Vibrational spectra and structure of mixed alkali borotungstate glasses: Evidence of mixed alkali effect

Vibrational Spectroscopy 71 (2014) 91–97

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Vibrational spectra and structure of mixed alkali borotungstate glasses: Evidence of mixed alkali effect A. Edukondalu ∗ , Syed Rahman, K. Siva Kumar, D. Sreenivasu Department of Physics, Osmania University, Hyderabad 500 007, India

a r t i c l e

i n f o

Article history: Received 30 October 2013 Received in revised form 26 December 2013 Accepted 25 January 2014 Available online 2 February 2014 Keywords: Amorphous FTIR Melt quenching Mixed alkali effect Raman scattering X-ray diffraction

a b s t r a c t Mixed alkali borotungstate glasses with xLi2 O–(30 − x)Na2 O–10WO3 –60B2 O3 (0 ≤ x ≤ 30) composition were prepared by melt quench technique. FT-IR and Raman spectroscopic studies were employed to investigate the structure of all the prepared glasses. Acting as complementary techniques, both IR and Raman measurements revealed that the network structure of the present glasses mainly based on BO3 and BO4 units placed in different structural groups. Raman spectra confirm the IR results regarding the presence of tungsten ions mainly as WO6 groups. In the present work, the mixed alkali effect (MAE) has been investigated in the above glass system using FTIR and Raman studies. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Raman and infrared spectroscopy have proven to be powerful and effective tools for characterizing the structure of local arrangements in glasses [1–3]. 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 nonlinear variations 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, while total alkali content in the glass being constant [4]. The boron atom in borate glasses is usually coordinated with either three or four oxygen atoms forming [BO3 ] or [BO4 ] structural units. These two fundamental units can be arbitrarily combined to form either the so-called super-structure or different Bx Oy structural groups like boroxol ring, pentaborate, tetraborate, diborate groups, etc. Alkali borate glasses with high ionic conductivity are receiving considerable attention because of their unique properties and their potential applications in the field of optoacoustical electronics, non-linear devices for frequency conversion in the ultraviolet region, piezoelectric actuator, as electrolytes for lithium batteries and good candidates for the optically induced elastoopticity [5]. In the alkali borate glass systems, each alkali

∗ Corresponding author. Tel.: +91 40 27095200; fax: +91 40 27090020. E-mail address: [email protected] (A. Edukondalu). 0924-2031/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.vibspec.2014.01.010

oxide is associated with a proportional quantity of B2 O3 so that the number of the structural units depends on both the nature and the total concentration of the added modifiers [6–10]. Tungsten ions are expected to have profound influence on physical properties of zinc bismuth phosphate glasses, for the simple reason that the tungsten ions exist in different valance states viz., W6+ , W5+ and also in W4+ state, regardless of the oxidation state of the tungsten ion in the starting glass batch as per the following thermo reversible disproportionate reaction: W5+ + W5+ ⇔ W4+ + W6+ Tungsten oxide participates in the glass network with different structural units like WO4 (Td ) and WO6 (Oh ) of W6+ ions and W5+ O3 − (Oh ) of W5+ ions [11]. The concentration of different structural groups of tungsten 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. W5+ ions are well known paramagnetic ions. The presence of W5+ ions confers to the glasses colors that change with composition [12,13]. To reveal the role of WO3 in the glassy borate network El-Kheshen and El-Batal [14] reported the effect of WO3 on spectroscopic, thermal, density properties and structure of high lead borate glasses. El Batal [15] performed optical, infrared, EPR and Raman spectral studies of some lithium borate glasses containing varying WO3 contents before and after gamma ray irradiation. Deal et al. [16] reported Raman and luminescence studies on

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452

2.92

448 444

2.88

440 2.84

436 432

2.80

428 424

Density (g/cc)

Na2 O–B2 O3 –WO3 glasses. Tungsten oxide with lithium borate glass could be used for electro-optic and related properties [17]. The crystallization, electrical conductivity, optical UV–vis, infrared, Raman and electron spin resonance spectra of WO3 based glasses such as WO3 –TeO2 , K2 O–WO3 –TeO2 and some alkali borotungstate glasses have been studied independently by different authors [18–24]. Many investigations have been reported on ternary alkali tungstate in phosphate, borate, tellurite, bismuthate and niobate glasses [25–28]. Salem et al. [29] presented physical, structural, optical and dielectric properties of Li2 O–Bi2 O3 –GeO2 –WO3 glasses. A recent investigation of density, glass transition temperature and electrical properties in Li2 O–Na2 O–WO3 (MoO3 )–P2 O5 glass system showed the mixed alkali effect [30,31]. More recently, Gaffar et al. [32] studied the structural and mechanical properties of Li2 O–Na2 O–K2 O–B2 O3 doped with cobalt. To the best of our knowledge, there are no reports on mechanical and structural studies on mixed alkali borotungstate glasses. In the present paper, the DSC and density data of xLi2 O–(30 − x)Na2 O–10WO3 –60B2 O3 (0 ≤ x ≤ 30 mol%) glasses are presented. In addition, the present study attempt to correlate the change in elastic data to the anticipated structural changes in the mixed alkali borotungstate glassy network using FTIR and Raman techniques. The compositional parameter RLi , which is defined as RLi = Li2 O (mol%)/(Li2 O + Na2 O) (mol%). RLi takes the values 0, 0.166, 0.33, 0.5, 0.66, 0.83 and 1.

Glass transition temperature(°C)

92

2.76

420 0.0

0.2

0.4

0.6

0.8

2.72

1.0

Compositional parameter RLi Fig. 1. Variation of density (filled circles) and glass transition temperature (empty squares) as a function of compositional parameter RLi .

0.51 0.50 0.49

Fragility

0.48

2. Experimental The present glasses with composition xLi2 O–(30 − x)Na2 O–10WO3 –60B2 O3 (0 ≤ x ≤ 30) were prepared through the standard melt quenching method. Details of the preparation and characterization of the glass specimens are given in ref. [33]. Infrared spectra of the powdered glass samples were recorded at room temperature in the range 400–1500 cm−1 using a spectrometer (Perkin-Elmer FT-IS, model 1605). These measurements were made on glass powder dispersed in KBr pellets. The room temperature Raman measurements were performed in the range 100–1800 cm−1 on a micro Raman system from Jobin-Yvon Horiba (LABRAM HR-800) spectrometer. The system is equipped with high stability confocal Microscope to focus the laser beam. Ar+ laser beam of 488 nm (E = 2.53 eV) was used for excitation. The incident laser power is focused in a dia. of ∼1–2 ␮m and a notch filter is used to suppress Rayleigh light. In the present system Raman shifts are measured with a precision of ∼0.3 cm−1 and the spectral resolution is of the order 1 cm−1 . 3. Results and discussion 3.1. Density and differential scanning calorimetry Fig. 1(a) presents the room temperature density of glasses measured by xLi2 O–(30 − x)Na2 O–10WO3 –60B2 O3 Archimedes method as a function of compositional parameter RLi . From the above figure, it is clear that the density varies non-linearly. The composition-dependent density appears to be ‘waveshaped’, featuring two maxima and one minima. This non-linear behavior is a consequence of mixed alkali effect. The density of the present glasses varies from 2.724 to 2.918 g/cm3 . The glass transition temperatures (Tg ) were determined based on the DSC curves using the onset method. The uncertainty in Tg is ±0.1 ◦ C. Fig. 1(b) plots the variation of glass transition temperature as a function of compositional parameter RLi . The glass transition temperature varies non-linearly and exhibited negative deviation indicating the presence of mixed alkali effect. The thermodynamical parameters such as glass transition temperature (Tg )

0.47 0.46 0.45 0.44 0.43 0.42 0.41

0.0

0.2

0.4

0.6

0.8

1.0

Compositional parameter RLi Fig. 2. Variation of thermodynamic fragility with glass composition.

and glass transition width Tg were determined. The thermodynamical parameters thus determined are given in Table 1. The negative deviation in glass transition temperature is explained due to the decrease in cross link density, hence decrease in Tg when compared with the end members. The Li+ and Na+ ion disturb the structural arrangement of the planar BO4 units considerably and favors the formation of non-bridging oxygen in glass system. The formation of non-bridging oxygen units cause the depolymerization of the oxide network. The decrease in Tg when compared with end members is attributed to the formation of non-bridging oxygen BO3 units in present glass system [33]. The thermodynamic fragility, F can be calculated by using the relation [34]: F=

0.151 −  , 0.151 + 

where  =

Tg Tg

(1)

The variation of thermodynamic fragility with glass compositional parameter is shown in Fig. 2 for the present glasses. The thermodynamic fragility is found to vary non-linearly with composition. In glass physics, fragility characterizes how rapidly the dynamics of a material slow down as it is cooled toward the glass transition. Materials with a higher fragility have a relatively narrow glass transition temperature width (Tg ), while those with low fragility have a relatively broad glass transition temperature

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93

Table 1 MDSC parameters such as glass transition temperature (Tg ), crystallization temperature (Tc ), glass stability S, heat capacity at Tg (Cp ), change in heat capacity (Cp ), glass transition temperature width (Tg ) and fragility (F) of the xLi2 O–(30 − x)Na2 O–10WO3 –60B2 O3 glass system. Parameters

x=0

x=5

x = 10

x = 15

x = 20

x = 25

x = 30

Tg (◦ C) (±1) Tc (◦ C) S (◦ C) Cp (J/mol ◦ C) Cp (J/mol ◦ C) Tg (◦ C) F

451 470 19 3.66 5.67 24.42 0.473

428 454 26 3.93 3.27 23.54 0.466

427 456 29 3.99 7.42 26.27 0.424

432 460 28 3.77 4.26 22.44 0.488

428 475 47 3.86 5.75 21.84 0.495

420 447 27 3.85 6.85 24.80 0.438

443 461 18 3.92 5.65 23.50 0.480

range. Physically, fragility may be related to the presence of dynamic heterogeneity in glasses. From Table 1 it is observed that the 10Li2 O–20Na2 O–10WO3 –60B2 O3 glass have the lowest fragility value of 0.424 and highest glass transition width of 26.3 ◦ C. Heat capacity (Cp ) can be measured using the ac calorimetric method [35] and modulated differential scanning calorimetry (MDSC). Although Debye theory describes the temperature dependence of heat capacity for metals and alkali halide crystals, it is not applicable for glasses or chain and layered structured polymers. A different approach to estimate the heat capacity from glass composition is examined by use of the three-band theory. According to Hirao et al. [36,37], the three-band theory considers heat capacity to be composed of separate contributions from the glass network formers and from the network modifiers. Therefore the empirical equation for the heat capacity of oxide glasses can be written as Cp = 3R



1 − exp

 −1.5T 

(2)

D

where R = 8.314 J/(mol-◦ C) is the gas constant, T is temperature and  D is the Debye temperature. The heat capacity at glass transition temperature can be evaluated from the above equation by substituting T with Tg and taking the Debye temperature of the respective glass (from Table 1). Thus evaluated heat capacity at glass transition temperature of the present glasses is given in Table 1. 3.2. IR spectra The IR absorption spectra of the present glasses were recorded in the range 300–2000 cm−1 . Fig. 3 shows the normalized FTIR absorption spectra of xLi2 O–(30 − x)Na2 O–10WO3 –60B2 O3 glasses. The observed infrared spectra of these glasses arise largely from the modified borate networks and are mainly active in the spectral range 400–1600 cm−1 ; therefore the spectra are shown in 350–1800 cm−1 range for better clarity. Each spectrum was deconvoluted by using 12 Gaussian functions considering peak assignment as reported earlier [30,38–40]. An example of the fitting for 5Li2 O–25Na2 O–10WO3 –60B2 O3 glass

composition is shown in Fig. 4. The infrared spectra of the present glasses show 11–12 absorption peaks. All the glass compositions show absorption peaks at 467, 540, 697, 762, 874, 940, 1020, 1080, 1228–1266, 1346–1380, 1438, and 1637 cm−1 . The peaks are sharp, medium and broad. Broad bands are exhibited in the oxide spectra, most probably due to the combination of high degeneracy of vibrational states, thermal broadening of the lattice dispersion band and mechanical scattering from powder samples. For the present glasses the IR band positions and area under the peak are presented in Table 2. According to the Krogh Moe’s the structure of the boron oxide glass consists of a random network of planer BO3 triangles with a certain fraction of six membered (boroxol) rings [41]. X-ray and neutron diffraction data suggests that glass structure consists of a random network of BO3 triangles without boroxol rings. The vibrational modes of the borate network are active mainly in three regions: the first region lies between 600 and 800 cm−1 and is due to bending vibration of various borate segments, the second region lies between 800 and 1200 cm−1 and is due to stretching vibrations of tetrahedral BO4 units and third region lies between 1200 and 1600 cm−1 and is due to stretching vibrations of B O in BO3 triangles [42–44]. Alkali oxides like Li2 O and Na2 O are well known glass modifiers and may enter the glass network by transforming sp2 planar BO3 units into most stable sp3 tetrahedral BO4 units and may also create non-bridging oxygens. Both BO3 and BO4 units co-exist in these glasses, which is evident from Fig. 4. The broad IR bands as shown in Fig. 4 are the overlapping of some individual bands with each other. Each individual band has its characteristic parameter such as its center which is related to some type of vibration of a specific structural group. A weak IR band around 467 cm−1 is assigned to the vibrations of Li cations through glass network. This IR band increases and then decreases in intensity as Li2 O content is increasing [45]. The present IR spectra showed nonexistence of band at 806 cm−1 , which reveals the absence of boroxol

0.40 0.35 0.30

Intensity (a.u)

x = 10

Intensity (a.u)

x = 20 x = 15 x = 30

0.25 0.20 0.15

x=5

0.10 x=0

0.05 0.00

1800 1600 1400 1200 1000

800

600

400

-1

wavenumber (cm ) Fig. 3. Infrared spectra of xLi2 O–(30 − x)Na2 O–10WO3 –60B2 O3 glass system.

2000

1800

1600

1400

1200

1000

800

600

400

-1

Wavenumber (cm ) Fig. 4. Deconvoluted FTIR spectrum of 5Li2 O–25Na2 O–10WO3 –60B2 O3 glass.

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Table 2 Deconvoluted parameters of the FT-IR spectra of xLi2 O–(30 − x)Na2 O–10WO3 –60B2 O3 glass system. x=0

x=5

x = 10

x = 15

x = 20

x = 25

x = 30

C

A

C

A

C

A

C

A

C

A

C

A

C

A

465 539 697 762 874 934 1020 1074 1266 1377 1407 1637

57 16 78 34 33 13 101 34 11 72 70 125

464 532 697 764 874 938 1015 1050 1266 1363 1460 1629

59 69 81 22 10 15 22 40 03 31 140 14

456 533 694 761 868 945 1010 1075 1228 1346 1433 1638

74 121 115 44 15 17 07 58 11 14 71 20

– 535 695 762 877 944 1015 1081 1234 1350 1438 1636

– 66 128 40 24 76 10 15 52 49 66 18

467 539 697 764 873 940 1017 1080 1261 1371 – 1632

63 192 119 82 51 86 24 24 138 149 – 22

467 – 698 764 876 940 1023 1075 1254 1360 – 1632

75 – – 32 67 47 59 46 147 160 – 32

444 540 695 – 879 – 1021 – 1246 1381 1452 1641

61 97 79 – 78 – 150 – 116 23 67 13

C, component band center (cm−1 ); A, relative area (%) of the component band.

699

880 878

697

876

-1

698

Peak position (cm )

-1

Peak position (cm )

(a)

874

696

872

695

870

694

868 693

0.0

0.2

0.4

0.6

0.8

1.0

Compositional parameter RLi

1024

1384

(b)

-1

-1

1020

Peak position (cm )

1376

Peak position (cm )

rings in glasses and hence it consists of only BO3 and BO4 groups [46]. In the present study, the IR peak around 697 cm−1 is assigned to bending vibrations of pentaborate groups, which are composed of BO4 and BO3 units in the ratio 1:4. The intensity of this band increases and then decreases with Li2 O content [47]. Since WO3 is a conditional glass former, with the substitution of WO3 with alkali oxides in borate glass network the intensity of vibrational band due to the BO3 groups is observed to increase at the expense of BO4 structural units [48]. In the present IR spectra the peak at around 940 cm−1 is assigned to the stretching vibrations of B O linkages in BO4 tetrahedra overlapping with the stretching vibrations of WO6 units [49]. Boudlich et al. [38] reported a mixture of WO4 and WO6 units at 880 and 900–950 cm−1 , respectively, in alkaline tungsten phosphate glasses. In the present study the IR peak around 874 cm−1 is assigned to starching vibration of tri-, tetra- and pentaborate groups and also due to the starching vibration of non-bridging oxygens of BO4 groups overlapping with the stretching vibrations of WO4 units [47]. A broad band around 1020 cm−1 is assigned to stretching vibrations of B O bonds in BO4 units from tri, tetra and pentaborate groups [48]. The weak peak at about 762 cm−1 can be attributed to B O B bending vibrations of BO3 and BO4 groups with W O W vibrations in the borate network [49]. The IR band around 1080 cm−1 is assigned to pentaborate groups [50]. The peak lying in 1346–1380 cm−1 is attributed to asymmetric stretching vibrations of the B O of trigonal (BO3 )3− units in meta-, pyroand orthoborate units [48]. The band around 1438 cm−1 is assigned to antisymmetrical stretching vibrations with three non-bridging oxygens of B O B linkages [51]. The weak band observed around 1637 cm−1 indicates a change from BO3 triangles to BO4 tetrahedra, and this peak may also be assigned to OH bending mode of vibrations [52]. The IR band in the range 1228–1266 cm−1 is assigned to B O stretching vibrations of (BO3 )3− unit in metaborate chains and orthoborates and these groups contain large number of nonbridging oxygens (NBO’s) [53]. This suggests the conversation of the BO4 tetrahedrons to the non-bridging oxygen containing BO3 triangles. The peak at around 540 cm−1 can be attributed to the borate deformation modes such as the in-plane bending of boronoxygen triangles [54]. Fig. 5(a) and (b) shows the compositional dependence of various peak position of the IR bands in the present study. The above figures depict a non-linear variation in peak positions for IR bands centered around 697, 874, 1020 and 1346 cm−1 exhibiting mixed alkali in present glasses. The assignments of IR bands are given in Table 3. To quantify the inter-alkali variation effect in the relative population of tetrahedral and triangular borate units we have calculated the fraction of four-coordinated boron atoms, N4 and

1368 1016 1360

1012

1008

1352

1344 0.0

0.2

0.4

0.6

0.8

1.0

Compositional parameter RLi Fig. 5. Compositional dependence of various IR peak positions in the present glasses.

three coordinated boron atoms containing NBOs, N3 which were estimated as follows [55]: N4 =

[A4 ] and N3 = 1 − N4 [A3 ] + [A4 ]

(3)

where A3 and A4 denotes the areas of BO3 units (the areas of component IR bands from 1200 to 1600 cm−1 ) and BO4 units (the areas of component bands from 800 to 1200 cm−1 ), respectively. The amount of four-coordinate boron atoms, N4 , and three-coordinate boron atoms is plotted as a function of interalkali variation in Fig. 6. It is clear from the above figure that the non-bridging oxygen

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Table 3 Assignment of infrared and Raman bands in the spectra of present glasses. Band positions (cm−1 )

Band assignments

IR

Raman

IR

Raman

∼467

∼329

Li cation vibrations

∼540

∼546

∼697

∼675

∼762

∼774

∼874

∼873

∼940

∼951

Borate deformation modes such as the in-plane bending of boran-oxygen triangles Bending vibrations of pentaborate groups, which are composed of BO4 and BO3 units in the ratio 1:4 B O B bending vibrations of BO3 and BO4 groups with W O W vibrations in the borate network Stretching vibration of tri-, tetra- and pentaborate groups also due to the starching vibration of non-bridging oxygen’s of BO4 groups overlapping with the stretching vibrations of WO4 units. Stretching vibrations of B O linkages in BO4 tetrahedra overlapping with the stretching vibrations of WO6 units Stretching vibrations of B O bonds in BO4 units from tri, tetra and pentaborate groups Pentaborate groups B O stretching vibrations of (BO3)3− unit in metaborate chains and orthoborates Asymmetric stretching vibrations of the B O of trigonal (BO3)3− units in meta-, pyro- and ortho-borate units Antisymmetrical stretching vibrations with three OH bending mode of vibration and change from BO3 triangles to BO4 tetrahedra

Bending vibrations of W O W in the WO6 units In plane bending mode of BO3− units 3

∼1020 ∼1080 ∼1266 ∼1346

∼1438

∼1464

∼1637

containing BO3 units, N3 , varies non-linearly. There is some sort of ordering that occurs which leads to a lessening of NBOs at RLi = 0.5. 3.3. Raman spectra Raman spectroscopy is one of the techniques used to investigate the structure of a glass. The room temperature Raman spectra of the present glass system is shown in Fig. 7. There are three regions clearly visible in the Raman spectra: (i) 250–500 cm−1 , (ii) 500–1100 cm−1 and (iii) 1250–2000 cm−1 . In all the present glasses studied the total alkali content is 30 mol%. At this concentration of the alkali content in borate containing glasses, the boroxol rings get converted mostly into pentaborate groups. This is observed clearly by the strong presence of peaks around 787 and 684 cm−1

units Denotes the existence of BO2 O3− 2

Ring breathing vibration of six membered ring contains both BO3 triangles and BO4 tetrahedral Stretching vibrations of W O W in the WO4 or WO6 units

W O− stretching vibrations in WO4 tetrahedra

Stretching of B O− bonds attached to large number of borate groups

resembling the localized breathing motions of oxygen atoms in the boroxol ring. Each Raman spectrum was deconvoluted by using 7–8 Gaussian functions to identify the exact position of peak and their intensity variation. An example of the fitting for 30Li2 O–10WO3 –60B2 O3 glass composition is shown in Fig. 8. All the glass compositions show Raman peak at around 333, 554, 684, 787, 873, 957 and 1464 cm−1 . The Raman band positions of all the glasses under study are given in Table 4. The vibrational Raman bands at 329–340, 873–904, and 951–960 cm−1 belonging to tungstate groups undergo complex changes. In the Raman spectra of all the studied glasses, there is a strong peak observed at ∼957 cm−1 which is assigned to W O− stretching vibrations in WO4 tetrahedral. The peaks around

0.725 0.44

0.425

0.399

0.393

0.392

x=20

0.650 0.625

0.375

0.600 0.575

0.350

x=15 x=0

x=25 x=10

0.550

0.325 0.300

x=5

0.675

N3

N4

0.400

0.427

0.700

Raman intensity (a. u)

0.450

x=30

0.328

0.0

0.2

0.324

0.4

0.6

0.8

0.525 1.0

2000

1800

1600

1400

1200

1000

800

600

400

200

-1

wavenumber (cm )

Inter alkali variation (RLI) Fig. 6. Plots of N4 and N3 as a function of inter alkali concentration.

Fig. 7. Room temperature Raman spectra of xLi2 O–(30 − x)Na2 O–10WO3 –60B2 O3 glass system.

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Raman intensity

of tetrahedral groups which involve three-dimensional bonding, which results in an increase of network coherence. The decrease in Tg is due to the formation of planar BO3 units containing nonbridging oxygens, which decreases the network coherence. The above IR and Raman discussion clearly indicates that the formation of BO4 group in Li2 O–Na2 O–WO3 –B2 O3 glasses has been observed with W O W vibration and conversion of WO4 to WO6 groups. These results agree with the density and glass transition temperature results due to compactness of glass structure, which results the conversion of BO3 into BO4 group. 4. Conclusions 2000 1800 1600 1400 1200 1000

800

600

400

200

-1

Wavenumber (cm ) Fig. 8. Deconvoluted Raman spectra of 30Li2 O–10WO3 –60B2 O3 glass. Table 4 Observed Raman band positions in xLi2 O–(30 − x)Na2 O–10WO3 –60B2 O3 glass system. Glass

Raman band positions (cm−1 )

x=0 x=5 x = 10 x = 15 x = 20 x = 25 x = 30

339 334 339 340 329 337 333

546 564 – 554 559 – 561

677 675 691 692 – 684 –

788 764 776 – 792 764 776

874 – 874 867 873 886 879

934 938 946 944 940 940 –

1407 1460 1433 1438 – – 1452

329–340 cm−1 are due to the bending vibrations of W O W in the WO6 units [57]. The Raman band around 873–904 cm−1 is assigned to stretching vibrations of W O W in the WO4 or WO6 units. In the Raman spectra of all the glassy specimens, there is a peak observed around 774–793 cm−1 which is characteristic of a six-membered ring with one or two BO4 tetrahedra. Earlier, Brill [56] assigned this peak to the formation of six-membered rings containing one BO4 tetrahedron, and the shift of this peak toward lower frequency has been assigned to six-membered rings with two BO4 tetrahedra. The six-membered rings with one BO4 tetrahedron can be in triborate, tetraborate or pentaborate forms, and rings with two BO4 tetrahedra can be in diborate, di-triborate or dipentaborate forms. In the studied glasses, the presence of Raman band in the range at 675–692 cm−1 has been attributed to the pentaborate groups in the borate glasses [45]. The Raman bands in the high frequency range 1429–1547 cm−1 has been assigned to stretching of B O− bonds attached to a large number of borate groups by Kamitsos and Chryssikos [57]. In the present study the 1464 cm−1 Raman band was assigned to stretching of B O− bonds attached to a large number of borate groups [59]. The Raman band in the range 546–564 cm−1 is assigned to in-plane bending mode of BO3 3− units [58]. Raman spectroscopic studies of alkali borate glasses for different concentrations of R2 O reveal the possibility of two chemical processes by which the alkali ion can be dispersed in the glasses. The first process, operative at lower concentrations of R2 O, leads to the formation of boron in fourfold coordination, i.e. BO4 − units, with the positive alkali ion (R+ ) adjacent to the negative BO4 − unit to provide local charge neutrality. The second process is the formation of a non-bridging oxygen (O− ) adjacent to the positive alkali ion. The dependence of Tg on N4 and N3 can also be understood from the two chemical mechanisms discussed above. It has been found that Tg first decreases and then increases with compositional parameter [59]. The increase of Tg is due to the formation

Mixed alkali tungsten borate glasses in the form of xLi2 O–(30 − x)Na2 O–10WO3 –60B2 O3 (0 ≤ x ≤ 30) were prepared, and their structural properties have been studied. The following conclusions were made: (i) Glass transition temperatures were determined and found to vary non-linearly with the compositional parameter exhibiting the mixed alkali effect. (ii) The infrared studies indicate the presence of BO3 , BO4 , WO3 , WO6 and Li units in the structure of the studied glasses. The intensities and their peak positions were affected by the alkali concentrations in each glass. The peak positions of a few IR bands showed non-linear variation with alkali content manifesting mixed alkali effect. (iii) The Raman spectra of the investigated glasses exhibit several bands which are attributed to BO3 , BO4 tetrahedra and pentaborate groups linked to BO4 tetrahedra. Raman spectra confirm the IR results regarding the presence of tungsten ions mainly as WO6 groups. (iv) The amount of four-coordinate boron atoms, N4 , and the nonbridging oxygen containing BO3 units, N3 , varies non-linearly as a function of inter-alkali variation. Acknowledgement One of the authors (A. Edukondalu) wishes to thank the Director, Consortium for Scientific Research, Indore, for providing MDSC and Raman facilities. References [1] V.N. Sigaev, I. Gregora, P. Pernice, B. Champagnon, E.N. Smelyanskaya, A. Aronne, P.D. Sarkisov, J. Non-Cryst. Solids 279 (2001) 136. [2] D. Ilieva, B. Jivov, G. Bogacev, C. Petkov, I. Penkov, Y. Dimitriev, J. Non-Cryst. Solids 283 (2001) 195. [3] R. Iordanova, Y. Dimitriev, V. Dimitrov, S. Kassabov, D. Klissurski, J. Non-Cryst. Solids 231 (1998) 227. [4] D.E. Day, J. Non-Cryst. Solids 21 (1976) 343. [5] Y.B. Saddeek, A.M. Abousehly, S.I. Hussein, J. Phys. D: Appl. Phys. 40 (2007) 4674. [6] J. Shelby, Introduction to Glass Science and Technology, The Royal Society of Chemistry, UK, 1997. [7] A. Varshneya, Fundamentals of Inorganic Glasses, Academic Press INC, New York, 1994. [8] T. Yano, N. Kunimine, S. Shibata, M. Yamane, J. Non-Cryst. Solids 321 (2003) 137. [9] J. Loörösch, M. Couzi, J. Pelous, R. Vacher, A. Levasseur, J. Non-Cryst. Solids 69 (1984) 1. [10] M. Kodama, J. Mater. Sci. 26 (1991) 4048. [11] A.G. Bergman, Z.N. Machavar, V.T. Maltsev, Zh.N. Khim, Russ. J. Inorg. Chem. 17 (1972) 3106. [12] I. Shaltout, Y. Tang, R. Braunstein, A.M. Abu-Elazm, J. Phys. Chem. Solids 56 (1995) 141. [13] C.C. de Araujo, W. Strojek, L. Zhang, H. Eckert, G. Poirier, S.J.L. Ribeirob, Y. Messaddeq, J. Mater. Chem. 16 (2006) 3277. [14] A.A. El-Kheshen, F.H. El-Batal, Indian J. Appl. Phys. 46 (2008) 225. [15] F.H. El Batal, Indian J. Pure Appl. Phys. 47 (2009) 471. [16] D. Deal, M. Burd, R. Braunstein, J. Non-Cryst. Solids 54 (1983) 207. [17] G.S. Murugan, K.B.R. Varma, Mater. Res. Bull. 34 (1999) 2201.

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