-6

)

(�-cm) -1 Austin Butcher Pollak

Activation

energy for

conduction

(eV)

C0 1.77 2.86 1.19 2.91 0. 577

C5 0.99 2.14 0.89 2.17 0. 610

C10 2.80 3.59 1.50 3.65 0.546

C15 3.70 4.13 1.72 4.20 0.519

C20 4.71 4.66 1.94 4.74 0.494

�

� By the initial doping of CoO to the host the glass matrix, the values of ε�, loss

(tanδ) and σac are found to decrease at any temperature (except in the relaxation region,

which is explained later) and frequency. Beyond that concentration all the parameters

are gradually increased at any any temperature and frequency. Generally for amorphous

materials like glasses space charge polarization is responsible for the increase of

dielectric constant with increase in temperature at any frequency. The decrease in

dielectric parameters at initial concentration (except in the relaxation temperature

region in the case of � might be due to the formation of CoO4 tetrahedral units in the

glass matrix. At higher concentrations, the modifying ions in the present study Ba2+

,

Bi3+

, Co2+

and Co3+

units with octahedral coordinations generate bonding defects

�

�

��������

(dangling bonds), structural defects (disorder in ideal random structure), donor and

ionized donor pairs (like Bi3+

-Bi5+

). These defects create easy path way for the

migration of charges and build up space-charge polarization and facilitate an increase

in the dielectric parameters [30]. Furthermore the activation energy for conduction is

observed to decrease from C5 to C20. These results are indication of increase in the

space charge polarization, which in turn caused by the increase in modifying nature of

Co2+

and Co3+

ions with CoO6 octahedral units. In CoO free glasses the observed

dielectric relaxation and its shifting with frequency suggests existence of dipoles in

these glasses. These dipoles may be formed by the association of reduced Bi3+

radicals

with their cationic vacancies in BaO mixed glasses. Further broadly spreading of

relaxation region with CoO doping is due to formation of new dipoles between and

electron which is localized (or mainly so) in a ligand orbital to a positive central metal

ion. The oscillating electric dipole moment associated with electron transfer over such

a distance is quite large and transitions are, therefore intense, having broad relaxations

in the glass matrix [32].

The density of energy states N(EF) near the Fermi level, has been evaluated

using the following equation and presented in Table 6.6.

4

2 2 5( ) [ ( )] lnph

e FKT N Eν

σ ω η α ωω

−� � �

= � �� �� �� �

----- (6.5)

�

�

�������

where for Austin = /3, Butcher = 3.66 2/6 and Pollack = 4/96 with the usual

meaning of remaining symbols reported [33]. From the Table 6.6 with initial doping of

cobalt the N(EF) values are decreased and increased beyond that concentration. It

suggests that, at higher concentrations of CoO, the disorder and hence modifying nature

in the glass network increases. Therefore, the number of localized states near Fermi

level is found to increase and is supported by the increase of Urbach energy �E values

at higher concentrations of dopant CoO in the glass matrix (Table 6.3).

�

�

�������

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�

�

��������

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�

�

��������

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