Structural changes in the ZnF2 – Bi2O3 – GeO2 glass system

doped with Fe2O3 by spectroscopic and dielectric

investigations

4.1 Introduction:

Glasses doped with transition metal ion, Fe2O3 are used in

electrochemical, electronic and electro-optic devices [1]. Sound investigations

were carried out previously the environment of iron ion in various inorganic

glass systems viz; silicate, borate, phosphate and telluride glasses [2-5]. Iron

ions are considered as effective and useful dopant ions because of the fact that

they exist simultaneously in different valence states in the glass network as

Fe3+ with both tetrahedral and octahedral and as Fe2+ octahedral

coordination[6]. Fe3+ and Fe2+ both are paramagnetic ions; Fe2+ ions posses a

large magnetic anisotropy due to its spin-orbit interaction of the 3d orbital

where as such anisotropy of Fe3+ ions is small since the anisotropic energy of

Fe3+ ions is small as the angular momentum of these ions is zero[7]. Several

studies were reported on the valence states and influence of iron ion on the

physical and electrical properties of different glass systems doped with Fe2O3

transition metal oxide[8-10] but most of them are related to borate, phosphate

and silicate glass formers, few studies are found on germanate glasses.

Germanate glasses mixed with heavy metal oxides are very promising

materials for IR transmitting windows [11], non-linear optics, and laser devises

122

[12]. For the present investigation germinate glasses mixed with heavy metal

oxide are chosen due to their remarkable optical properties like high refractive

index, dispersion and transmission in the IR region [13, 14] germanium is

imparted to the present investigation. Glasses containing heavy metal oxides

(HMO) are found to be excellent candidates for IR transmission compared with

conventional glasses [15, 16]. Bismuth oxide glasses have wide range of

applications in the field of optical and electronic devices, thermal and

mechanical sensors, reflecting windows [17, 18]. Bi2O3 participate in the glass

structure with two different coordinations as (BiO3) pyramidal which acts as

glass former and as BiO6, octahedral units act as modifier [19-21]. Recently,

spectroscopic and dielectric studies on the VA group elements mixed with

germanate glasses doped with CuO have been reported [22] from our

laboratory in which Bi2O3 effectively modified the germanate glass network

among VA group elements. Fluoride glasses typically have infrared absorption

edge wavelengths in the range 6 to 8 µm; with the addition of ZnF2 (metal

fluoride) to heavy metal germanate glasses widens their transmitting range into

the infrared [23].

NIR properties of GeO2 – PbO – Bi2O3 glasses doped with Yb3+ ions

were reported by Kasaab et al [24]. Salem and Mohammad [25] studied dc, ac

and dielectric properties of Bi2O3 – GeO2 – MoO3 glasses and observed that

molybdenum ions hinder the electron motion. Srinivasa Rao et al [26] have

reported the structural properties of ZnF2-Bi2O3 – P2O5 glass system.

123

The objective of the present study is aimed to find out the structural

changes in ZnF2 – Bi2O3 – GeO2 glass network doped with different

concentrations of Fe2O3 by means of spectroscopic and dielectric studies. The

prepared glasses were characterized by XRD; Analysis of spectroscopic

properties (viz FTIR, Raman, optical absorption, EPR) and dielectric properties

(viz; dielectric constant ε', loss tanδ and ac conductivity σac) over a wide range

of frequency has been reported in this chapter.

The glass samples with following composition were prepared using

melt-quenching method. In the present study the glass system 20ZnF2 –

40Bi2O3 – (40-x) GeO2: x Fe2O3; 0 ≤ x ≤ 2.5 wt% in steps of 0.5 wt% were

prepared using analytical grade reagents of GeO2, Bi2O3, ZnF2 and Fe2O3

(Sigma Aldrich 99.99% pure) in suitable proportions. The mechanically

homogenized mixtures were melted in a silica crucible at 1050 oC for 20

minutes until a bubble free liquid is formed. The melt was then poured on a

brass mould and subsequently annealed at 400 oC. Transparent yellow (pure)

and brown (doped) glasses are obtained; color of prepared glasses increases

with increase in the dopant concentration.

The prepared samples were labelled as

F0: 20ZnF2 – 40Bi2O3 – 40GeO2

F1: 20ZnF2 – 40Bi2O3 – 39.5GeO2: 0.5 Fe2O3

F2: 20ZnF2 – 40Bi2O3 – 39GeO2: 1.0 Fe2O3

124

F3: 20ZnF2 – 40Bi2O3 – 38.5GeO2: 1.5 Fe2O3

F4: 20ZnF2 – 40Bi2O3 – 38GeO2: 2.0 Fe2O3

F5: 20ZnF2 – 40Bi2O3 – 37.5GeO2: 2.5 Fe2O3 (all are in wt%)

Fig. 4.1 shows the physical appearance of the glasses in the present

investigation.

Fig. 4.1 Physical appearance of the 20ZnF2 – 40Bi2O3 – (40-x) GeO2: xFe2O3

glasses

4.2 Brief review of the previous work on the Iron doped glasses

Sanjay et al [27] have reported the study of structural, optical and

transport properties of semiconducting Fe2O3-PbO-B2O3 glasses and the dc

conductivity of these samples was measured in the temperature range 473-623

K. Horea et al [28] have reported structural investigation of xFe2O3- (100-x)

[P2O5-TeO2] glass system by FT-IR study and EPR spectroscopy. In these

glasses the addition and the increasing of Fe2O3 content modify progressively

the structure of the glass matrix. Effect of divalent metal oxides on absorption

spectra of some sodium borate glasses containing mixed nickel and iron oxides

125

have reported by El-Betal et al [29]. The electrochemical behaviour of

Fe2+/Fe3+ redox couple in sodium disilicate glasses has been studied by Mariac

et al. [30]; from the results they have concluded that Fe3+ acts both as network

former and network modifier while Fe2+ acts as network modifier. Stefan and

Simon [31] have reported EPR of Fe3+ ions doped in bismuth borate glasses

and their studies indicate various sites for Fe3+ ions in environments

characterized by different crystalline field intensities. Baiocchi et al. [32]

studied the optical and magnetic properties of iron ions in lead silicate glasses;

they have assigned the bands observed in the optical absorption spectrum to the

corresponding transitions by taking into account the selection rules and on the

basis of ligand field energy calculations. They have also concluded that the

four-fold coordination of Fe3+ ions is more common than the six fold in silicate

glasses. Dance et al [33] have investigated ESR of Fe3+ ions in fluoro aluminate

glasses and attributed the single line centered at g=4.3 in the ESR spectrum to

the presence of Fe3+ ions in sites of fully rhombic symmetry. Hazra and Ghosh

[34] studied structural and physical properties of Fe2O3 doped lead vanadate

glasses; they concluded that there is a strong role of iron both in the glass

network and in the conduction mechanism of the glasses.

Muralidhara [35] et al. carried out electron paramagnetic resonance

(EPR) and optical absorption spectral investigations on Fe3+ ions doped sodium

boro phosphate glasses. The optical absorption spectrum of sodium boro

phosphate glasses exhibits four bands characteristics of Fe3+ ions in an

126

octahedral symmetry. The value of inter-electronic repulsion parameter B

obtained in the present work suggests that the bonding is moderately covalent.

Pascuta [36] et al. Structural investigation of xFe2O3 - (100 - x) [3B2O3 - SrO]

glass system, with 0 ≤ x ≤ 40 mol%, was performed by means of X-Ray

diffraction (XRD), Fourier transform infrared (FTIR) and Raman spectroscopy.

At higher concentrations the iron ions determinate the break of regulate glass

network structure and determines the appearance of BO4 isolated tetrahedral.

Therefore, with the increasing of iron ions concentration, the structure of the

glasses is modifying and the number of non-bridging oxygens in these glasses

increases. Spectroscopic studies of Fe2O3 and CeO2 doped ZnO–Bi2O3–B2O3

glasses were carried out by Singh et al. [37]. From EPR and optical studies it is

observed that iron ions are present in trivalent state with distorted octahedral

symmetry. The EPR spectra of Fe3+ ions exhibit two resonance signals at g ≈

4.2 and g ≈ 6.4 which are attributed to Fe 3+ ions in rhombic and axial

symmetry sites, respectively. The optical bandgap (Eopt) decreases with

increase of transition metal. Mixed alkali effect in Li2O–Na2O–B2O3 glasses

doped with Fe2O3 was studied by Chakradhar et al. [38]. The EPR spectra in

these glasses exhibit three resonance signals at g = 7.60, 4.20 and 2.02. The

resonance signal at g = 7.60 has been attributed to Fe3+ ions in axial symmetry

sites whereas the resonance signal at g = 4.20 is due to isolated Fe3+ ions in

rhombic symmetry site. The resonance signal at g = 2.02 is due to Fe3+ ions

coupled by exchange interaction. The existing theories of MAE have been

127

proposed mainly on the basis of transport properties in mixed alkali glasses,

particularly electrical. Fe concentration dependent transport properties of LiI–

AgI–B2O3 glass system were reported by Srilatha et al. [7]. Optical absorption

and ESR studies have indicated that iron ions exist in Fe2+ state in addition to

Fe3+ state. DC conductivity is increased up to 0.9 mol% of Fe2O3 and beyond

that the conductivity is found to decrease. The analysis of the DC conductivity

results indicated that there is a mixed conduction (both ionic and electronic)

and the ionic conduction seems to prevail over polaron hopping in the glasses

containing Fe2O3 more than 0.9 mol%.

Srinivasarao et al. [39] studied the role of iron ions on the structure and

physical properties of PbO – As2O3 glasses by means of DTA, optical

absorption, FTIR, magnetic susceptibility and dielectric parameters. The

optical absorption measurements indicate iron ion in Fe3+ state in lower

concentrations and Fe2+ state when the concentration is beyond 0.25%.

Dielectric and studies on physical properties indicate that the structure of PbO

– As2O3 glass is more stable at low concentration of Fe2O3. Raghavaiah et al.

[40] studied the thermoluminescence studies on PbO–Sb2O3–As2O3 glasses

doped with iron ions. Thermoluminescence (TL) studies coupled with data on

optical absorption, ESR and magnetic susceptibility measurements have been

carried out. The TL light output has been observed to decrease with increase in

the concentration of Fe2O3 up to 0.6 mol%. For further increase in the content

of Fe2O3, the TL light output has been observed to increase. Crystallization and

128

the physical properties of Fe2O3-induced lead arsenate glasses are studied by

Nagarjuna et al. [41] Optical absorption, FTIR, ESR and magnetic

susceptibility measurements were also carried out. The optical absorption

studies together with ESR and magnetic susceptibility measurements indicated

the dominant presence of iron ions in the trivalent state when the concentration

of nucleating agent Fe2O3 is less than 0.3 mol%. The analysis of the FTIR

spectra indicated a gradual transformation of iron ions from tetrahedral sites to

octahedral

4.3. Characterization

4.3.1 X- Ray diffraction

The absence of sharp Bragg peaks from X-ray diffraction pattern of all

glass samples as shown in Fig. 4.2 confirms the amorphous nature of the

prepared glasses.

4.3 Physical parameters

The measured density value of F0 is 5.9324 g/cm3 and it is found to increase

gradually with increase in the concentration of Fe2O3 as presented in Table 4.1.

Simultaneously the molar volume is decreased from F0 to F5. From the

measured values of density ‘ρ’ and calculated average molecular weight M of

the glasses, some physical parameters such as mean iron ion concentration Ni,

mean iron ion separation Ri, and polaron radius Rp in the glass network

evaluated and also presented in Table 4.1.

129

10 20 30 40 50 60 70 80

F3

F5

F4

F2

F1

Fo

Cou

nts

2θ

Fig. 4.2 XRD patterns of ZBiG glasses doped with Fe2O3

130

Table 4.1

Physical Parameters of ZBiG glasses doped with Fe2O3.

Sample x

wt%

M ρ (g/cm3)

(±0.0001)

Vm

(m3/mol)

(±0.001)

Nix1021

ions/cm3

Iron ion

separation

Ri (Ǻ)

Polaron

radius

Rp (Ǻ)

F0 0 248.918 5.9234 42.022 -- -- --

F1 0.5 249.192 5.7701 43.186 2.79 7.10 2.86

F2 1 249.467 5.7431 43.437 5.56 5.64 2.27

F3 1.5 249.743 5.7125 43.718 8.29 4.93 1.99

F4 2 250.019 5.6787 44.027 11.00 4.49 1.81

F5 2.5 250.294 5.6297 44.459 13.63 4.18 1.68

131

4.4 Results

4.4.1 Optical absorption spectra

Optical absorption spectra of ZBiG glasses doped with Fe2O3 recorded

at room temperature in the wave length region 200-1400 nm are shown in Fig

4.3. From the figure it is clear that pure sample F0 shows two small peaks

observed at 630 nm and 860 nm and the samples doped with Fe2O3 show two

peaks at about 831 nm and 964 nm. The UV absorption edge or cut off

wavelength, λc of F0 sample is observed at 386 nm. The cutoff wavelength is

found to be increased from F0 to F5 samples.

Fig. 4.3 Optical absorption spectra of ZBiG glass sample doped with Fe2O3.

400 600 800 1000 1200 14000.0

0.2

0.4

0.6

0.8

1.0

400 600 800 10000.10

0.15

0.20

0.25

0.30

860

630

F0

681.5F3

F2

F1

5Eg

5T2g

F4

F3

F2

F1

F0 F

5

Abs

orpt

ion(

cm-1)

Wavenumber(cm-1)

132

Table 4.2

Cutoff wavelength (λc), Optical band gap (Eg) and Urbach energy (∆E) of

ZBiG glasses doped with Fe2O3.

Sample λc (nm)

(±0.1)

band position (nm) (±0.1) Eg (eV)

(±0.001)

∆E (eV)

(±0.001) 6A1 → 4T1

2B1g→2B2g

F0 386 -- -- 2.848 0.201

F1 422.5 831 964 2.712 0.163

F2 438 829 963 2.663 0.202

F3 448.5 825 964 2.611 0.227

F4 465.5 825 964 2.556 0.238

F5 469 825 967 2.544 0.248

From Tauc’s plots, drawn between hν and (αhν)1/2 as shown in Fig. 4.4,

optical band gap (Eg) of all the samples are determined by the extrapolation of

the linear portion of the curve to the x-axis [(αhν)1/2 = 0]. The data pertinent to

cut off wavelength (λc), absorption band position and band gap (Eg) energies

for the glasses under investigation are presented in Table 4.2.

133

2.0 2.2 2.4 2.6 2.8 3.0 3.20

2

4

6

8

10

F5 F

4F

3 F2 F

1 F0

(αhν

)1/2 (c

m-1eV

)1/2

hν(eV)

Fig. 4.4 Tauc’s plots of ZBiG glasses doped with Fe2O3

The absorption coefficient α(ν) in Urbach’s exponential tail region is

evaluated from the following equation

α(ν) = C exp (hν/∆E) -- (1)

Where C is a constant and ∆E is the Urbach’s energy defined as the

energy gap between localized tail states in the forbidden band gap [42]. Urbach

tail plots drawn between ln(α) and hν are shown in Fig. 4.5. From these plots

∆E values are evaluated by taking the reciprocal of the slopes of the linear

portion of the curves and are also presented in Table 4.2. Inset of Fig. 4.5

134

shows the variation of Eg as well as ∆E with respect to dopant concentration of

Fe2O3.

2.0 2.2 2.4 2.6 2.8 3.0 3.20

1

2

3

4

5

0 1 2 3

0.18

0.24

Conc. of Fe2O

3

∆E

(eV

)

2.6

2.7Eg(eV

) F5

F4

F3

F2

F1

F0

ln(α

)

hν(eV)

Fig. 4.5 Urbach energy (∆E) of ZBiG glasses doped with Cobalt. Inset of the

figure shows the variation of Urbach energy (∆E) and Optical band gap (Eg)

with concentration of Fe2O3.

4.4.2 EPR spectra

EPR spectra of ZBiG glasses doped with Fe2O3 recorded at room

temperature are shown in Fig. 4.6. All samples show an intense signal at g =

4.2±0.1 and a broad resonance peak at g = 2.1±0.1 along with a shoulder peak

135

in the region g = 6.8±0.1. The intensity of signals at 4.2 and 6.8 decreases with

increase in the concentration of Fe2O3; whereas the intensity of the signal at 2.1

remains same up to F4 and the signal changes to an intense peak in F5 sample.

0 250 500

g = 6.8

g = 4.2

g = 2.1

F5

F4

F3

F2

F1

Firs

t Der

ivat

ive

of A

bsor

ptio

n

Magnetic Field(mT)

Fig. 4.6 EPR signals of ZBiG glasses doped with Fe2O3.

136

4.4.3 FTIR spectra

FTIR spectra of all glasses recorded in order to identify the structural

units in the glass network are shown in Fig. 4.7. The observed vibrational

bands and their corresponding assignments are given in Table 4.3. The glass

samples show four vibrations around 945 nm, 745 nm, 452 nm and 428 nm.

1100 1000 900 800 700 600 500 400

945 745 452

428

F5

F4

F3

F2

F1

F0

%T

ν(cm-1)

Fig. 4.7 FTIR spectra of ZBiG glasses doped with Fe2O3.

137

Table 4.3

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

of ±0.1cm-1) of the ZBiG glasses doped with Fe2O3.

F0 F1 F2 F3 F4 F5 Assignment

428 422 413 423 417 417 Bi-O bonds in BiO6 units

452 471 466 471 469 472 Bi-O bonds in distorted BiO6 octahedra units and ZnO4 units

745 754 750 751 751 749 Bi-O symmetrical stretching of BiO3 and Ge – O– asymmetric stretching of GeO6 units

945 958 959 961 961 963 Bi-O stretching vibrations in BiO6 units and GeO4 units

4.4.4 Raman Spectra

Fig. 4.8 shows the Raman spectra of the all glass samples under

investigation. The main Raman feature shifts (RF) and the corresponding bond

vibrations are present in Table 4.4. A broad and intense RF is observed around

391 cm-1and at 782 cm-1 with feeble features around ~ 276, ~ 555, ~ 654 and ~

891 cm-1. By the addition of Fe2O3 the area of RF782 is found to be decreased,

where as the area under Raman feature at 391 cm-1 is observed to increase.

138

1000 900 800 700 600 500 400 300 200

276

555891782

654

391

Ram

an In

tens

ity (a

.u.)

F5

F4

F3

F2

F1

F0

ν(cm-1)

Fig. 4.8 Raman spectra of ZBiG glasses doped with Fe2O3.

139

Table 4.4

Assignment of Raman Features (with a probable error of ±0.1cm-1) of ZBiG

glasses doped with Fe2O3.

F0 F1 F2 F3 F4 F5 Assignment

276 282 288 284 280 284 Bi-O-Bi stretching vibrations in distorted BiO6 units

391 391 399 397 397 395 Bi-O-Bi bending vibrations

-- -- 458 458 -- 471 Symmetrical stretching vibrations of oxygen in Bi – O – Ge and Ge – O – Ge along with ZnO4 units

555 -- 552 554 560 575 Bi – O vibrations in distorted BiO6 units

654 -- 649 639 654 686 Ge – O – Ge bending modes associated with ring strains

782 781 780 781 771 792 Symmetric stretching vibrations of Ge – O– bonds in GeO4 units

891 875 873 883 898 896 Ge – O– bonds associated with Q2 GeO4 units and Ge –O – Ge asymmetrical stretching

-- 932 928 943 939 -- Ge – O– bonds associated with Q3 GeO4 units

140

4.4.5. Dielectric properties

Fig. 4.9 shows the variation of dielectric constant ε' with the temperature

of F3 sample at different frequencies. The variation of dielectric constant with

temperature for different concentrations of Fe2O3 at 100 KHz is shown in the

inset of Fig. 4.9. Among all the samples F0 sample has

0 50 100 150 200 250 300

18

24

30

36

0 50 100 150 200 250 30015

20

25

30

35

40

F0

F1

F2

F3

F4

F5

Die

lect

ric c

onst

ant ε

'

Temparature oC1 MHz

500 KHz

100 KHz

10 KHz

1 KHz

Die

lect

ric c

onst

ant ε

'

Temparature oC

Fig. 4.9 Variation of dielectric constant ε' with temperature at different

frequencies of F3 sample. Inset shows the variation of ε' with temperature at

100 KHz for different concentrations of Fe2O3 in ZBiG glasses.

141

exhibited minimum value 13.78 for the dielectric constant ε' at 1 MHz and at

the temperature 30 oC. The sample F5 has shown the maximum value 44.45 for

ε' at 1 KHz and at the temperature 300 oC.

The variation of dielectric loss(tanδ) with temperature at 100 KHz for

different concentrations of Fe2O3 is shown in Fig. 4.10 and its inset shows the

temperature dependence of loss(tanδ) at different frequencies of F5 sample. The

curves of both pure and Fe2O3 doped glasses exhibit distinct maxima in the

dielectric loss(tanδ) verses temperature plot and such maxima are found to be

shifted towards lower temperature region with increase in the concentration of

Fe2O3. Such maxima in the dielectric loss (tanδ), is the characteristic of

relaxation of the dipoles present in the sample. Further, the variation of

dielectric loss with temperature for different concentrations of Fe2O3 indicates

a gradual increase in the broadening of relaxation curves. At the same time

(tanδ)max of relaxation curves is found to be increased with increase in the

concentration of Fe2O3. The effective activation energy, wd for all glass

samples is calculated for all the samples [26] using the following equation

f = f0 exp (-Wd/KT) -- (2)

Where f is the frequency and f0 is a constant. The summary of data

pertinent to loss (tanδ), activation energy for dipoles (A.E.) and breakdown

strength for different concentrations of Fe2O3 are presented in Table 4.5. From

observations, it is found that wd is maximum for F0 sample and is decreased

with increase in Fe2O3 doping. Since insulating strength of the glass sample

142

depends upon the electric field applied on it, the dielectric breakdown strength

of the samples is determined. The value of the breakdown strength 12.92

KV/cm is obtained for F0 sample and its value is found to be decreased with

increase in the Fe2O3 concentration.

0 50 100 150 200 250 300

0.04

0.06

0.08

0.10

0 50 100 150 200 250 3000.03

0.06

0.09

0.12

1 MHz

500 KHz

100 KHz

10 KHz

1 KHz F5

F4

F3

F2

F1

F0

tanδ

Temparature oC

Temparature oC

tanδ

Fig. 4.10 Variation of dielectric loss Tanδ with temperature at 10 KHz for

different concentrations of Fe2O3 in ZBiG glasses. Inset shows the variation of

loss(tanδ)with temperature at different frequencies of F5 sample.

The a.c. conductivity values for all glasses at different temperatures and

frequencies are evaluated [26] using the following equation

σac = ωε'ε0tanδ -- (3)

143

Where ω is the angular frequency, ε0 is the dielectric constant of

vacuum. The variation of conductivity with 1/T of all investigated glasses at 1

KHz is shown in Fig. 4.11 and its inset shows the variation of conductivity with

1/T at different spot frequencies of F1 sample.

Table 4.5

Summary of data on dielectric loss of ZBiG glasses doped with Fe2O3 at 1

KHz.

Sample (Tan δ)max.ave

x(10-1)

Temp. region of relaxation

AE for dipoles

(eV)

Breakdown Strength (kV/cm)

F0 -- -- -- 12.92

F1 0.3625 125-165 3.55 12.37

F2 0.378 110-155 3.38 12.09

F3 0.406 95-145 3.22 11.72

F4 0.425 85-145 3.06 11.28

F5 0.453 75-150 2.91 11.16

144

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

1E-7

1.6 2.0 2.4 2.8 3.21E-8

1E-6

1E-4 1 MHz

500 KHz

100 KHz

10 KHz

1 KHz

F0

F1

F2

F3

F4

F5

1000/T

σac

Fig. 4.11 Variation of σac with 1000/T at 1 KHz for different concentrations of Fe2O3

in ZBiG glass samples and inset shows the variation of σac with 1000/T for the Sample

F1.

The activation energy (A.E.) for conduction in the linear region of logσac with

1/T plots, observed at higher temperature, has been evaluated. Such σac values

at 70o C, density of defect energy states N(EF) and A.E. values for conduction

of all the glass matrices are incorporated in Table 4.6.

145

Table 4.6 Summary of data on ac conductivity of ZBiG glasses doped with Fe2O3 at 1 KHz.

4.5 Discussion:

ZnF2 – Bi2O3 – GeO2 glass containing iron ions is a complex

composition and an admixture of modifier, intermediates and network formers.

The observed decrease in the density and increase of molar volume by the

doping of Fe2O3 is ascribed to the following: (i) due to the gradual decrease in

the concentration of GeO4 structural units and simultaneous increase in Fe3+

(0.49 Å) and Fe2+ (0.78 Å) ion concentration in the composition (ii) formation

of octahedral Ge4+ [0.53 Å] ions at the expense of tetrahedral Ge4+ [0.39 Å]

Sample σa.c at 70oC x(10-8)

( Ωcm)-1

N(EF) x 1019 (eV-1/cm3) AE for conduction

(eV) Austin Butcher Pollak

F0 3.10 3.05 1.27 3.10 0.546

F1 3.52 3.25 1.36 3.30 0.451

F2 3.81 3.38 1.41 3.44 0.415

F3 4.27 3.58 1.49 3.64 0.357

F4 4.89 3.83 1.59 3.89 0.314

F5 5.94 4.22 1.76 4.29 0.305

146

ions in the glass network; since the density of a glass is sensitive to the ionic

size, atomic weight and amount of different elements present [23, 43]. From

the measured values of density ‘ρ’ and calculated average molecular weight

M of the glasses, physical parameters such as mean iron ion concentration Ni,

mean iron ion separation Ri, and polaron radius Rp of the glasses are evaluated

and presented in Table 4.1.

The presence of absorption peaks at 630 nm and 860 nm are ascribed to

bismuth radical Bio in Bi2O3. At higher temperature due to thermal effect Bi3+

ions may be reduced to Bi2+ → Bi+ → Bio bismuth radicals. Hence, the

observed absorption bands around 630 nm and 860 nm are ascribed to 3P0 →

1D2 and 3P0 → 3P2 transitions of Bi+ ions [42]. A small peak observed at around

681.5 nm in doped samples from F1 to F3 due to the presence of bismuth

radicals.

The absorption peak observed at 831 nm for F1 sample is blue shifted

with increase in concentration of Fe2O3. Using-Tanabe diagrams for d5 ions the

peak at 831 nm is due to 6A1 → 4T1 spin forbidden transitions of Fe3+ ions [44].

The observed red shift from the literature is perhaps due to the presence of high

polarizing nature of bismuth radicals which generally yield high crystal field

splitting. The band observed at 964 nm is ascribed to 5T2g → 5Eg transition of

Fe2+ (d6) ions [45]. Based on the selection rules and ligand field calculations

the first band at 831 nm is due to FeO6 group and the first band at 964 nm is

due to FeO4 group [44]. The Fe3+ ion sites considered as interaction between its

147

external orbital and the p-orbital of the neighbouring oxygens [46]. When the

concentration of Fe2O3 increased, the intensity of the peak at 964 nm is

increased at the expense of peak around 831 nm. Further the Fe2+ ions occupy

only interstitial positions, since the ratio of cation-oxygen radii is 0.63 Ǻ for

Fe2+ ion, which is far from the value of 0.19 Ǻ to be possessed by an ion to

occupy tetrahedral or substitutional sites [47] and create more disorder by

creating dangling bonds in the glass network.

The absorption edge λc observed at 386 nm for F0 sample and increases

with Fe2O3 concentration suggests that the increase in number of non-bridging

oxygens (NBOs). This increase leads to degree of localization of electrons and

hence the donor centres in the glass matrix. The study of variation of short

wave length absorption edge (SWAE) and optical band gap in oxide glasses

give vital information to understand the electronic band structure. The ionic

size of iron ion (Fe3+ - 0.49 Ǻ, Fe2+ - 0.78 Ǻ ) is larger than those of Ge ions

(GeO4 – 0.39 Ǻ, GeO6 – 0.53 Ǻ ). Therefore bond length of Fe-O is greater

than bond length of Ge-O and hence doping of Fe2O3 effectively opens the

glass network. The addition of Fe2O3 increases the average bond length which

narrows the optical band gap [48]. Among the doped samples F1 shows

minimum tail energy and increased with concentration of Fe2O3. The increase

in Urbach energy is reasonably explained because of the inducement of defects

like fluctuations in bond angle distortions and wrong bonds. Hence, the density

of localized states N(EF) of these defects increases and leads to tailing of the

148

states into the gap at the band edges. This tailing also gives the evidence for

decrease in Eg.

EPR spectra of ZnF2 – Bi2O3 – GeO2: Fe2O3 glasses exhibited two

signals centred at g = 4.2 and 2.1. The signal at g = 4.2 arises from the

tetrahedral environment of Fe3+ ions and the signal at g = 2.1 arises due to Fe3+

– O – Fe3+ spin-pair [59-51] and the shoulder peak at g = 6.8 is attributed to

the isolated Fe3+ ions in rhombic and axial symmetry sites [52-55]. These large

g values arise due to presence of certain symmetry elements in the glass matrix.

The theory of these large g values is usually expressed by the spin-Hamiltonian

[56]

2 2 2( 1)( )

3Z x y

S SH g BS D S E S Sβ

+ = + − + − --- (4)

Where S = 5/2. Here D and E are the axial and rhombic structure parameters

respectively, λ = E/D lies within the limits 0 < λ < 1/3 [57]. The iron ions in

Fe3+ state belong to d5 configuration with 6S as the ground state in the free ion

and there is no spin-orbit interaction [58]; when Fe3+ ions placed in a crystal

field environment, the 6S ground state splits into three Kramer doublets |±1/2>,

|±3/2> and |±5/2>. The resonance signal at g = 4.2 arises due to the middle

Kramer doublet |±3/2> [59]. The decay of the intensity of EPR signals at g =

4.2 and 6.8 indicates the decrease in the concentration of Fe3+ ions in the glass

network. The intense peak at g = 2.1 for F5 sample arises with further increase

149

in concentration of Fe2O3 and is ascribed to dipole – dipole interactions due to

Fe3+ ions in the sites of less distorted octahedral field.

The inferences drawn from the analysis of optical absorption and EPR

spectra suggest that the trivalent iron ions decrease with increase in the

concentration of Fe2O3 and the redox ratio (conc. of Fe2+/conc. of Fe3+)

increases. Hence, the modification of the glass network increases.

FTIR spectra are helpful to understand the structural changes in the

present investigation. Researchers report that vitreous GeO2 exhibits IR

transmissions at ~ 915 cm-1, ~ 750 cm-1 and ~ 584 cm-1 and are assigned to

asymmetric stretching vibrations of GeO4 units, asymmetric vibrations of Ge-

O– bonds of GeO6 and bending vibrations of Ge4 – O – Ge4 respectively[60-

63]. The observed bands in the pure sample F0 at 945 cm-1, 745 cm-1 are

ascribed to stretching vibrations of BiO3 [61] combined with Ge-O–

asymmetric stretching of GeO6 units [63]. Further, the bond at 452 cm-1 is due

to ZnO4 structural units overlapped by Bi-O bonds in distorted BiO6 units [64]

and GeO6 units [63]. A small at 428 cm-1 is due to Bi-O bonds of different

length in distorted BiO6 polyhedra [65].

From Fig 4.6 it is observed that the band at 945 cm-1 is slightly red

shifted, which indicates the increase in ionic nature of the glass matrix with

Fe2O3 doping. Further the areas of bands at 745 cm-1 and 452 cm-1 concluded

that the increase of octahedral GeO6 structural units at the expense of

tetrahedral GeO4 structural units; which results in the cross-linking bonds such

150

as Ge-O-Bi; Ge-O-Zn and Bi-O-Zn. The increase in the area of bands is due to

the increase of NBO’s. The red shift at 945 cm-1 may ascribed to the increase in

bond length when Ge-O is replaced by Fe-O, this augments the conductivity

of the glass network.

From the literature it is observed that trigonal GeO2 has germanium in

four fold coordination [67] and rutile structure of GeO2 has six-fold

coordination [67]. The Raman spectra of vitreous GeO2 consist of bands around

910, 835, 810 and 765 cm-1 due to asymmetric stretching of oxygen in GeO4

tetrahedra with BOs = 4, 3, 2 and 1 respectively [68]. In the present

investigation the Raman spectra of the glass samples show an intense peak

around 391 cm-1 is assigned to the combination of ZnO4 structural units

combined with Bi-O-Bi bending vibrations and symmetric stretching of

bridging oxygens in Ge-O-Ge in GeO6 [66, 69]. The small shoulder at around

555 cm-1 is due to Bi-O vibrations in BiO6 units [70]. An intense peak at 782

cm-1 is assigned to symmetric stretching vibrations of Ge-O– bonds in GeO4

units [71] along with two small peaks at 891 cm-1 and 964 cm-1 are ascribed to

Ge-O-Ge asymmetrical stretching in GeO4 structural units [72, 73]. From the

Fig. 4.7 it is observed that the area of the peak at around 782 cm-1 is decreased

and the area of the peak at 391 cm-1 is observed to increase with increase in the

concentration of Fe2O3, which indicates the increase of GeO6 (octahedral) units

in proportion with GeO4 (tetrahedral) units.

151

The inferences drawn from the analysis of FTIR spectra of the present

glasses are also supporting those obtained from Raman spectra; such inferences

indicate clearly the decrease in GeO4 structural units and increase in GeO6

structural units with increase in concentration of Fe2O3. Hence, iron ions

occupy octahedral positions in the glass structure with increase in

concentration; such ions act as a network modifier in decreasing the rigidity of

glass matrix.

Space charge polarization contributes more to the dielectric constant

among all polarizations (electronic, ionic, dipolar and space charge

polarization) and it depends on the purity and perfection of the glass [8]. The

observed increase in the value of ε', loss (tanδ) and σac at any frequency are

found to be increased with temperature. The increase of these dielectric

parameters attributed to (i) the high polarizing nature of Bi3+ ions (ii) increase

in the ionic environment of Fe2+ ions and (iii) enhancement of Bi3+- Bi5+ pairs.

In the present investigation iron exists as both Fe3+ and Fe2+ states, in which

Fe3+ ions take part in network forming positions and Fe2+ ions acts as modifiers

[39]. The activation energy decreases with increase in concentration of Fe2O3,

which is an indication of space charge polarization. The gradual increase in

octahedral positioned Fe2+ ions in the glass network leads to its modification.

These ions weaken the glass network by creating bonding defects and create

path ways for migration of ions that build up space charge polarization and

leads to an increase in the dielectric parameters. This is also supported by

152

decrease in the optical band gap energy, Eg and intensity of resonance signal in

EPR studies. At higher concentration, Fe2+ ions in octahedral positions lead to

more space charge polarization and hence increase the disorder in the glass

network [62].

The observed dielectric relaxation and its shift with frequency may be

attributed to association of Fe2+ ions, Zn2+ ions with a pair of Ge – O– groups or

F– ions in analogy, such that the association of the divalent ion with a pair of

cationic vacancies occurs in conventional glasses [74]. The dielectric relaxation

effects can be ascribed to Bi2+ ions with addition with Zn2+ ions. Further,

spreading of relaxation region with Fe2O3 doping is due to Fe2+ ions. From

Table 4.6, the increase in (tanδ)max.ave and decrease in activation energy for

dipoles with Fe2O3 content suggests that an increase in degree of freedom for

dipoles to orient into the field direction leads to a decrease rigidity of the glass

network. This enhancement is directly related to the mobility of charge carriers.

The observed decrease in activation energy and increase in conductivity of the

present glasses is an obvious change [75]. By correlating activation energy with

mean separation (Ri), and polaron radius (Rp) with increase in Fe2+ ion

concentration, the mean site-to-site separation between iron ions and polaron

radius are found to decrease, which cause to decrease in activation energy Wd

and increase of σac as reported previously [76].

The low temperature part of the conductivity can be explained on the

basis of quantum mechanical tunneling model [73]. The density of energy

153

states N(EF) near the Fermi level, has been evaluated using the equation (5)

and presented in Table 4.6.

4

2 2 5( ) [ ( )] ln ph

e FKT N Eν

σ ω η α ωω

− =

--- (5)

where for Austin η = π/3, Butcher η = 3.66 π2/6 and Pollack η = π4/96 with the

usual meaning of remaining symbols reported [73]. The increase in the number

of localized states near Fermi level is supported by increase of Urbach energy

∆E values suggests that, the increase in ionic radii with Fe2O3 doping is caused

to increase the disorder thus modifying the glass network.

The breakdown strength of all the samples is determined at room

temperature. Since the breakdown strength is inversely proportional to the

value ε'(tanδ) [52], the result suggests that the rate of increase of ε'(tanδ) with

temperature is lowest for F0 sample. Therefore, the experiments on dielectric

breakdown strength of ZnF2 – Bi2O3 – GeO2 glasses revealed the conversion of

a part of Fe3+ ions into Fe2+ ions with increase in concentration of Fe2O3 which

act as modifiers, thus modifying the glass network.

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