Journal of Alloys and Compounds 460 (2008) 699–703
Role of Bi2O3 content on physical, optical and vibrational studies in Bi2O3–ZnO–B2O3 glasses Shashidhar Bale a , Syed Rahman a,∗ , A.M. Awasthi b , V. Sathe b a
Department of Physics, Osmania University, Hyderabad, India b Consortium of Scientiﬁc Research, Indore, India
Received 2 April 2007; received in revised form 25 June 2007; accepted 25 June 2007 Available online 5 July 2007
Abstract Glasses with composition (85 − x)Bi2 O3 –xZnO–15B2 O3 with 15 ≤ x ≤ 40 have been prepared by conventional melt quench technique. Systematic variation in density and molar volume in these glasses indicates the effect of Bi2 O3 on the glass structure. The parameters glass transition temperature (Tg ), change in the transition temperature (Tg ) and specific heat capacity difference (Cp ) in the glass transition range were measured. The values of optical band gap and theoretical optical basicity are also reported. Raman and infrared studies have been employed on these glasses in order to obtain information regarding the competitive role of Bi2 O3 in the formation of glass network. © 2007 Elsevier B.V. All rights reserved. Keywords: Amorphous materials; Thermal analysis; Heat capacity; Raman studies; Infrared spectroscopy
1. Introduction The interest in heavy metal oxide (HMO) glasses is due to their long infrared (IR) cut-off and optical non-linearity [1,2]. Bismuth based oxide glasses attracted the scientific community due to their important applications in the field of glass ceramics, thermal and mechanical sensors, reflecting windows, etc. . Especially zinc oxide based glasses/ceramics have special applications in the area of varistors, dielectric layers and transparent dielectric and barrier ribs in plasma display panels [4–6]. Despite the fact that Bi2 O3 is not a classical glass former, due to high polarizability and small field strength of Bi3+ ions, in the presence of conventional glass formers (such as B2 O3 , PbO, SiO2 , etc.) it may build a glass network of [BiOn ] (n = 3, 6) pyramids . However, the structural role played by Bi2 O3 in glasses is complicated and poorly understood. This is because the [BiOn ] polyhedra are highly distorted due to the lone pair electrons. Several techniques have been employed in an attempt to identify the local environment of the different elements in bismuthate glasses. X-ray and infrared studies have shown that Bi3+ ions participate in the glass network structure above 45 mol%
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Bi2 O3 . The addition of Cd, Th, Li, Ba, Zn, and Fe oxides results in large glass formation domain. Raman and infrared spectroscopy provide important information regarding the local structure in vitreous and ceramic materials [9–15]. Hazra et al.  studied the role of lithium ions in lithium bismuthate glasses by Raman and IR techniques. Chahine et al.  reported infrared and Raman spectra of sodium–bismuth–copper phosphate glasses, which reflect the structural role of bismuth. Recently, Yin et al.  studied the structure and crystallization kinetics of Bi2 O3 –B2 O3 glasses and Radu et al.  employed infrared and Raman studies to investigate the structural units in bismuth based glasses. Xinyu et al.  studied the correlation among electronic polarizability, optical basicity and interaction parameter of Bi2 O3 and B2 O3 glasses. More recently Abdeslam et al.  investigated the ZnO–TeO2 –Bi2 O3 system. The aim of the present study is to obtain by means of Raman and infrared spectroscopy, specific data regarding the local structure of Bi2 O3 –ZnO–B2 O3 ternary glass system. Further, some physical properties such as density and glass transition temperature have also been studied. Also optical absorption studies were performed on these glass samples. The presence of two glass-forming oxides, the classical B2 O3 and the unconventional Bi2 O3 , increase the interest of the present study.
S. Bale et al. / Journal of Alloys and Compounds 460 (2008) 699–703
2. Experimental procedure Glass samples of compositions (85 − x)Bi2 O3 –xZnO–15B2 O3 (15 ≤ x ≤ 40) were prepared by melt quench technique using reagent grade chemicals Bi2 O3 , ZnO, and H3 BO3 . The mixture of these chemicals taken in porcelain crucibles 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 ensure homogeneous mixture. The clear liquid (free of bubbles) was quickly cast in a stainless steel mould kept at 200 ◦ C and pressed with another steel disc maintained at same temperature. The samples were transparent and their colour varied from yellow to light brown as the content of bismuth is increased. Thus obtained glasses were annealed at 200 ◦ C for a duration of about 12 h to remove thermal stress and strain. The amorphous nature of all the samples was confirmed by the absence of Bragg’s peak in X-ray diffraction pattern. The density (ρ) of the glass samples was determined at room temperature by Archimedes method with xylene (ρ = 0.86 g/cm3 ) as the immersion liquid. The glass transition temperature, Tg , was measured in all samples using a temperature modulated differential scanning calorimeter (TA Instruments, DSC 2910). All samples were heated at the standard rate of 10 ◦ C min−1 in aluminum pans. The optical absorption spectra of the present glass samples were recorded at room temperature using a double beam Shimadzu spectrometer (model UV3100) in the wavelength range 400–800 nm. The uncertainty in the observed wavelength is about ±1 nm. The room temperature Raman measurements were performed in the range 100–1700 cm−1 using a micro Raman system from Jobin-Yvon Horiba (LABRAM HR-800) spectrometer. The system is equipped with high stability confocal Microscope for Micro Raman 10×, 50×, 100× objective lens 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 diameter 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 . Infrared spectra of the powdered glass samples were recorded at room temperature in the range 400–2000 cm−1 using a spectrometer (Perkin-Elmer FT-IS, model 1605). These measurements were made on glass powder dispersed in KBr pellets.
3. Results 3.1. Density The measured density (ρ) of the glass samples, the molar volume (VM ), oxygen packing density (O), ionic concentration (N) and inter ionic distance (R) are presented in Table 1. Substitution of Bi2 O3 for ZnO increases the density. This could be explained
by considering the fact that the higher molecular mass of bismuth oxide as compared to other oxides, therefore, this is an expected result. The molar volume of the glasses also increases with Bi2 O3 content. 3.2. Differential scanning calorimetry Fig. 1 illustrates non-reversible heat flow (NRHF) and heat capacity (Cp ) signals of a typical glass sample. The inset in the figure explains the determination of glass transition temperature Tg , change in the transition temperature Tg and specific heat capacity difference Cp values. Tg , Tg and Cp in the glass transition region were determined  for all the glass samples and are presented in Table 1. As can be seen from Table 1, Tg shows nonlinear variation, while there is an overall decrease in Tg with the increase in Bi2 O3 content. This result is interpreted in terms of with the increase in the number of non-bridging oxygen atoms. This tendency is the same as those of Bi2 O3 –LiBO2 , Li2 O–Bi2 O3 –B2 O3 and ZnO–P2 O5 glass systems [23–25]. 3.3. Theoretical optical basicity The theoretical optical basicity (Λth ) for the glass system under study has been calculated using the relation  Λth = X(Bi2 O3 )Λ(Bi2 O3 ) + X(ZnO)Λ(ZnO) +X(B2 O3 )Λ(B2 O3 ) where X(Bi2 O3 ), X(ZnO), X(B2 O3 ) are the equivalent fractions of the different oxides, i.e., the proportion of the oxide atom they contribute to the glass system and Λ(Bi2 O3 ), Λ(ZnO) and Λ(B2 O3 ) are optical basicity values assigned to the constituent oxides. Here the values of Λ(Bi2 O3 ) = 1.19, Λ(ZnO) = 0.82 and Λ(B2 O3 ) = 0.425 have been taken from literature . The calculated values of Λth are presented in Table 1. 3.4. Optical absorption spectra Fig. 2 shows the optical absorption spectrum of a typical glass composition in the (UV–vis–NIR) region. A distinct cut-off was
Table 1 Physical parameters of the glass system (85 − x)Bi2 O3 –xZnO–15B2 O3 Parameter
x = 40
x = 35
x = 30
x = 25
x = 20
x = 15
Average molecular weight (g/mol) Density (g/cm3 ) Molar volume (cm3 /mol) Oxygen packing density (g-atm/l) Zn2+ ion concentration (×1021 /cm3 ) ˚ Inter ionic distance (A) Optical basicity Cut-off wavelength (nm) Optical band gap (eV) Glass transition temperature (◦ C) Tg (◦ C) Cp (J/mol ◦ C)
252.67 5.91 42.71 51.51 5.64 5.61 0.927 414 2.994 501 28 1.5
271.90 5.98 45.41 50.64 4.64 5.99 0.945 415 2.987 491 25 8.0
291.11 6.12 47.53 50.48 3.80 6.40 0.964 417 2.973 488 22 2.9
310.36 6.22 49.88 50.11 3.01 6.92 0.982 418 2.966 491 23 1.7
329.59 6.24 52.75 49.28 2.28 7.59 1.001 419 2.958 492 26 3.5
348.81 6.32 55.13 48.97 1.63 8.49 1.019 420 2.951 495 24 2.7
S. Bale et al. / Journal of Alloys and Compounds 460 (2008) 699–703
Fig. 1. Typical modulated DSC (NRHF and Cp ) results during a heating scan in the 70Bi2 O3 –15ZnO–15B2 O3 glass sample.
observed. All other glass compositions have similar behaviour. The cut-off wavelength (λc ) and the optical band gap (Eopt ) were determined for different glass samples and are presented in Table 1.
Fig. 3. Raman spectra of (85 − x)Bi2 O3 –xZnO–15B2 O3 glass system. The deconvoluted peaks are represented by dashes curves.
3.5. Raman spectra
Fig. 4 illustrates the infrared spectra of the present glass system. For all glass compositions, bands around 420–450 cm−1 , 480 cm−1 and 700 cm−1 were observed. The observed IR band
Fig. 3 shows the Raman spectra of present ternary glass system in the spectral range 100–1700 cm−1 consisting of broad peaks and shoulders. The broadening of peaks is due to the disorderness. To find out the exact mode of vibrations and also the Raman shifts, the spectrum with superimposed broad peaks was deconvoluted into six peaks using a Gaussian distribution. In the Raman spectra the strong band that appears at all concentrations around 133 cm−1 becomes stronger as Bi2 O3 content increases. Weak shoulder is observed for all compositions around 254 cm−1 and 586 cm−1 . The band around 394 cm−1 that grows in intensity shifts towards lower wavenumbers with increasing Bi2 O3 content. For all compositions a strong band around 925 cm−1 and a weak band around 1278 cm−1 are observed that grows in intensity and shifts towards lower wave numbers with increase in Bi2 O3 concentration.
Fig. 2. Optical absorption spectrum of a typical glass in the present study.
3.6. IR spectra
Fig. 4. Infrared spectra of (85 − x)Bi2 O3 –xZnO–15B2 O3 glasses.
S. Bale et al. / Journal of Alloys and Compounds 460 (2008) 699–703
at 900 cm−1 increases in intensity and shifts to higher wave numbers as Bi2 O3 content is increased. Also a convoluted band around 855 cm−1 and bands around 1258 cm−1 and 1319 cm−1 were observed.
observations were reported in BaO(SrO)–Bi2 O3 –B2 O3 glasses .
The vibrational Raman and infrared spectra of the investigated glasses with 45–70 mol% bismuth oxide content are dominated by bands associated to the structural units of the heaviest cation, Bi3+ . The Raman bands due to heavy metal oxides such as Bi2 O3 , can be classified into four regions: (1) low wave number Raman modes (<100 cm−1 ); (2) heavy metal ion vibrations in the range 70–160 cm−1 ; (3) bridged anion modes in the intermediate 300–600 cm−1 region; (4) non-bridging anion modes at higher wave numbers . An evidence for the existence of [BiO3 ] and [BiO6 ] polyhedra in the glass structure is the presence of band around 135 cm−1 in the Raman spectra . In the present Raman spectra this band was observed at 133 cm−1 and its intensity increases with Bi2 O3 content. Therefore we assume that Bi3+ cations are incorporated in [BiO3 ] and [BiO6 ] groups. This is confirmed by the IR spectra as the shift of the band from 480 cm−1 to 523 cm−1 which is due to the change of local symmetry in [BiO6 ] polyhedra as the bismuth oxide content decreases. Similar observations were found in bismuth-based glasses . Having in view that the bismuth group vibrations appear at significantly smaller wave numbers in comparison with the boron units vibrations, we can assume that in the 120–650 cm−1 spectral region of the Raman spectra, only the bismuth unit vibrations as bridged anion and angularly cation–anion–cation constrained modes appear [34,37]. Thus, the broad but strong band occurring in the present Raman spectra centered around 394 cm−1 can be attributed to Bi–O–Bi vibrations of both [BiO3 ] and [BiO6 ] octahedral units, while the shoulder at 586 cm−1 can be attributed to Bi–O− stretching vibrations in distorted linked [BiO6 ]. In the Raman spectra the band around 927 cm−1 can be ascribed to isolated orthoborate group  while the weak band from 1258–1278 cm−1 is due to BiO− (NBOs) of BiO3 units in the entire composition range. This is confirmed from the fact that in the present glasses, the band observed at 696–720 cm−1 in infrared spectra is assigned to B–O–B bending vibrations in [BO3 ] triangles  while the strong bands in the range 1258 cm−1 and 1319 cm−1 arises from Bi–O− stretching vibrations of BiO3 units. The band in the present IR spectra around 855 cm−1 represents the convolution of the absorbance bands reported for different bismuthate glasses at 840 cm−1 and 860 cm−1 , assigned to the total symmetric stretching vibrations of the [BiO3 ] and [BiO6 ] polyhedra, respectively [40,41]. In the present study this band is observed at 900 cm−1 which increases in intensity and shifts to 966 cm−1 as Bi2 O3 content is increased from 45 mol% to 70 mol%. The presence of Raman peak at 254 cm−1 and IR band in the range 400–600 cm−1 indicates the presence of Zn–O tetrahedral bending vibrations in the present glass system [42,43]. It is clearly observed that with increase in the zinc oxide content the peak around 254 cm−1 increases in intensity which indicates the formation of the ZnO4 units.
4.1. Optical absorption spectra The observed values of cut-off wavelength (λc ) shift towards higher wavelength as the content of Bi2 O3 increases. Values of Eopt are dependent in a systematic manner. Boron and bismuth are known to have more than one stable configuration, i.e., boron triangles and tetraborate for boron anomaly  and bismuth pyramidal [BiO3 ] and octahedral [BiO6 ] units for bismuth . At higher concentrations of Bi3+ the BO3 , BiO− of [BiO6 ] and [BiO3 ] units with non-bridging oxygens (NBOs) are formed. The electronic shell of O2− ion is affected by the highly polarizing action of modifying cation, i.e., Bi3+ . Therefore increase in Bi2 O3 concentration results in progressive increase of NBOs which in turn decreases the bridging oxygens. It is known that the NBOs are bonded to only one framework cation, B3+ in three or four co-ordination and bridging oxygens are bonded to two network cations . Therefore it can be understood that, with the increase in the Bi2 O3 content Eopt decreases. Thus, creation of NBOs seem to be the reason for λc shifting towards longer wavelength. It has been reported earlier  that bismuth oxide in the form of thin film shows Eopt in the range 2.5–2.6 eV. Now, if viewed with respect to ZnO, it is evident that introduction of Zn2+ ions in the bismuth oxide network increases optical band gap up to 2.99 eV. However presence of sharp cut-off in these glasses may make them useful in spectral devices as optical windows. 4.2. Theoretical optical basicity Optical basicity expresses the basicity of glass in terms of electron density carried by oxygen. Many physical and chemical properties of oxide glasses have been related to basicity. From Table 1 it can be observed that the calculated theoretical optical basicity Λth values increase with increase in Bi2 O3 content. This may be understood according to the relation , Λth = 1.67 1 −
where α2− 0 is the oxide ion polarizability. This equation shows that with increase in polarizability, the basicity also increases. It is well known in the literature that Bi3+ ions are highly polarizable, which is due to their large ionic radii and small cation unit field strength and also Bi3+ ion possesses a lone pair of valence shell. Further, the optical band gap in the present glass system is found to decrease with increase in optical basicity (Table 1). Therefore, with increase in the Bi2 O3 content, the number of non-bridging oxygens increase which cause a decrease in Eopt and increase in optical basicity. Similar
4.3. Raman and IR spectra
S. Bale et al. / Journal of Alloys and Compounds 460 (2008) 699–703
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