Table of Contents
ISRN Optics
Volume 2012, Article ID 193185, 7 pages
http://dx.doi.org/10.5402/2012/193185
Research Article

Effect of Stepwise Replacement of LiF by Bi2O3 and of Annealing on Optical Properties of LiF·B2O3 Glasses

1Department of Physics, Maharshi Dayanand University, Rohtak 124 001, India
2Department of Electronic Science, Kurukshetra University, Kurukshetra 136 119, India

Received 29 December 2011; Accepted 29 January 2012

Academic Editors: M. Farries, L. R. P. Kassab, and Y. Tsuji

Copyright © 2012 Susheel Arora et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Bismuth fluoroborate glasses with compositions 𝑥Bi2O3(40𝑥)LiF60B2O3(𝑥=0,5,10,15,and20) are synthesized by melt-quench method. Optical characterization was carried out to examine variation of optical band gap energy (𝐸𝑔) and Urbach energy (𝐸𝑈) with respect to the concentration. It reflects the effect of stepwise replacement of non-oxide and less polarizable LiF by an oxide and more polarizable (Bi2O3) group on the optical properties of the samples. The value of 𝐸𝑔 decreases with increase in concentration of Bi2O3. The samples were subjected to annealing at different temperatures (300°C, 350°C, and 400°C), and the effect of annealing on the optical properties of various samples was again studied. Annealing affects remarkably the values of 𝐸𝑔 and 𝐸𝑈 in the samples with 𝑥=0.

1. Introduction

Fast cooling of the molten form of a substance makes it difficult to form a crystalline phase. Instead, it forms the amorphous phase of the substance. Glassy nature of the substance, philosophically, is just the lack of time required for the atoms or molecules to form a long range order. The sheet-like structure of boron-oxygen triangles in borate glasses, with their ability to connect themselves to form a network, has popularized B2O3 as one of the best glass former [1, 2]. The glass formation demands a random arrangement of various atomic and molecular species which easily form in borate glasses. A number of modifications in the properties of the borate glasses, with the addition of alkali halides, have been reported so far [39]. The inclusion of LiF in the borate glass network brings out some structural changes [3], which in turn become responsible for the change in various properties. Some oxygen atoms get replaced by fluorine ions in the network to form new units like BO2F, BO2F2, BOF3, and BO3F [46]. Also, there may be an increase in the number of polyhedral groups of boron and oxygen, which in turn increases the number of nonbridging oxygen atoms [79]. Because of high third-order nonlinear optical susceptibility of Bi2O3, caused by high density and refractive index [10, 11], it has numerous nonlinearity applications such as optical switching [12, 13], supercontinuum generation [14], wavelength conversion [15], and so forth. Also, bismuth ion acts as an efficient luminescent activator with applications in lasers as a sensitizer for some rare earth ions [16, 17].

Various properties like optical, physical, electrical, and structural, and so forth are always sensitive to the variations in the microstructure of the substance. Due to nonhomogeneous cooling of the melt, the structure and, therefore, optical behavior of the material are affected. B2O3 glasses change their characteristics remarkably, when subjected to different annealing temperatures [1822]. In these glasses, it is never discouraged to expect the formation of B3O6 rings called boroxol rings, by the combination of BO3 groups. Raman spectroscopy reveals that for annealing temperature greater than the glass transition temperature, the concentration of boroxol rings increases with decrease in temperature [21]. There appears a residual stress in such materials. During annealing of a glass, a thermodynamical and mechanical steady state is achieved after a specific time and temperature.

The purpose of this paper is to report the change in optical properties of LiF·B2O3 glasses with the stepwise replacement of LiF by Bi2O3. The addition of Bi2O3 provides an opportunity for the new molecular units to be formed with more number of NBOs. It also affects the overall polarizability of the glass network. Both of these factors result in a change in optical band gap energy.

2. Experimental Details

2.1. Sample Preparation

Bi2O3 containing fluoroborate glasses with compositions xBi2O3· (40 − x)LiF·60 B2O3  (𝑥=0,5,10,15 and 20) glasses were synthesized through melt-quench method using Bi2O3, LiF, and H3BO3, reagent grade powders. Various powdered materials were taken in grams equal to their molecular masses and then were mixed uniformly according to their percentage presence in various samples. The uniform mixture was heated at 1273 K for 30 minutes. The bubble free-melt formed was pressed between two carbon plates at room temperature. The glassy samples were thus obtained in form of thin pallets with an average thickness of 1 mm each.

2.2. Optical Characterization

Optical characterization was carried out using Perkin-Elmer UV-Visible spectrophotometer in the range 200–1000 nm at room temperature. A plot between the absorption and wavelength of incident radiations is obtained for further analysis.

2.3. Annealing

Samples were annealed at 300°C, 350°C, and 400°C for 2 hrs each. The samples so obtained were again tested for optical and physical properties.

3. Results and Discussion

3.1. Optical Analysis

A series of samples in xBi2O3·(40 − x)LiF·60 B2O3 compositions with 𝑥=0,5,10,15, and 20 were tested for UV-visible absorption at room temperature. The absorption profiles of as-prepared and annealed samples are depicted in Figures 1, 2, 3, and 4, which contain plots of absorption coefficient 𝛼(𝜈) versus wavelength.

193185.fig.001
Figure 1: Optical absorption coefficient versus wavelength plots for samples as prepared with 𝑥=0,5,10,15, and 20 in the compositions xBi2O3·(40 − x)LiF·60 B2O3.
193185.fig.002
Figure 2: Optical absorption coefficient versus wavelength plots for samples annealed at 300°C with 𝑥=0, 5, 10, 15, and 20 in the compositions xBi2O3·(40 − x)LiF·60 B2O3.
193185.fig.003
Figure 3: Optical absorption coefficient versus wavelength plots for samples annealed at 350°C with 𝑥=0,5,10,15, and 20 in the compositions xBi2O3·(40 − x)LiF·60 B2O3.
193185.fig.004
Figure 4: Optical absorption coefficient versus wavelength plots for samples annealed at 400°C with 𝑥=0, 5, 10, 15, and 20 in the compositions xBi2O3·(40 − x)LiF·60 B2O3.

The absorption coefficient 𝛼(𝜈) is formulated in terms of transmitted intensity (𝐼𝑡), incident intensity (𝐼𝑖), and the thickness of the sample (𝑡) [23] and is given as1𝛼(𝜈)=𝑡𝐼ln𝑖𝐼𝑡.(1) From Figures 1, 2, 3, and 4, one can observe that there is an absence of sharp absorption edge in all the plots. This confirms that the samples are amorphous in nature. Calculation of optical band gap energy explores the optical behavior of a sample in terms of its transparency towards electromagnetic radiations. The optical band gap energy (𝐸𝑔) is related to the absorption coefficient 𝛼(𝜈) [23] as𝛼𝜈=B𝜈𝐸𝑔𝑟.(2) In this equation, 𝜈 is the frequency of incident radiation and B is a constant named as band tailing parameter. Value of the index 𝑟 suggests the nature of transitions taking place in the sample. For indirect allowed and forbidden transitions 𝑟=2 and 3, respectively, and for direct allowed and forbidden transitions 𝑟 equals 1/2 and 2/3, respectively. Tauc’s plots [(𝛼𝜈)1/𝑟versus𝜈,𝑟=2] for the reported samples are shown in Figures 5, 6, 7 and 8. The values of optical band gap energy 𝐸𝑔 are obtained by extrapolating the linear regions of tauc’s plots. The variation of 𝐸𝑔 for as-prepared and annealed samples against composition is listed in Table 1 and plotted in Figure 9.

tab1
Table 1: Cutoff wavelength (λcutoff), optical band gap energy 𝐸𝑔 and Urbach energy (𝐸𝑈) for the samples with 𝑥=0,5,10,15, and 20 in the compositions xBi2O3·(40 − x)LiF·60 B2O3.
193185.fig.005
Figure 5: Tauc’s plots with 𝑟=2 for samples as prepared with 𝑥=0, 5, 10, 15, and 20 in the compositions xBi2O3·(40 − x)LiF·60 B2O3.
193185.fig.006
Figure 6: Tauc’s plots with 𝑟=2 for samples annealed at 300°C with 𝑥=0,5,10,15, and 20 in the compositions xBi2O3·(40 − x)LiF·60 B2O3.
193185.fig.007
Figure 7: Tauc’s plots with 𝑟=2 for samples annealed at 350°C with 𝑥=0,5,10,15, and 20 in the compositions xBi2O3·(40 − x)LiF·60 B2O3.
193185.fig.008
Figure 8: Tauc’s plots with 𝑟=2 for samples annealed at 400°C with 𝑥=0,5,10,15, and 20 in the compositions xBi2O3·(40 − x)LiF·60 B2O3.
193185.fig.009
Figure 9: Variation of 𝐸𝑔 with composition for samples with 𝑥=0,5,10,15, and 20 in the compositions xBi2O3·(40 − x)LiF·60 B2O3.

There is a continuous decrease in the optical band gap energy starting from 𝑥=0 to 𝑥=20. In glassy materials, the decrease in optical band gap energy is often attributed to the increase in polarizability. Also in oxide glasses, it is supported by the increase in disorder due to increase in number of nonbridging oxygen atoms (NBOs). In case of presently reported samples, there is an increase in the polarizability as concentration of LiF, whose polarizability is less, is replaced stepwise by concentration of more polarizable Bi2O3. Also there is quite a good scope of increase in the number of NBOs, as there is a replacement of a non-oxide group (LiF) taking place by an oxide group (Bi2O3) from 𝑥=0 to 𝑥=20.

Interestingly, annealing at different temperatures has remarkably affected 𝐸𝑔 for the sample with no Bi2O3 added. For samples with 𝑥=5 also, it first decreases for annealing temperature 300°C and the increases for higher temperatures. 𝐸𝑔 increases with annealing temperature, in general, for all other concentrations. In samples with 𝑥=0, it is expected that annealing causes BO4 units to be transformed into BO3 units, which have one NBO each. Therefore, 𝐸𝑔 decreases on account of increasing no. of NBOs. In samples other than 𝑥=0, annealing causes more BO3 groups to connect themselves to form boroxol rings and to decrease the no. of NBOs. For 𝑥=5, the formation of boroxol rings overshoots the formation of BO3 units at 400°C. Although the trend of variation of 𝐸𝑔 in annealed samples remains almost same with respect to concentration as it is in the as-prepared samples.

In the low absorption region of tauc’s plot, the absorption coefficient is related to photon energy [24] as𝛼exp𝜈𝐸𝑈.(3) In this equation 𝐸𝑈 is named as urbach tail energy which corresponds to the width of the tail states in the mobility gap. 𝐸𝑈 arises due to presence of the disordered arrangement of atoms, which causes the mobility edges to enter in the mobility gap and give rise to the tail states. Formation of such localized states can thus be attributed to the random potential fluctuations [25]. Generally, in such states the phonon-assisted electronic transitions take place [26, 27]. The formation of such states in the materials makes it the indirect band gap material. Therefore, one can conclude that the reported samples are the indirect band gap materials.

The value of 𝐸𝑈 for the reported samples is calculated from the inverse of the slopes of the linear parts of the Urbach plots (ln𝛼v/s𝜈), shown in Figures 10, 11, 12, and 13. The values of 𝐸𝑈 for as-prepared and annealed samples are listed in Table 1, and the variation of 𝐸𝑈 is plotted against the composition in Figure 14.

193185.fig.0010
Figure 10: Urbach plots for samples as prepared with 𝑥=0,5,10,15, and 20 in the compositions xBi2O3·(40 − x)LiF·60 B2O3.
193185.fig.0011
Figure 11: Urbach plots for samples annealed at 300°C with 𝑥=0,5,10,15, and 20 in the compositions xBi2O3·(40 − x)LiF·60 B2O3.
193185.fig.0012
Figure 12: Urbach plots for samples annealed at 350°C with 𝑥=0,5,10,15, and 20 in the compositions xBi2O3·(40 − x)LiF·60 B2O3.
193185.fig.0013
Figure 13: Urbach plots for samples annealed at 400°C with 𝑥=0,5,10,15, and 20 in the compositions xBi2O3·(40 − x)LiF·60 B2O3.
193185.fig.0014
Figure 14: Variation of Urbach tail energy with composition for samples with 𝑥=0,5,10,15, and 20 in the compositions xBi2O3·(40 − x)LiF·60 B2O3.

Value of 𝐸𝑈 remains almost same for all as-prepared samples but increases remarkably for 𝑥=0 when annealed at different temperatures. The reason is the same as given for decrease in band gap energy that is, conversion of BO4 units to BO3 units, thereby increasing the no. of NBOs and hence disorder. For other samples, 𝐸𝑈 increases due to annealing, keeping the trend of variation similar with respect to the concentration.

4. Conclusions

(1)Optical band gap energy in the series of samples xBi2O3·(40 − x)LiF·60 B2O3 decreases from 𝑥=0 to 𝑥=20 due to increased polarizability and no. of NBOs in the samples.(2)Annealing at different temperatures decreases the value of 𝐸𝑔 for 𝑥=0 due to conversion of BO4 units into BO3 units, thereby increasing the no. of NBOs. For samples with 𝑥=5 also, annealing at 300°C firstly causes 𝐸𝑔 to decrease but it increases for increase in annealing temperature. This is because with increase in annealing temperature, formation of boroxol rings dominates over formation of BO3 units, hence decreasing NBOs. For other samples, 𝐸𝑔 the increases with annealing temperature, again due to increase in NBOs.(3)In case of as-prepared samples, Urbach energy varies slightly with concentration with maximum for 𝑥=0.(4)For same reasons explained above, after annealing, 𝐸𝑈 is affected appreciably for 𝑥=0 and 𝑥=5. For other samples, it is slightly increased keeping the trend of variation similar.

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