Physical, Optical and Structural Studies on binary mixed Alkali-

Alkaline Earth Oxide Borate Glasses

G. Srinivas, J. Shiva Kumar, Md. Shareefuddin, M.N Chary, R. Sayanna*.

Department of physics, Osmania University, Hyderabad, India.

E-Mail: srinu123@gmail.com

Abstract:

Mixed alkali alkaline earth oxide borate glasses of composition (25-x)

Li2O-xK2O-12.5BaO-12.5MgO-50B2O3 with (x= 0, 5, 10, 15 and 20 mol %) were

prepared by melt quenching method. The X-ray diffractograms of all the glass

samples were recorded at room temperature. Peak free X-ray spectra confirmed

the amorphous nature of the all glasses. Modulated differential scanning

calorimeter (MDSC) was used to determine the glass-transition temperature

(Tg). The mixed alkali effect (MAE) has been investigated in the glass system

through density, molar volume, modulated differential scanning calorimeter

(MDSC), FTIR, and optical absorption studies. The density and glass-transition

temperature (Tg) of the present glasses have shown nonlinear variation with

composition supporting the existence of MAE. FTIR spectra occurs due to change

in the dipole moment of the molecule. It involves the twisting, bending,

rotating and vibrational motions in molecules. The bands appeared ̴ 710cm−1 are

due to bending vibrations of various borate segments, bands found around

965cm−1 might be due to the B-O stretching vibration of tetrahedral BO4 units

and the bands found in the range 1200cm−1 to 1500cm−1 may be due to B-O

stretching vibration of trigonal BO3 units. From the optical absorption

studies the values of the optical band (Eopt) for indirect transition and

Urbarch energy (∆E) have been evaluated. The values of Eopt and ∆E also show

nonlinear behaviour with the compositional parameter, which also support the

MAE in the present glass system.

Keywords: Borate glasses, modulated differential scanning calorimeter (MDSC),

FTIR and optical absorption.

INTRODUCTION

Borate glasses are one of the most popular and excellent glass forming

materials. Upon addition of alkali and alkaline oxides to B2O3, the covalent

network of amorphous boron oxide causes considerable changes, resulting in the

creation of anionic sites that accommodate the modifying alkali cations.

Borate glasses containing alkali or alkali earth oxides exhibit high

mechanical strength and are relatively moisture-resistant when compared with

the pure borate glasses. Some of their applications include phosphors, solar

energy converters and optical devices [1]. When two types of alkali ions are

introduced into a glassy network, a phenomenon known as mixed alkali effect

(MAE) is observed. It represents the non-linear variation in many physical

properties associated with the alkali ion movement and structural properties,

when one type of alkali ion in an alkali glass is gradually replaced by

another, total alkali content in the glass being constant [2] Mixed alkali

effect is frequently occurs among properties associated with cations movements

such as, ionic conductivity, dielectric loss and alkali diffusion co-efficient

[3].

According to Narayana Reddy and Sreekanth Chakradhar [4], in borate glasses,

the structure of pure vitreous, B2O3 consists of a random network of boroxyl

rings and BO3 triangles connected by B–O–B linkage. The addition of alkali

oxides modifies the boroxyl rings; complex borate groups with one or two four

co-ordinate borate atoms are formed. Fast ion conducting lithium based borate

glasses have a variety of technological applications [5]. The small size,

light weight and highly electropositive character of lithium ions gives rise

to high voltage and high energy density microbatteries. Apart from these

technological applications, structural studies on these glasses help to

understand how the structure of the host glass in which the ions present,

influences their mobility. There have been several structural studies, which

deal with the structure of lithium based borate glasses [6].Borate glasses

with great variability in composition, structure, and properties have a

promising future in the fields of linear and nonlinear optics. Borate glasses

can be used as thermal insulators and textile fiberglass. Bearing in mind the

numerous applications of borate glasses, the authors are interested in

studying the MAE in glasses with two types of alkali ions. This work presents

the results of investigations on the variation of the density and glass-

transition temperature in (25-x)Li2O-xK2O-12.5BaO-12.5MgO-50B2O3with (x= 0, 5,

10, 15 and 20 mol %) glasses as a function of the compositional parameter RLi

In our papers, we reported on measurements of density, glass-transition

temperature, optical absorption of mixed alkali-alkaline borate glasses.

Experimental

Preparation Method

The oxide borate glasses of (25-x) Li2O-xK2O-12.5BaO-12.5MgO-50B2O3 with

(x= 0, 5, 10, 15 and 20 mol %) were prepared by melt quenching method. The

starting materials were Merck (GR grades) H3BO3, lithium oxide (Li2O),

potassium carbonate (K2CO3), magnesium oxide (MgO), barium oxide (BaO).The

calculated amounts by mol% of these compounds are thoroughly mixed and grind

in Agate mortar with pestle. The ingradients were taken in to a porcelain

crucible and melted in an electrically heated furnace maintained at 11500C for

40 minutes. To obtain homogeneity, the melt was shaken frequently. The

homogeneous melt was then quickly poured onto a stainless steel plate and

pressed with another stainless steel plate, both being maintained at 2000C.

The glass samples so obtained were subsequently annealed at 3800C for 12hrs to

relieve the strains. The glass samples thus obtained were clear, transparent

and bubble free. The absence of any Bragg peaks in the x-ray diffraction

pattern confirmed that the prepared glasses were amorphous. The density

measurements were carried out at room temperature using the Archimedes method

with xylene (density = 0.86 g/cc) as immersion liquid. The uncertainty in

density measurement is ±0.001.The thermal behavior of the glass samples were

investigated using a modulated differential scanning calorimeter (MDSC) (model

2910; TA Instruments) with a heating rate of 100C/min. The uncertainty in

glass-transition temperature is ±10C. Infrared spectra of the powdered glass

samples dispersed in ATR pellets were recorded at room temperature in the

wavelength range from 600 cm─1 to 2000 cm─1 using a Bruker spectrometer.

Optical absorption spectra of all the glass samples were recorded on a

(2092PIUV/VIS-Analytical Technologies Limited) UV–VIS spectrometer in the

wavelength region 200 nm to 1000 nm. The accuracy of measured band position is

±1 nm.

Result and Discussion

X–Ray Diffraction

Bruker D8 Advance X-ray diffractometer with copper Kα tube target with

nickel filter operated at 40 kV, 30 mA was used to record the X-ray

diffractograms. All the X-ray diffractograms were recorded at room

temperature. Peak free X-ray diffractograms of all the glass samples confirmed

the amorphous nature of

the glasses. Fig.1 shows

the X- ray diffractogram of

the present glass

samples.

20 40 60 80

0

1000

2000

3000

4000

5000

6000

7000

8000

x=20

x=15x=10

x=5

Intens

ity(A

rb.U

)

2(deg)

x=0

Fig.1 shows the XRD patterns of (25-x) Li2O-xK2O-12.5BaO-12.5MgO-50B2O3

glasses.

Density

Fig.2 shows the nonlinear variation of molar volume (Vm) and density

(ρ) of the mixed alkali-alkaline (25-x) Li2O-xK2O-12.5BaO-12.5MgO-50B2O3 with

(x= 0, 5, 10, 15 and 20 mol %) borate glasses as a function of Li2O content.

Fig. 2.Variation of Molar volume and density as a function of Li2O content.

4 6 8 10 12 14 16 18 20 22 24 26

24

26

28

30

Li2O (m ol% )

Molar volum

e(V M)

(cc/mol)

V M

2.68

2.70

2.72

2.74

2.76

2.78

Density (g/cc)

From the fig.2. It is clear that the density and molar volume varies

nonlinearly. The composition dependent density appears to be ‘‘wave’’ shaped,

featuring two maxima and two minima. The molar volume (Vm) is M/ρ, where M is

the molecular weight of the glass expressed as the mole fractions of the

oxides multiplied by their molecular weights. The density and molar volume

parameters of the present glasses are presented in Table I. In the present

glass system, the molar volume of the glasses decreases as the concentration

of Li2O increases.

Table I. Physical and optical parameters of (25-x) Li2O-xK2O-12.5BaO-12.5MgO-

50B2O3 glasses.

Glass composition LKBMB-1 x = 20

LKBMB-2 x=15

LKBMB-3 x=10

LKBMB-4x=5

LKBMB-5 x=0

Average molecularweight(M)(g/mol)

79.348 76.132 72.916 69.7 66.484

Density(ρ)(g/cc) 2.694 2.735 2.725 2.701 2.779

Molar volume (VM)(cc/mol)

29.45 27.836 26.758 25.805 23.923

Oxygen Packing Density(g.atm/l)

67.9 71.84 74.74 77.5 83.59

Direct optical band gap(eV)

3.702 3.638 3.671 3.655 3.639

Indirect optical band gap(eV)

3.373 3.355 3.27 3.303 3.373

Urbach energy∆E(eV)(±0.02)

0.245 0.233 0.206 0.212 0.241

Cut- off wavelength (nm)

351 360 365 366 362

Glass transition temperature(Tg)(0C)

466 471 463 470 468

Specific Volume (Vs) (cm3/g)

0.371 0.365 0.367 0.37 0.359

Glass-Transition Temperature

Modulated differential scanning calorimetry (MDSC) was used to

characterize the glasses and to determine the glass-transition temperature

(Tg). Fig. 3 presents the MDSC thermograms of the present glass samples. The

glass-transition temperatures (Tg) are given in Table 1.

350 400 450 500 550

-2.2

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

X= 10X= 15

X= 5

X= 20

End

o E

xo

T em perature (°C )

X= 0

Fig. 3. MDSC thermograms of (25-x) Li2O-xK2O-12.5BaO-12.5MgO-

50B2O3 glasses.

The variation of the glass-transition temperature as a function of Li2O

content is shown in Fig.4. The glass-transition temperature varied nonlinearly

and exhibited negative deviation from linearity, indicating the influence of

the mixed alkali effect (MAE) on the thermal properties within the glass-

transition region.

Fig .4. Glass-transition temperature as a function of

Li2O content.

Density and Tg

Fig.5 shows the variation of glass-transition temperature and density as

a function of Li2O content. In general the density of glass is explained in

terms of competition between the masses and sizes of the various structural

groups present. Accordingly, the density is related to how tightly the ions

5 10 15 20 25462

464

466

468

470

472

Glass

tran

sitio

n te

mpe

rartu

re (°

C)

Li2O (m ol% )

and ionic groups are packed together in the substructure. In mixed-alkali

glass systems, the density may exhibit either positive or negative deviation

from linearity. Stevels [7] visualized the glass structure as containing

interstices of varying diameter so that alkali ions of different sizes are

more easily accommodated than when all the alkali ions have the same size.

Thus, in Stevels’ theory, the higher density of mixed-alkali glasses is due to

a more efficient packing. In the present (25-x) Li2O-xK2O-12.5BaO-12.5MgO-

50B2O3glass system, when lithium ions are replaced by potassium ions of larger

ionic radius, the glass system expands to accommodate the potassium ions. The

density exhibits positive deviation from linearity, which is supported by

Stevels’ theory. In mixed alkali-alkaline borate glass systems, the glass-

transition temperature (Tg) can exhibit positive or negative deviation from

linearity. The glass-transition temperature of borate glasses is linked with

the atomic arrangements present in the glass system. Shelby [8] Button et al

[9] and Martin and Angell [10] linked the decrease in Tg with the growth of

boron with non-bridging oxygens in the high-alkali region. The decreasing in

Tg value of the present glass systems may be attributed to the formation of

non-bridging oxygens.

4 6 8 10 12 14 16 18 20 22 24 26462

464

466

468

470

472

Li2O (m ol% )

Glass

tran

sitio

n te

mpe

rartu

re (°

C)

T g

2.68

2.70

2.72

2.74

2.76

2.78

Density (g/cc)

Fig.5. Variation of Glass-transition temperature and density as a function of

Li2O content.

FT-IR studies

FTIR spectra gives significant information about molecular vibrations

as well as rotations associated with covalent bonding. The FTIR spectra occurs

due to change in the dipole moment of the molecules. The spectra involves

twisting, bending, rotation and vibrational motions in molecules. Fig.6. sows

FTIR spectra of (25-x) Li2O-xK2O-12.5BaO-12.5MgO-50B2O3 with (x= 0, 5, 10, 15

and 20 mol %) glasses at room temperature in the wavelength range of 600–

2000cm−1. Table.2 shows absorption bands of all the glass samples. From the

Fig.6.it was observed that LBMB-5 glass system has shown feeble shallow bands

at 705cm−1,778cm−1,902cm−1, 1092cm−1 and sharp, deep absorption bands at 1218cm−1,

1366cm−1, 1437 cm−1, and 1738 cm−1. The remaining four glass systems LKBMB-1,

LKBMB-2, LKBMB-3 and LKBMB-4 have shown a sharp deep band around 710cm−1, and

two deep broad bands ̴ 965cm−1, 1365 cm−1. In addition to these bands slight

kinks at 1216cm−1, 1470cm−1 were appeared around 1365cm−1 broad deep absorption

band. A sharp deep band found at 1738cm−1 in LBMB-5 glass appeared as small

shallow band in remaining four glass samples.

From the Fig.6.(Table.2) it was observed that the absorption bands in

LBMB-5 glass sample are sharp and deep while other four glass compositions

LKBMB-1 to LKBMB-4 have shown broad shallow absorption bands. The 710cm−1

band found in all four glass samples (LKBMB-1 to LKBMB-4) resulted into two

shallow bands around 705cm−1,778cm−1 in LBMB-5glass sample and also the broad

absorption band found ̴ 965cm−1 in these four glasses seens to be resulted into

two shallow bands at 902cm−1, and 1092cm−1 in LBMB-5 glass sample. But the

sharp peaks found ̴ 1218cm−1, 1366cm−1, 1437cm−1, and 1738cm−1 in LBMB-5 glass

sample appeared as kinks around 1365cm−1 broad absorption band in all four

glasses.

From the above discussion it is clear that K2O absent composition

i.e. LBMB-5 glass have shown clear sharp peaks where as other four glasses

LKBMB-1 to LKBMB-4 in which K2O content is present have shown broad shallow

absorption bands and some bands appeared as kinks, this may be due to the

mixed alkali effect. The band appeared ̴ 710cm−1 are due to bending vibrations

of various borate segments, bands found around 965cm−1 might be due to the B-O

stretching vibration of tetrahedral BO4 units and the bands found in the range

1200cm−1 to 1500cm−1 might be resulted due to B-O stretching vibration of

trigonal BO3 units [11]. The bands found around 1738cm−1 in all the glass

samples are attributed to the bending and stretching vibrations of B-O-B in

[BO3] triangles [12-16]. The bands found around 3420cm−1 in all the glass are

assigned to presence of OH groups.

Fig.6. FT-IR spectra of (25-x) Li2O-xK2O-12.5BaO-12.5MgO-50B2O3

glasses.

Table 2. The FTIR band positions of the (25-x) Li2O-xK2O-12.5BaO-12.5MgO-50B2O3 glass

system.

Sample code Absorption peaks (cm─1)Bending

vibrations

of borate

segments

B-O stretching

vibration of

tetrahedral

BO4 units

B-O stretching

vibration of trigonal

BO3 units

LKBMB-1 716 900 968 - 1246 1367 1442

LKBMB-2 714 - 964 - 1216 1368 1446

LKBMB-3 710 - 969 - 1231 1369 1472

LKBMB-4 705 - 965 - 1254 1395 1490

LBMB-5 705 778 902 1092 1218 1366 1437

Optical Absorption

The optical absorption spectra of (25-x) Li2O-xK2O-12.5BaO-12.5MgO-

50B2O3 glasses are shown in Fig. 7.

Fig. 7. Optical absorption spectra of (25-x) Li2O-xK2O-12.5BaO-12.5MgO-

50B2O3 glasses

From the spectra optical absorption coefficient α (υ) near the fundamental

absorption edge of the curve (Fig. 7) was determined using the relation.

α(ϑ)=1dlog¿),

(1)

where I0 andIt are the intensities of the incident and transmitted beams,

respectively, and d is the thickness of the glass sample. The factorlog¿)

corresponds to the absorbance. Davis and Mott [17] and Tauc and Menth [18]

relate this data to the optical band gap (Eopt) through the following general

relation proposed for amorphous materials:

α(ϑ)=B (hϑ−Eopt ) nhϑ, (2)

¿¿

where B is a constant andhϑ is the incident photon energy. The indexn = 1/2 for direct allowed transitions and n = 2 for indirect

transitions. From Equation (2) and (3) by plotting (αhϑ )2, (αhϑ )1 /2as a

function of photon energyhϑ, optical energy band gap (Eopt) for direct and

indirect transitions can be estimated respectively. The respective values

of Eopt were obtained by extrapolating to (αhϑ )2= 0 for direct transition

s∧(αhϑ )12= 0 for indirect transitionFig.8a, 8b and 9 represents the Tauc

plots [(αhϑ )2, (αhϑ )1 /2versushϑ] for the present all glass samples.

Fig. 8a, 8b Tauc plots [(αhϑ )2 versus hϑ] for the (25-x) Li2O-xK2O-12.5BaO-12.5MgO-50B2O3 glasses.

Fig. 9. Tauc plots [(αhϑ )1 /2 versushϑ] for the (25-x) Li2O-xK2O-12.5BaO-12.5MgO-50B2O3 glasses.

From Fig.9 the values of the indirect inter optical band

transitions between conduction and valence bands were obtained by

extrapolation of the linear region of the Tauc plots and are presented in

Table I. Similar behavior was also observed by other workers [19, 20].

The variation of the direct and indirect band gap energy (Eopt) with the

compositional parameter is shown in Fig. 10 and 11.

1 2 3 40

1

2

3

4

5

(h)1/

2 (cm-1 eV

)1/2

h (eV )

X =20% X =15% X =10% X =5%X =0 %

Fig.10.Compositional dependence of the direct optical

band

5 10 15 20 253.26

3.28

3.30

3.32

3.34

3.36

3.38

Indire

ct b

and ga

p(eV

)

Li2O (m ol% )

Fig. 11. Compositional dependence of the indirect

optical band gap.

It is observed that Eopt varies nonlinearly with composition. The

main feature of the absorption edge of amorphous materials is an

exponential increase of the absorption coefficient α (ϑ)with the photon

energyhϑ, as given by the Urbach rule [21].

α (ϑ)=Cexp( hϑ∆E )(3)

where C is a constant and ∆E is the Urbach energy, which is a measure ofband shift. Fig.12.shows the variation of ln (α) as a function of photonenergyhϑ. The values of the Urbach energy (∆E¿ were determined by taking the

reciprocal of the slopes of the linear portion of the ln (α) versus hϑcurves. Fig.12 illustrates a fitted curve used to determine the Urbach energy

for a typical glass sample. The Urbach energy values of the present glass

samples are presented in Table I.

Fig. 12. Urbach energy of the ((25-x) Li2O-xK2O-12.5BaO-12.5MgO-50B2O3

glasses.

Fig. 13.Compositional dependence of the Urbach energy (∆E) in the present glasses.

5 10 15 20 250.20

0.21

0.22

0.23

0.24

0.25

Li2O (m ol% )

Urb

ach

ener

gy (e

V)

The compositional dependence of the Urbach energy ∆E with

compositional parameter Li2O content is shown in Fig. 13. From this Figure it

is observed that the nonlinear variation of the Urbach energy with the

compositional parameter indicates the existence of the mixed alkali-alkaline

effect.

Conclusions

The oxide borate glasses of composition (25-x) Li2O-xK2O-12.5BaO-

12.5MgO-50B2O3 revealed the composition dependent density appears to be wave

shaped, featuring two maxima and two minimum. The glass-transition temperature

(Tg) values from 4710C to 4630C in the present glass system is attributed to

the formation of non-bridging oxygen.

I. From FTIR spectra it was observed that the absorption bands in LBMB-5

glass sample are sharp and deep while other four glass compositions LKBMB-1

to LKBMB-4 have shown broad absorption bands. The 710cm−1 band found in all

four glass samples (LKBMB-1 to LKBMB-4) resulted into two shallow bands

around705cm−1,778cm−1 in LBMB-5glass sample and also the broad shallow

absorption band found ̴ 965cm−1 in these four glasses seens to be resulted into

two shallow bands at 902cm−1, and 1092cm−1 in LBMB-5 glass sample. But the

sharp peaks found ̴ 1218cm−1, 1366cm−1, 1437cm−1, and 1738cm−1 in LBMB-5 glass

sample appeared as kinks around 1365cm−1 broad absorption band in all four

glasses.

II. From the above discussion it is clear that K2O absent composition i.e.

LBMB-5 glass have shown clear sharp peaks where as other four glasses LKBMB-1

to LKBMB-4 in which K2O content is present have shown broad shallow

absorption bands and some bands appeared as kinks, this may be due to the

mixed alkali effect. The band appeared ̴ 710cm−1 are due to bending vibrations

of various borate segments, bands found around 965cm−1 might be due to the B-O

stretching vibration of tetrahedral BO4 units and the bands found in the

range 1200cm−1 to 1500cm−1 are resulted due to B-O stretching vibration of

trigonal BO3 units.

III. From the optical absorption studies the values of the optical band (Eopt)

for indirect transitions between conduction and valence bands and Urbarch

energy (∆E) have been evaluated. The values of Eopt, ∆E and density also shownonlinear behaviour with the compositional parameter, which also support the

presence of mixed alkali-alkaline effect (MAE) in the present glass system.

Acknowledgements

One of the authors, G.SRINIVAS, thanks the DST New Delhi for awarding

Fellowship under DST-PURSE programme and RFSMS (Junior Research Fellow)

Programme UGC. The authors thanks UGC-DAECSR Indore-Centre, Head Department of

Physics O.U, CFRDOU Director for his encouragement and proving facilities.

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