PHYSICAL AND OPTICAL POLARIZABILITY AND TRANSPORT PROPERTIES OF BISMUTHATE GLASSES

Final ReadingSeptember 1, 2009 16:38 WSPC/147-MPLB 02078

Modern Physics Letters B, Vol. 23, No. 22 (2009) 2665–2679c© World Scientific Publishing Company

PHYSICAL AND OPTICAL POLARIZABILITY AND TRANSPORTPROPERTIES OF BISMUTHATE GLASSES

SHASHIDHAR BALE∗ and SYED RAHMAN

Department of Physics, Osmania University, Hyderabad 500 007, India∗sss [email protected]

Received 28 November 2008Revised 9 January 2009

Bismuth-based glasses containing ZnO, B2O3 and Li2O are investigated through differ-ent physical, polarizability and transport properties. Raman spectroscopy reveals thatthese glasses are built from [BiO3] and [BiO6] units. Zinc in tetrahedral form is alsoobserved. Density and glass transition temperature increase with the bismuth content.The refractive index, oxide ion polarizability and optical basicity also increase with theBi2O3 content, whereas the interaction parameter decreases. The DC electrical con-ductivity increases and the activation energy decreases with the increase in the Li2Ocontent.

Keywords: Differential scanning calorimetry (DSC); Raman spectroscopy; infrared (IR)spectroscopy; optical absorption; optical materials.

1. Introduction

Bismuth-oxide-containing glasses have attracted a great deal of interest, owing totheir important applications in the field of glass ceramics, layers for optical andelectronic devices, thermal and mechanical sensors, reflecting windows, plasma dis-play panels (PDPs), etc.1 It is a known fact that Bi2O3 is not a classical glassformer, due to its high polarizability and small field strength of Bi3+ ions, in thepresence of conventional glass formers (such as B2O3, PbO, SiO2) it may builda glass network of [BiOn] pyramids.2 The structure of the glasses is determinedmostly by the anionic network, whereas the effect of the cations is usually smaller.The complex geometry and rapid change in temperature and viscosity as the glasscools during its formation pose difficulty in analyzing most of the glass-formingoperations. Moreover, the structural role played by Bi2O3 in glasses is complicatedand poorly understood. This is because the [BiOn] polyhedra are highly distorteddue to the lone pair electrons. X-ray and infrared studies have shown that Bi3+

ions participate in the network structure above 45 mol% Bi2O3. The addition ofCd, Th, Li, Ba, Zn and Fe oxides to the bismuthate glasses results in a large glassformation domain.3,4

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2666 S. Bale & S. Rahman

Among the various properties of glasses, refractive index is one of the mostimportant. Therefore, a large number of researchers have carried out investigationsto ascertain the relation between the refractive index and glass composition. Ingeneral, it is recognized that the refractive index and density of many glasses canbe varied by changing the base glass composition.5

Polarizability is another property of glass materials that governs the nonlinearresponse of the material. The optical nonlinearity is caused by electronic polariza-tion of the materials on exposure to intense light beams. Polarizability is relatedto many macro- and microscopic physical and chemical properties, such as opticalUV absorption of metal ions and electro-optical effect.

Since the first report that the bismuth-based glasses are thermally stable,3 manystudies have been devoted to understanding the structure of Bi2O3-containingglasses, mainly because of their interesting properties. Ionic conduction is wellknown in crystalline solids,6,7 and both cations and anions can carry current(sometimes simultaneously) under certain circumstances. The mechanisms of trans-port in glassy ionic conductors are not well understood, principally because thestructure — both of the framework and the local environment of the ions — isin general not known. The factors that are believed to control the magnitude ofconductivity in glasses are associated with the composition of the glass networkin addition to the binding energies holding the charge carriers in their equilibrium(metastable) sites and the migration barriers that the carriers face during theirtransfer.

Here, we present and correlate our results related to the density, glass transi-tion temperature, refractive index, optical spectra, oxide ion polarizability, basicity,interaction parameter, Raman spectra and DC electrical conductivity of the qua-ternary glass series (60 − x)Bi2O3–25ZnO–15B2O3–xLi2O, where x in mol% takesthe values 0 ≤ x ≤ 15.

2. Experiment

Glass samples of compositions (60− x)Bi2O3–25ZnO–15B2O3–xLi2O (0 ≤ x ≤ 15)were prepared by the melt quench technique using reagent grade chemicals Bi2O3,ZnO, H3BO3 and Li2CO3. The mixture of these chemicals was placed in porcelaincrucibles, and was calcinated at 450◦C for 1 h and then melted at 1100–1200◦C,depending on the glass composition. The liquids were agitated for 1 h to ensurehomogeneity of the mixture. The clear liquid was quickly cast in a stainless steelmould kept at 200◦C and pressed with another steel disk to obtain glass disks of∼ 1 mm thickness and ∼ 10 mm diameter. All samples were transparent and yellowin color. The glasses were chemically stable and nonhygroscopic. Thus, obtainedglasses were annealed at 200◦C for 12 h to remove thermal stress and strain.

The amorphous nature of all samples was confirmed by the absence of Bragg’speak in the X-ray diffraction pattern. The density of the glass samples wasdetermined at room temperature by the Archimedes method, with xylene as theimmersion liquid.

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Physical and Optical Polarizability and Transport Properties of Bismuthate Glasses 2667

The glass transition temperature, Tg, was measured in all samples using atemperature-modulated differential scanning calorimeter (TA Instruments, DSC2910). All samples were heated at the standard rate of 10◦C min−1 in aluminumpans.

The room temperature dielectric measurements at 10 MHz were performed usingan HP-4192A impedance analyzer.

The room temperature Raman measurements were performed in the range of100–1700 cm−1 on a micro-Raman system from a Jobin-Yvon Horiba (LABRAMHR-800) spectrometer. This system is equipped with a high stability confocalmicroscope for a micro-Raman 10×, 50×, 100× objective lens to focus the laserbeam. An Ar+ laser beam of 488 nm (E = 2.53 eV) was used for excitation. Theincident laser power was focused on a diameter of ∼ 1–2 µm and a notch filter wasused to suppress Rayleigh light. In the present system Raman shifts are measuredwith a precision of ∼ 0.3 cm−1.

The optical absorption spectra of the present glass samples (thickness ∼ 1 mm)were recorded at room temperature using a double-beam Shimadzu spectrometer(model UV-3100) in the wavelength range of 400–800 nm. The uncertainty in theobserved wave length is about ±1 nm.

The electrical measurements were made by the usual technique of two electrodes.Silver paste was painted on the polished circular disk surface of the samples with∼ 1 mm thickness and ∼ 10 mm diameter. With painted silver paste, good ohmiccontacts were found. The electrical conductivity was measured as a function oftemperature using spring-loaded sample in a cylindrical furnace. The DC electricalconductivity measurements were made using Keithley electrometer model 614. Thetemperature of the specimen was recorded between 200◦C and below the glasstransition temperature of each glass sample, with a chromel–alumel thermocouplekept in close thermal contact with the specimen surface.

3. Results and Discussion

3.1. Raman spectra

The Raman spectra of the glassy samples have been investigated to give informationon the structure and arrangement of building structural groups with respect to eachother and the type of bonds present in the glass. The recorded Raman spectra weredeconvoluted into six peaks as shown in Fig. 1, using Gaussian distribution to findthe exact mode of vibration.

(1) The Raman spectra are dominated by the bands associated with the structuralvibrations of the heaviest cation, Bi3+. The dominating peak around 135 cm−1

is an evidence for the existence of [BiO6] octahedral and [BiO3] pyramidal unitsin the entire composition range.8 The peak increases in intensity with increasein the Bi2O3 content.

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Fig. 1. Raman spectra of the present glass samples. The deconvoluted peaks are represented bydashed curves.

(2) The broad but strong band occurring in the present Raman spectra around394 cm−1 is attributed to the Bi–O–Bi vibrations of [BiO6] octahedral units,while the shoulder at 602 cm−1 can be attributed to BiO−/Bi–O–Zn stretchingvibrations.9

(3) In the present glass system, B2O3 is constant at 15 mol%. Only orthoborateand pyroborate units may exist.10 Therefore, in the present glass system, theRaman band around 930 cm−1 is due to orthoborate groups, and a very weakbroad band around 1280 cm−1 may be due to pyroborate groups and also toBiO− stretching vibrations of BiO3 units.

(4) The presence of the Raman peak at 254 cm−1 indicates the existence of Zn–Otetrahedral bending vibrations in the present glass system.11

3.2. Density

The density data for these glasses as a function of the Bi2O3 content are pre-sented in Table 1. It is observed that the density increases with the increase in theBi2O3 content. This is due to the higher molecular mass of Bi2O3 as compared toother oxides. A similar result has been reported for the Bi2O3–ZnO–B2O3 (Ref. 12)and Bi2O3–LiBO2 glass systems.13 The molar volume (Vm) is defined as the mean

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Table 1. Physical and optical parameters of the (60 − x)Bi2O3–25ZnO–15B2O3–xLi2O glass system.

Parameter x = 15 x = 10 x = 5 x = 0

Average mol wt (g/mol) 244.95 266.75 288.56 310.36

Density (g/cc) 5.381 5.656 5.953 6.222

Molar volume (cc/mol) 45.52 47.16 48.47 49.88

Oxygen packing density (g-atm/l) 48.33 48.77 49.51 50.12

Li+ concentration ×1021 (cc−1) 3.96 2.55 1.24 0

Li+ interionic distance (A) 6.32 7.31 9.30 —

Polaron radius (A) 2.54 2.94 3.74 —

Tg (◦C) 458 471 484 491

∆Tg (◦C) 26.5 23.8 22.4 24.8

Cp (J/mol ◦C) 30.1 19.5 21.9 24.3

∆Cp (J/mol ◦C) 2.8 1.1 1.0 2.9

ε 6.173 6.296 6.398 8.299

n 2.484 2.509 2.529 2.881

λc (nm) 412 415 416 418

E0 (eV) 3.01 2.99 2.98 2.97

αO2−(n) (A3) 2.221 2.303 2.362 2.423

αO2−(E0) (A3) 2.138 2.199 2.239 2.287

Λ(n) 0.916 0.944 0.963 0.980

Λ(E0) 0.899 0.911 0.924 0.940

Λth 0.954 0.963 0.973 0.982

A(n) 0.088 0.079 0.072 0.065

A(E0) 0.096 0.087 0.080 0.075

Ath 0.067 0.062 0.056 0.051

Rm(n) (cm3) 28.81 30.10 31.15 32.22

Rm(E0) (cm3) 27.86 28.93 29.76 30.67

α(n) (A3) 11.41 11.93 12.34 12.76

α(E0) (A3) 11.04 11.46 11.79 12.15

Edc (eV) 1.20 1.24 1.25 1.29

molecular weight of the glass constituents divided by its density (ρ):

Vm =M

ρ,

where M is the average molecular weight of the glass expressed as the mole fractionsof the oxides multiplied by their molecular weights. The oxygen packing density iscalculated from the density data using the following formula14: Oxygen packingdensity = (ρ/M)× number of oxygen atoms per formula unit. The values of themolar volume and oxygen packing density are presented in Table 1. It can be ob-served that both the molar volume and the oxygen packing density of the presentglasses increase with increasing Bi2O3 content. The above results indicate the in-crease in the tightness of the glass structure with the increase in the Bi2O3 content.

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Fig. 2. MDSC profile of (60 − x)Bi2O3–25ZnO–15B2O3–xLi2O glasses.

3.3. Glass transition temperature

Figure 2 shows the thermograms of 55Bi2O3–25ZnO–15B2O3–5Li2O and 60Bi2O3–25ZnO–15B2O3 quaternary glasses. Figure 3 illustrates nonreversible heat flow(NRHF) and heat capacity (Cp) signals of a typical glass sample. The inset of thefigure explains the determination of the glass transition temperature Tg, change inthe transition temperature ∆Tg and specific heat capacity difference ∆Cp values.Tg, ∆Tg and ∆Cp in the glass transition region were determined15 for all the glasssamples and are presented in Table 1. The value of glass transition temperatureincreases with the Bi2O3 content. This tendency is the same as those reported forthe Bi2O3–ZnO–10B2O3–Na2O and Bi2O3–ZnO–10B2O3–K2O glass systems.16 Itis considered that Tg is dependent on the strength of the chemical bond in theglass structure. In general, lithium metal oxide play the role of network modifierin silicate and borate glasses and nonbridging oxygen increases with increase inLi2O in these glasses. It is widely known that the bond between the lithium atomand oxygen is ionic, and is in general weaker than a covalent bond. Hence, Tg de-creases with increasing lithium oxide content or increases with the Bi2O3 content.The smaller values of ∆Cp are an indication of strong resistance to any structuralchange, showing that the glass-forming liquid has a number of configurational de-grees of freedom associated with it. The glass transition temperature of the presentglasses is found to be higher when compared with Bi2O3–B2O3–Li2O (300–400◦C)12

and Bi2O3–Li2O (25–280◦C).9 This is due to the addition of ZnO, which takes anetwork-forming position in the glassy network and increases the glass stability, asmentioned earlier.

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Fig. 3. Typical modulated (NRHF and Cp) results during a heating scan in the 50Bi2O3–25ZnO–15B2O3-10Li2O glass.

3.4. Dielectric constant and optical absorption spectra

The room temperature dielectric constant (ε) of the present glasses was determinedat a frequency of 10 MHz using the relation

ε =C − C0

ε0

(t

A

),

where C is the capacitance of the sample, C0 (=6 pf) the capacitance in air, ε0 =8.854× 10−12 F/m the permittivity of free space, t the thickness and A the area ofthe glass sample. Further, the refractive index (n) was calculated from the dielectricconstant of the glass using17

ε = n2 .

The values of the dielectric constant (ε) and the refractive index (n) of the presentglasses are presented in Table 1. It can be seen that both ε and n increase withincrease in the Bi2O3 content.

Figure 4 shows the optical absorption spectrum of a typical glass composition.A distinct cutoff has been observed and other glass compositions have similar be-havior. However, the values of the cutoff wavelength (λc) for different glasses areslightly different; they are presented in Table 1. The optical band gap (E0) hasbeen estimated from the cutoff wavelength; it is also listed in Table 1. It can beseen from the table that λc increases and E0 decreases with increase in the Bi2O3

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Fig. 4. Optical absorption spectra of a typical glass sample in the present study.

content. It has been reported earlier18 that bismuth oxide in the form of thin filmshows Eopt in the range of 2.5–2.6 eV. Now, if viewed with respect to ZnO, it isevident that introduction of Zn2+ ions into the bismuth oxide network increases theoptical band gap up to 3.01 eV. However, the presence of a sharp cutoff in theseglasses may make them useful in spectral devices as optical windows.

3.5. Molar refraction and molar polarizability

The most familiar and widely used relationship that links molar refraction Rm todielectric constant ε and molar volume Vm is the Lorentz–Lorentz equation

Rm =(

ε − 1ε + 2

)Vm . (1)

This equation gives the average molar refraction for isotropic substances.Also, the molar refraction (Rm) of these glass samples has been estimated from

the following relationship linking Vm and E0, found empirically by Duffy19:

E0 = 20(

1 − Rm

Vm

)2

.

The values of molar refraction calculated from the refractive index (n) and energyband gap (E0) are designated as Rm(n) and Rm(E0) respectively, and are presentedin Table 1. It can be seen that good agreement exists between the values of Rm(n)and Rm(E0) and both increase with the Bi2O3 content.

The molar polarizability (αm) which is the sum of polarizability of oxide ions(αO2−) and cation polarizability (

∑αi), can be calculated taking into account the

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relationship

αm =(

34πNA

)Rm ,

where NA is Avogadro’s number, αm is in A3 and Rm is in cm3. The calculatedvalues of αm from Rm(n) and Rm(E0) are designated here as αm(n) and αm(E0)and are presented in Table 1. It can be seen that good agreement exists between thevalues of αm(n) and αm(E0) and both increase with the Bi2O3 content. It is notedthat the values of αm(n) are greater than those of αm(E0). This is due to fact thatαm(n) is the contribution of the electronic, atomic and dipolar polarization andαm(E0) is only due to the electronic polarization, and the electronic polarizationdominates the total polarization.20

3.6. Electronic polarizability of oxide ions (αO2−)

αO2− arises, in a crude way, from oxygen being able to exist not only in bridging ornonbridging form but also with a degree of negative charge, which may be varied.The electronic polarizability of oxide ions (αO2−) as originally proposed by Dimitrovand Sakka21 can be calculated on the basis of energy gap and refractive index datausing the equations

αO2−(E0) =[(

Vm

2.52

) (1 −

√E0

20

)−

∑αi

](NO2−)−1

,

αO2−(n) =[(

Rm

2.52

)−

∑αi

](NO2−)−1

,

where∑

αi denotes molar cation polarizability and NO2− denotes the number ofoxide ions in the chemical formula. The values of αBi = 1.508 A3 for Bi3+ ions,αLi = 0.024 A3 for Li+ ions, αB = 0.002 A3 for B3+ ions and αZn = 0.283 A3

for Zn2+ ions are used. The calculated values of αO2− estimated from refractiveindex and energy gap data, designated as αO2−(n) and αO2−(E0) are presented inTable 1. It can be seen that a good agreement exists between the values of αO2−(n)and αO2−(E0) and both increase with the Bi2O3 content.

3.7. Optical basicity (Λ)

The optical basicity (Λ) of an oxide medium as proposed by Duffy and Ingram22

is a numerical expression of the average electron donor power of the oxide speciesconstituting the medium. It is used as a measure of the acid base properties ofoxides, glasses, alloys, molten salts, etc. The optical basicity can be determinedexperimentally from optical absorption spectra of doped ions such as Tl+, Pb2+ orBi3+ and also from X-ray photoelectron spectroscopy (XPS). In acid base equilibriainvolving oxygen, three sorts of transitions are involved:

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(1) Transitions accompanied by an alteration of the co-ordination number of oxy-gen, but no change of the distributed oxidation state, for atoms with a highionization energy.

(2) Transitions which do not involve any change in the co-ordination number, but achange in the distributed oxidation state, for medium ionization energy atoms.

(3) Transitions which involve a change in both the co-ordination number and thedistributed oxidation state, leading to formation of isolated cations, for lowionization energy atoms.

In most circumstances, the characteristic process of acid base reactions in oxidesystems is “the transfer of an oxygen ion from a state of polarization to another”.In a chemically complex melt or glass the capability of transferring fractional elec-tronic charges from the ligands (mainly oxide ions) to the central cation dependsin a complex fashion on the melt structure, which affects the polarization stateof the ligand itself. The mean polarization state of the various ligands and theirability to transfer fractional electronic charges to the central cation are neverthelessrepresented by the “optical basicity” of the medium.23

An intrinsic relationship proposed by Duffy23 exists between electronic polariz-ability of the oxide ions αO2− and optical basicity of the oxide medium Λ, and isgiven by

Λ = 1.67(

1 − 1αO2−

).

This relation presents a general trend toward an increase in the oxide ion polar-izability with increasing optical basicity. The optical basicity values obtained byusing αO2−(n) and αO2−(E0) data are designated as Λ(n) and Λ(E0) respectivelyand are presented in Table 1. The Λ(n) and Λ(E0) values increase with increase inthe Bi2O3 content, indicating that the glasses containing high Bi2O3 contents showlarge optical basicities.

On the other hand, the so-called theoretical optical basicity (Λth) for the presentglasses can be calculated using the following equation, which is based on the ap-proach proposed by Duffy23:

Λth = XLi2OΛLi2O + XZnOΛZnO + XBi2O3ΛBi2O3 + XB2O3ΛB2O3 ,

where XLi2O, XZnO, XBi2O3 and XB2O3 are the contents of individual oxides inmol%. ΛLi2O, ΛZnO, ΛBi2O3 and ΛB2O3 are the theoretical optical basicity valuesassigned to oxides present in the glass. The values ΛLi2O = 1, ΛZnO = 8.2, ΛBi2O3 =1.19 and ΛB2O3 = 0.42 are used in the present study.22,24 The Λth values calculatedusing the above equation for the present glasses are given in Table 1. It can be seenfrom the table that the theoretical optical basicity increases slightly with increasingBi2O3 content. Good agreement exists between the values of Λ(n), Λ(E0) and Λth.Further, it should be noted that the theoretical optical basicity (Λth) shows larger

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values than the optical basicity Λ(E0) for glasses containing the least amounts ofBi2O3, which is similar to that found by Honma et al.25

3.8. Interaction parameter A

The interaction parameter is a quantitative measure for the interionic interactionof negative ions such as F− and O2− with the nearest neighbors. Dimitrov andKomatsu26,27 applied the interaction parameter A, proposed by Yamashita andKurosawa28 to describe the polarizability state of an average oxide ion in numeroussimple oxides and its ability to form an ionic covalent bond with the cation. Therefractive-index-based interaction parameter can be expressed as a sum of the partseach cation with the given oxide ion contributes to the total interaction for anaveraged cation anion pair in the glass matrix:

A = XLi2O

α−f − αO2−

2(αLi+ + α−f )(αLi+ + αO2−)

+ XZnO

α−f − αO2−

(αZn2+ + α−f )(αZn2+ + αO2−)

+ XBi2O3

α−f − αO2−

2(αBi3+ + α−f )(αBi3+ + αO2−)

+ XB2O3

α−f − αO2−

2(αB3+ + α−f )(αB3+ + αO2−)

,

where α−f = 3.921 A3, the electronic polarizability of the free oxide ion is used,

taking into account the value of ionic refraction of O2− theoretically determinedby Pauling29 and αO2− corresponds to αO2−(n). The energy-gap-based interactionparameter A(E0) can be expressed similarly. The refractive-index-based interactionparameter A(n) and the energy-gap-based interaction parameter A(E0) are deter-mined from above equation; they are presented in Table 1. It is seen that both A(n)and A(E0) decrease with increasing Bi2O3 content.

On the other hand, the so-called theoretical interaction parameter Ath is calcu-lated on the basis of the following equation27:

Ath = XLi2OALi2O + XZnOAZnO + XBi2O3ABi2O3 + XB2O3AB2O3 ,

where ALi2O, AZnO, ABi2O3 and AB2O3 are the interaction parameters of the re-spective oxides. The values ALi2O = 0.110, AZnO = 0.040, ABi2O3 = 0.008 andAB2O3 = 0.244 are taken from Ref. 26. The Ath values calculated using the aboveequation are given in Table 1. It can be seen from the table that the theoreticalinteraction parameter decreases with the increase in the Bi2O3 content.

It has been established that the Yamashita–Kurosawa interaction parameterA is closely related to the oxide ion polarizability and optical basicity of oxideglasses. That is to say, the larger the oxide ion polarizability and optical basicity,the smaller the interaction parameter. The correlation between A(n) and Λ(n) forthe present glasses is shown in Fig. 5 indicating almost a linear distribution of thebasicity with respect to the interaction parameter. The almost linear distributionof the optical basicity against the interaction parameter can be used as the optical

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Fig. 5. Relationship between optical basicity Λ(n) and interaction parameter A(n).

basicity scale for oxide glasses. The interaction parameter decreases with increasingoptical basicity. It is observed in the present study that the glasses containing largeamounts of Bi2O3 possess large values of Λ and low values of A.

Based on the good correlation among the electronic polarizability of oxide ions(αO2−), optical basicity (Λ) and interaction parameter (A), the various simple oxidescan be classified into three groups:

(1) Semicovalent acidic oxides, e.g. B2O3, P2O5, SiO2. These havelow αO2− — 1–2 A3;low Λ — < 1;large A — 0.20–0.25 A3;

(2) Normal ionic (basic) oxides, e.g. TeO2, In2O3. These haveintermediate αO2− — 2–3 A3;Λ — close to 1;intermediate A — 0.02–0.08 A3;

(3) Very ionic or very basic oxides, e.g. BaO, Bi2O3. These havehigh αO2− — > 3 A3;high Λ — > 1;small A — 0.003–0.008 A3.

The present glasses fall in group (2), i.e. normal ionic (basic) oxides.

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Fig. 6. Variation of DC electrical conductivity as a function of temperature in the present glasses.

3.9. DC electrical conductivity

The dependence of log σDC on the reciprocal temperature is shown in Fig. 6 for allinvestigated glass samples. However, all glasses follow the Arrhenius equation

σDC = σ0 exp(−EDC

kT

),

where EDC is the thermal activation energy, k the Boltzmann constant, σ0 the pre-exponential factor and T the absolute temperature. From Fig. 6, it is clear thatwith the increase in the lithium content the conductivity increases. Therefore, itis assumed that the conduction is ionic. The thermal activation energy is derivedfrom linear-fitting (first order) the above plots; see Table 1. It is seen that theactivation energy decreases with the increase in the conductivity in the presentglasses. According to the mechanism of ion transport, the conduction in glassescontaining lithium oxide is due to the successive jumping of Li+ ions from onenonbridging oxygen to another.30 The calculated data of Li+ concentration andthe polaron radius in the present glasses are listed in Table 1. It is obvious fromthe table that the Li+ concentration decreases with increase in the Bi2O3 content.The Li+ concentration and/or non-bridging oxygen number can both affect theglass conductivity. Therefore, in the present system, conductivity is due to Li+ andincreases with increase in the Li2O content.

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4. Conclusions

(1) The present glasses possess:(a) High density;(b) A higher refractive index than the conventional silicate and borate glasses;(c) A sharp UV cutoff;(d) A high glass transition temperature, and are stable up to 450◦C;(e) Low ∆Cp values — an indication of strong resistance to structural changes.

(2) These glasses are built from orthoborate, pyroborate, [BiO3], [BiO6] and [ZnO4]units.

(3) The refractive index, oxide ion polarizability and optical basicity increase withthe Bi2O3 content, whereas the interaction parameter decreases.

(4) The present glasses fall into the normal ionic (basic) oxide group.(5) The DC electrical conductivity increases and the activation energy decreases

with the increase in the Li2O content.

References

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PHYSICAL AND OPTICAL POLARIZABILITY AND TRANSPORT PROPERTIES OF BISMUTHATE GLASSES

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