Effect of swift heavy ions on structural and optical properties of bismuth based alumino-borosilicate glasses

Radiation Physics and Chemistry 86 (2013) 23–30

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Effect of swift heavy ions on structural and optical properties of bismuth based alumino-borosilicate glasses Ravneet Kaur a,n, Surinder Singh a, Kulvir Singh b, O.P. Pandey b a b

Department of Physics, Guru Nanak Dev University, Amritsar 143005, India School of Physics and Materials Science, Thapar University, Patiala 147004, India

H I G H L I G H T S c c c c

We have investigated SHI irradiation effects on the bismuth alumino borosilicate glass. The structural and optical properties have been studied by preparing the thin films of the glass. Comparative studies with two heavy ions: Li3 þ (50 MeV) and Ag14 þ (180 MeV) have been done. The results indicate that the structure of the glass corresponds to that of radiation hard glass.

a r t i c l e i n f o

abstract

Article history: Received 2 October 2011 Accepted 16 January 2013 Available online 4 February 2013

Borosilicate glass is considered to be one of the most suitable materials for immobilization of the high level radioactive waste (HLW). In the present investigation, glass of the composition Bi2O3 (15%)–Al2O3 (10%)–B2O3 (50%)–SiO2 (25%) was prepared by melting followed by quenching method. Optical and structural properties of these glasses were investigated using UV–visible absorption spectroscopy; X-ray diffraction and Fourier Transform Infrared (FTIR) spectroscopic techniques. Effects of heavy ion irradiation on glass network and structural units have been studied by irradiating glass thin film samples with heavy ions Li3 þ (50 MeV) and Ag14 þ (180 MeV) at different fluence rates ranging from 1012 ions/cm2 to 1014 ions/cm2. Irradiation of materials by swift heavy ions (SHI) results in highly excited lattice atoms due to inelastic collisions with atomic electrons. Atomic displacements and structural modifications of such a lattice, brings out interesting changes in the materials. It is seen that irradiation causes significant changes in compaction of the glass network. Changes in the atomic structure before and after the irradiation are observed and explained. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Borosilicate glass Bismuthate glass Ion irradiation Optical band gap Refractive index IR spectroscopy

1. Introduction Glasses based on heavy metal oxides ( Bi2O3, PbO, Al2O3, TeO2, Ag2O, GeO2, etc.) have always been an area of interest because of their characteristic structural and physical properties such as high refractive index, high thermal expansion, high density, low transformation temperature and excellent infrared transmission. This makes them a favorable candidate for potential applications in IR technologies, design of laser devices and non-linear optics (Pan and Ghosh, 2000; Stone et al., 2000; Upinder et al., 2010). Among heavy metal oxide (HMO) glasses, bismuth borosilicate glasses are the subject of growing and intense research. Bi2O3 containing glasses have attracted a considerable attention because

n Corresponding author. Tel.: þ91 183 2258802, 91 183 2258809x3342; fax: þ 91 183 2258820. E-mail address: [email protected] (R. Kaur).

0969-806X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.01.031

of their vast range of applications in the field of radiation shielding, glass-ceramics, reflecting windows, thermal and mechanical sensors, etc. (Venktaraman and Varma, 2006). Since, these glasses show long infrared (IR) cut-off, thus they are ideal to be used for optical transmission (Bale et al., 2008). Bi2O3 is also known to occupy both network-forming and network modifying positions in the glass matrix; however, when used as a modifier, the cation due to its highest valence state produces important structural effects (Baia. et al., 2003). Recently, the development of radiation-induced optical absorption in glasses has promoted the use of glass in the field of dosimetry as well as laser technologies (Bishay, 1970; Friebele, 1991; Lell et al., 1996). This is because the optical properties of the glass are very closely related to its interaction with the energy of electromagnetic radiation (El-Alaily and Mohamed, 2003). A good amount of literature is dedicated to the study of radiation hardness of various glasses to X rays, UV–vis, Gamma rays and neutrons (Griscom 1985; Griscom, 1991; Marshall et al., 1997).

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R. Kaur et al. / Radiation Physics and Chemistry 86 (2013) 23–30

The knowledge of glass structure before and after irradiation is necessary to understand the structural evolution of glasses under long term irradiation. This leads to better understanding of the intrinsic structure as well as the alterations, which results from interaction with radiation. Irradiation behavior of glass is a major parameter for determining its long term stability (Weber et al., 1997). In certain metallic and silicate glasses, various effects like dimensional changes (Hou et al., 1990), phase transformation (Dammak et al., 1993; Dunlop et al., 1989), damage creation (Paumier et al., 1989) have been observed as a result of an electronic energy loss caused by swift heavy ion irradiation. Particularly in borosilicate glasses the plastic flow of glass has been reported under heavy ion bombardment which leads to the ¨ anisotropic plastic deformation of the material (Klaumunzer et al., 1987) referred to as ion hammering effect (Hedler et al., 2005). The borosilicate glasses in particular are used for incorporating the high-level radioactive waste, throughout the world as they are homogeneous from chemical as well as micro structural point of view (Bishay, 1970). Thus, the present course of investigation is intended to study the possible scope of bismuth doped aluminoborosilicate glasses as the target materials for nuclear waste immobilization as well as for radiation dosimetry. A review of the literature (Cheng et al., 2006; Karthikeyan and Mohan, 2003) indicates that attempts have been made to investigate optical properties of bismuth borate glasses but to the best of our knowledge, here we report for the first time the effects of heavy ion irradiation on the structural and optical properties of the present glass system. The reason for choosing SHI to study the modifications in the present case was that in comparison to other ionizing radiations, heavy charged particles exhibit a totally different way of physical interaction with the target material (Mehta, 1997). SHI irradiation causes the modification of the thin films or near surface region of the bulk samples due to electronic excitation (Avasthi, 2000). These high energy, heavy ions lose energy in materials mainly through inelastic collisions with the atomic electrons of the material. Along the trajectory, a trail of defects (point defects, defect clusters, structural phase transformation) known as latent track may be formed depending on the type of ion, its energy as well as the physical property of the material undergoing interaction. This damage is always created in the close vicinity of the trajectory of projectile (Kanjilal, 2001). Since the energy of the heavy ions is of the order of few MeV and higher so these impinging ions do not get embedded in the thin substrates used as target due to their large range (typically a few tens of micrometer or larger). For this particular reason it is always advisable to use thin film samples for better understanding of interaction of SHI with matter as the elastic collision effects causing collision cascade can be safely neglected and the effect of the embedded ions do not come into picture (Avasthi, 2000).

assembly was initially heated in a pit furnace till a temperature of 900 1C was attained. After that the mixture was shifted to high temperature resistance furnace. The temperature of the mixture was raised to 1350 1C slowly and was held at this temperature for one hour to ensure homogeneity. Finally the melt was cast into preheated graphite mould of the dimensions 12  12  45 mm3. The sample was annealed for three hours at 300 1C in muffle furnace. The glass thus formed was hard and dark brown in color.

2.2. Thin film preparation and irradiation In order to study the effect of swift heavy ions, thin glass films of thickness 300 nm were deposited on Si and glass slide (1 cm  1 cm) substrate by electron beam gun evaporation method of the prepared glass. The pressure during evaporation was 10  6 Torr and the distance from the source to substrate was 135 mm. The thickness of the film being deposited was monitored by quartz crystal monitor. The samples were irradiated with Li3 þ (50 MeV) and Ag14 þ (180 MeV) ions using the 15 UD Pelletron tandem accelerator at Inter University Accelerator Centre, New Delhi. The samples were mounted on irradiation ladder in high vacuum irradiation chamber. In order to do homogeneous irradiation, the focused ion beam was carefully scanned over an area of 1 cm2 (1 cm  1 cm). The thin films were irradiated at different fluence ranging from 1012 ions/cm2 to 1014 ions/cm2. Table 1 shows the fluence ranges to which the samples were exposed.

2.3. SRIM calculations The trend of lithium and silver ion penetration through the borosilicate glass thin films was estimated using the SRIM calculations. The range of the ions in the glass is found to be 378.72 mm and 29.37 mm for Li3 þ and Ag14 þ ions, respectively which is sufficiently high than the thickness of the film due to which both the ions will pass through the film. The calculated electronic energy loss (dE/dx)e for Li ion of 50 MeV is found to be 79.2 keV/mm whereas for silver ion the value is 9.77Eþ03 keV/mm. The nuclear energy loss (dE/dx)n is 4.33E-02 keV/mm for Li ion and 30.4 keV/mm for Ag ion. As the nuclear energy loss for both the ions is very small as compared electronic energy loss, thus the ion-glass interaction is almost electronic in nature. Moreover, at high energies the inelastic collisions dominate and the displacement of atoms due to elastic collisions is almost insignificant. The thin films have been characterized using XRD, UV–vis and (FTIR). The samples marked S1-S7 represent the thin films of the glass sample deposited on silicon as a substrate and G1–G8 represent the thin film deposited on microscopic glass slide as a substrate. The unirradiated/pristine sample here refers to the thin film of the glass sample deposited on Si (or microscopic glass slide) as a substrate which has not been exposed to SHI.

2. Experimental 2.1. Sample preparation Alumino-borosilicate glass from the system Bi2O3 (15%)–Al2O3 (10%)–B2O3 (50%)–SiO2 (25%) have been synthesized (all compositions in mole percent). The glass was prepared from chemically pure materials in amount sufficient to produce 75 g glass. Analytical pure reagents of Bi2O3, Al2O3, B2O3 and SiO2 were taken as raw materials. B2O3 was introduced in the form of di-boron trioxide. Appropriate amount of the dried ingredients was mixed and ball milled in acetone medium for 90 min. The mixed powder was kept overnight for drying. The dried mixture was then transferred to alumina crucible of 120 ml capacity. The entire

Table 1 Showing the fluence ranges to which different samples were exposed. Ion used: Ag

14 þ

(180 MeV)

Ion used: Li 2

3þ

(50 MeV)

Sample Name

Fluence (ions/cm )

Sample Name

Fluence (ions/cm2)

G1 G2 G3 G4 S1 S2 S3

1  1012 5  1012 1  1013 3  1013 1  1012 5  1012 3  1013

G5 G6 G7 G8 S4 S5 S6 S7

5  1012 1  1013 5  1013 1  1014 5  1012 1  1013 5  1013 1  1014

R. Kaur et al. / Radiation Physics and Chemistry 86 (2013) 23–30

25

2.4. X-ray diffraction

3. Results and discussion

The X-ray powder diffraction data for all glasses was obtained from I.U.A.C., New Delhi, India using Bruker Axs diffractometer. X-ray diffraction technique was used to check for possible crystallanity of the sample after quenching and annealing. The pattern was recorded at a scanning rate of 21 min  1 and at angular rate (2y) of 15–601. The X-ray diffraction patterns of the glass samples before and after irradiation with Ag 14 þ and Li 3 þ are shown in the Figs. 1 and 2 respectively.

The purpose of the present course of investigation is to identify and characterize the radiation induced defects in the above mentioned glasses. The results of the different techniques performed on the glass thin film samples are discussed below.

2.5. UV–vis absorption measurements The optical absorption measurements performed on the thin film glass samples (G1–G8) before and after heavy ion irradiation, were recorded using a UV–vis spectrophotometer (Hitachi UV-300) in the range 200–900 nm. Only those glass thin film samples were exposed to UV–visible radiations which were deposited on glass slide as a substrate i.e. samples G1–G8. 2.6. Infrared spectra The FTIR spectrum of the glass samples (S1–S7) was recorded from 400–4000 cm  1. The data was recorded by Shimazdu FTIR8400S spectrophotometer at room temperature.

Intensity (a.u.)

G4 G3 G2 G1

30

40 2 Theta

50

60

Fig. 1. Showing the XRD spectra of Ag14 þ irradiated samples.

Intensity (a.u.)

G7

The XRD spectra of Ag14 þ irradiated glass thin film samples as well as that of the pristine sample is shown in figure1. The broad humps appearing in the unirradiated glass thin film sample are indicative of the amorphous nature of the parent thin film sample. As the fluence ranges of Ag14 þ ion is increased to 5  1012, 1  1013 and 3  1013 ions/cm2 for the glass thin film samples ˚ G2, G3 and G4 respectively three distorted peaks at d ¼3.967 A, 3.277 A1, 2.369 A˚ appear in the structure of the glass. A large broad peak around 2y ¼25–301 which is observed in the present case after irradiation with Ag ion, is a typical feature of borate glass Kim et al., 2007. This shows that the irradiation of the glass samples with high energy Ag14 þ ion has caused the periodic alignment of the atoms which is realistic with the SRIM calculations. It can be seen from Fig. 1 that as the fluence rate is increased; the atoms tend to get aligned in periodic manner within the lattice resulting in the appearance of crystalline phases. This is due to the fact that at such a high fluence (3E13) Ag14 þ ions were able to penetrate the thin films and thereafter have undergone inelastic collisions with atomic electrons which resulted in the displacement of the atoms leading to the periodicity of the samples. In contrast, the thin film glass samples G5, G6, G7 and G8 which were irradiated using Li3 þ (50 MeV) ion as well as the unirradiated one show amorphous nature which is clearly indicated by the broad humps appearing in their spectra as shown in the Fig. 2. This shows that the low energy Li3 þ on interacting with the target material could not result in major dislocations. 3.2. Optical measurements

Unirradiated

20

3.1. X-ray diffraction

G8

3.2.1. Theory The primary electronic effect of the cations in a glass network is to form an optical mobility gap similar to the energy-band gap of crystals. This results in the leakage of charge carriers across the mobility gap thereby, forming a tail of energy-density states. On absorbing the ultraviolet and visible light, the outer electrons of the glass sample are excited to higher energy levels. The amount of energy required for this excitation is equal to the difference in energy between the two electron energy bands. These energy values are unique and quantized for all species in the sample and correspond to the light angular frequencies, o, as shown in the expression below E ¼ :o

ð1Þ

where h is Planck’s constant and when divided by 2f becomes :. A more accurate model was proposed by Mott and Davis (1979) which takes into account even the material with small band gap. According to this model, the absorption coefficient alpha (a) varies with the angular frequency o in the following manner:

G6 G5 Unirradiated

hoaðoÞ ¼ B½:o2E0 2 20

30

40 2 Theta

50

Fig. 2. Showing the XRD spectra of Li3 þ irradiated samples.

60

ð2Þ Eopt g ,

can be where B is a constant. The optical mobility gap, calculated by plotting (aE)1/2as a function of E( ¼ho). The value of Eopt was then calculated from a linear extrapolation to zero g ordinate.

R. Kaur et al. / Radiation Physics and Chemistry 86 (2013) 23–30

To calculate the width of the energy tail, also designated as Urbach energy Eu, the model proposed by Urbach (1953)and Tauc (1987) is followed. According to this model, the following relation is valid: ð3Þ

where C is a constant. Eu can be then obtained from the reciprocal of the slope of the plot of the natural logarithm of a versus the photon energy. Both of these parameters i.e. band gap energy Eopt and Urbach energy Eu is very useful for describing g the structural disorder. 3.2.2. Measurement of refractive index The refractive index (n) of the glass can be calculated from the transmission measurements (Schwarz and Ticha´, 2003) given by following relation n ¼ ½1þ ð12T 2 Þ1=2 =T

ð4Þ

Absorbance (a.u.)

lnðaÞ ¼ C þ :o=EU

G5 G6 G7 G8

Unirradiated

Absorbance (a.u.)

26

300

400

500

600

700

800

900

Wavenumber (nm)

300

400

500 600 700 Wavenumber(nm)

800

900

Fig. 4. Showing the UV–vis spectra of Li3 þ irradiated samples. UV–vis spectra of the unirradiated glass sample.

where T is transmission. 3.2.3. Optical properties (optical band gap and Urbach energy) 3.2.3.1. Origin of UV bands in unirradiated glass thin film. The UV–vis spectrum was recorded only for the glass thin film samples which were deposited on microscopic glass slide as a substrate. The glass films deposited on Si as a substrate were not chosen for UV–vis spectroscopy as Si is not transparent to UV–Visible light. The optical absorption spectrum of the samples G1, G2, G3 and G4 which were irradiated within the fluence ranges of 1  1012, 5  1012, 1  1013 and 3  1013 ions/cm2 respectively with the Ag14 þ ion is shown in Fig. 3. On the other hand, the optical spectra of the samples G5, G6, G7 and G4 which were irradiated within the fluence ranges of 5  1012, 1  1013, 5  1013 and 1  1014 ions/cm2 respectively with the Li3 þ ion is shown in Fig. 4. The unirradiated parent bismuth aluminoborosilicate glass thin film reveals (inset of Figs. 3 and 4) strong UV absorption extending from 300–400 nm. The absorption bands are created due to centers formed by electron or hole trapping. Precursors for these centers can be either pre-existing defects or they may be created by atomic displacements or radiolytic process resulting from high energy radiations. Many workers (Parke and Webb, 1973; Paul, 1972) have shown even when traces of Bi3 þ ions are added in borate and phosphate glasses a UV peak is observed and the transition of this peak was related to 1S0-3p1. Duffy and Ingram (1970 and 1974) agreed to this assignment. Reisfeld and Boehm (1974) compared the absorption

Unirradiated

Absorbance (a.u.)

Absorbance (a.u.)

G1 G2 G3 G4

300

400

500

600

700

800

900

Wavenumber (nm)

300

400

500 600 Wavelength(nm)

700

800

900

Fig. 3. Showing the UV–vis spectra of Ag14 þ irradiated samples. Inset: UV–vis spectra of the unirradiated glass sample.

spectrum of Bi3þ ions in phosphate, borate and germanate glasses and showed that the UV absorption peak lies in the three glasses at 43010, 41322 and 36764 cm  1, respectively. Also it has been accepted by several authors (Duffy, 1997; Ehrt, 2000; Moncke and Ehrt, 2004; Natura and Ehrt, 1999; Sigel and Ginther, 1968) that the ultra violet charge transfer bands are usually observed in most of the commercial glasses at about 210, 270 and 310 nm, and even extending to near visible region at 380 and 420 nm. The basic reason for their origin is the presence of transition metal impurities in the raw materials even if present in ppm level and specifically to the predominance of trivalent Fe3 þ ions and the combined effect of absorption due to the presence of high Bi3 þ . It can be concluded from above results that UV absorption band appearing in unirradiated parent bismuth aluminoborosilicate glass film is due to absorption by bismuth ions and also due to the presence of unavoidable trace iron impurities in the raw materials for glass preparation (Duffy, 1997; Ehrt et al., 2000; Ebendorff-Heidepriem and Ehrt, 2000; Natura et al.,2000; Natura and Ehrt, 1999) similar reasoning was given in undoped sodium phosphate, bioglasses (Marzouk and ElBatal, 2006; Marzouk et al., 2007) and cabal (El-Batal et al., 2006) glasses. 3.2.3.2. Contribution of the effect of swift heavy ions (SHI). The values of Urbach energy (Eu) and optical band gap Eopt for the g pristine and heavy ion irradiated samples of different fluences with two different ions is given in Table 2. The irradiation of the thin film samples with heavy ions does not lead to the formation of any new bands. However, slight variation in the slope is observed. It can be seen from Figs. 3 and 4 that the absorption edges are not sharp, which is a clear indication of the glassy or amorphous nature of the samples. These figures show that absorption edge is shifting slightly towards the longer wavelength side with the increasing heavy ion fluence. The shifting of absorption edge in oxide glasses corresponds to the transition of an electron belonging to an oxygen ion to an excited state (Stevals, 1953). Thus, more weakly these electrons are bound to the lattice, more easily absorption occurs (Sandhu et al., 2009).This effect leads to the increased oxygen environment within the glass with increasing fluence of incident beam. Fig. 5 represents the values of band gap of the samples G1–G4 and in a similar way band gap values were calculated for G5–G8 (Fig. 6). The optical band gap (Eopt g ), calculated from the absorption edge of the UV spectra varied from 3.76 to 2.77 eV for

R. Kaur et al. / Radiation Physics and Chemistry 86 (2013) 23–30

Table 2 Representing the values of fluence range, Urbach energy and optical band gap for the Ag14 þ and Li3 þ ion irradiated samples. Ag14 þ ion irradiated

Li3 þ ion Irradiated

Fluence (ions cm  2)

Eopt g (eV)

Eu (eV)

Fluence (ions cm  2)

Eopt g (eV)

Eu (eV)

0 1  1012 5  1012 1  1013 3  1013

3.76 3.42 3.23 3.11 2.77

0.22 0.61 0.75 0.78 0.97

0 5  1012 1  1013 5  1013 1  1014

3.76 3.57 3.35 3.25 3.08

0.22 0.59 0.64 0.71 0.76

0.35

(alpha E)1/2 (cm-1/2 eV-1/2)

0.30 0.25 G4

0.20

G3 G2

0.15

G1

0.10 0.05 0.00 1.5

2.0

2.5

3.0

3.5 4.0 4.5 5.0 Band gap (eV)

5.5

6.0

6.5 7.0

Fig. 5. Showing band gap plot of samples G1–G4.

(alpha E)1/2 (cm-1/2 eV-1/2)

0.30

0.25

G8 G7 G6 G5

0.20

0.15

0.10

0.05

0.00 1.5

2.0

2.5

3.0 3.5 4.0 Band gap (eV)

4.5

5.0

5.5

6.0

Fig. 6. Showing band gap plot of samples G5-G8 irradiated with Li ion.

pristine and various irradiated samples, respectively. The decrease in band gap of the glass can be explained on the basis of changes taking place in the structure of glass. The interaction of glass with nuclear radiations can cause the breaking up of three dimensional network leading to the transformation of bridging oxygen (BO) to non-bridging oxygen (NBO) which do not participate in the network (Kingery et al., 1976). The ultraviolet transparency of oxide glasses is explained in terms of electro negativity of the constituent oxygen atoms by Duffy (2001).

27

In glasses, the negative charges on NBOs have a larger magnitude as compared to that of bridging oxygen. Increasing the ionicity of oxygen ions by transforming them from bridging oxygen ions to non bridging oxygen ions raises the top of the valence band resulting in the reduction of band gap energy. Consequently, the UV absorption occurs at lower photon energies as the oxygen atoms become NBOs. Further, it can be seen from Table 2 that the band gap decreases from 3.76 eV to 2.77 eV for the glass thin film samples irradiated with Ag ion showing an overall decrease of 26.3%. Whereas in the case of Li ion the band gap value is found to reduce by 18.08%. This is also true for the glass samples of same fluence (G2 and G5, G3 and G6). A comparative higher decrease in the band gap values in case of Ag ion can be attributed to the fact that high electronic energy losses of Ag ion has resulted in a highly cross linked network thereby decreasing the optical mobility gap and causing densification of the glass structure. Decreasing band gap indicates a compaction of the glass network after irradiation. Radiation compaction includes displacements, electronic defects and breaks in the B–O bonds allowing the structure to relax and fill the large interstices in the interconnected network of boron and oxygen atoms (Sharma et al., 2006). This result is further supported by the shifting of absorption band towards higher wavelength and is attributed to the increasing number of non-bridging oxygen (NBO) atoms after breaking. Similar results with gamma irradiation on microscopic glass slide have been shown by Singh and Sandhu (2006) and Singh et al. (2007). The fast heavy ions when moving through matter are capable of producing excitations and ionizations that have significant effects on the structure. These ions because of their high energy and heavy mass can cause the radiolysis cleavage of Si–O–Si as well B–O bonds thereby displacing the oxygen out of its normal position and resulting in increase of NBOs (Mcswain et al., 1963) The width of the energy tail is a measure of the disorder or the variety of environments affecting the cations. The increase in values of the Urbach energies (Table 2) both in case of Li ion and Ag ion shows that the disorder in the glass structure increases resulting even more amorphicity of the glass samples after irradiation (Sandhu et al., 2008). Eu value increases from 0.22 eV to 0.97 eV for the thin film samples irradiated with Ag14 þ ion. The results also reveal that the Urbach’s energy has increased from an initial value of 0.22 to a final value of 0.76 eV in case of lithium ion irradiation. This indicates that 180 MeV silver ion has caused greater damage to the glass network structure. When an ion slows down in the material it loses (in fact, stores) most of its energy in electrons which carry the energy to areas away from the trajectory or just de-excites in time depending on the type of the material. This results in dislocations of the atomic electrons resulting in changes in the structure of the material (Avasthi, 2000).

3.2.4. Effect of SHI beam on refractive index Glasses containing elements like Al2O3 and Bi2O3 have relatively high values of refractive index as observed in the present case (Griscom, 1991). Figs. 7 and 8 represents the refractive index calculated from the transmission measurements respectively for Ag 14 þ ion and Li3 þ ion irradiated of the glass thin film samples. It can be seen (from Figs. 7 and 8) that the refractive index decreases with the increasing wavelength of the incident photon, and on the contrary increases with the increase in the fluence of ion. Introducing HMO such as bismuth in the glass network results in the increase in coordination number of the glass because the BO4 units will increase resulting in creation of more NBOs. Consequently, the average coordination number of the studied glasses increases which further increases the refractive

28

R. Kaur et al. / Radiation Physics and Chemistry 86 (2013) 23–30

5.5

Pristine 12 1*10 12 5*10 13 1*10 13 3*10

5.0

Refractive index (n)

4.5 4.0 3.5 3.0 2.5 2.0 1.5 400

450

500

550

600 650 700 Wavelength(nm)

750

800

850

900

Fig. 7. Shows the value of refractive index of pristine and Ag14 þ irradiated samples as a function of increasing wavelength.

5.5 5.0

Pristine 12 5*10 13 1*10 13 5*10 14 1*10

Refractive Index(n)

4.5 4.0 3.5 3.0 2.5

glass sample with high energy SHI leads to the creation of the NBOs which further creates more ionic bonds because of the energy transferred to the atoms by incident ions. This results in the larger polarizability of the ionic bonds over the covalent bonds of bridging oxygen providing a higher refractive index value (Saddeek et al., 2008). The refractive index of the glass can also be linked to the glass density according to Fanderlik (1983). The significant increase in the refractive index indicates that the density of the glass sample has also increased. The incident high energy beam causes induced damage which results in the compaction of B2O3 by breaking the bonds between the trigonal elements allowing the formation of the different ring configuration. Thus a transition takes place from BO3-BO4 groups which are strongly bonded than triangular BO3 groups resulting in compact structure. The average ring size is decreased thereafter with increase in the fluence leading to increase in the density and observed increase in the refractive index (Min et al., 1995). Since it was not possible to measure the density of thin films by immersing it in a medium, following Archimedes Principle, as it would have caused the stripping of the film from the substrate, so in the present case density of the thin film samples was not calculated. However, an increase in refractive index does indicate the increase in density of thin films with increasing fluence. The change in the refractive index is more significant in the case of Ag ion as compared to Li ion. For the same fluence of 1013 ions/cm2 the value of n for Ag ion irradiated sample is 5.17 and Li ion irradiated sample is 4.77. Whereas at fluence 5  1012 ion/cm2 the value of n is 5.04 and 4.63 respectively for Ag ion and Li ion exposed samples. This is due to the fact that the higher energy loss of Ag ion causes the displacement of the ions within the glass matrix to an extent thereby expanding the structure of glass by increasing the concentration of NBOs and eventually increasing the refractive index of the glass. This is further supported by the band gap results.

2.0

3.3. IR Measurements 1.5 400

450

500

550

600 650 700 Wavenumber(nm)

750

800

850

900

Fig. 8. Shows the value of refractive index of pristine and Li3 þ irradiated samples as a function of increasing wavelength.

index. The creation of NBOs creates more ionic bonds resulting in larger polarizability of ionic bonds over the mostly covalent bonds of bridging oxygen providing a higher index value (Sadeek et al., 2010). Also it is well known that the Bi3 þ ions are highly polarizable, therefore, presence of Bi2O3 in the network causes an increase in optical basicity in these glasses. The optical basicity calculated for the bulk glass using the method given elsewhere (Choudhury et al., 2006) is found to be 0.567 which is sufficiently high. Higher the optical basicity, higher is the oxide polarizability and in turn a high refractive index. In case of Ag ion as in Fig. 7 the refractive index initially decreases from 4.54 to 4.22 for the fluence 1012 ions/ cm2 but then with the increase in wavelength the value of n increases from that of pristine sample. As the fluence is increased to 3  1013 ions/cm2 the value of refractive index attains a high value of 5.38. This is also true in the case of Li ion where the value of refractive index reaches to 5.01 at the highest fluence of 1014 ions/cm2. The increase in the refractive index with the increasing fluence may be due to the atomic displacements or ionization that resulted from the collision of SHI with the glass which probably caused the material alterations or changes in the internal structure in the glass (El-Alaily, 1991). Irradiating the

FTIR is very beneficial technique to have a clear insight into the structure of glass as it provides information regarding the local units constituting the glass network as well as the anionic sites hosting the modifying metal cations. The FTIR transmittance spectrum of the glass samples is recorded between 400–4000 cm  1. However, the active vibrations of the borate and silicate units are in the region of wave number higher than 550 cm  1. The pristine glass thin film sample shows (Figs. 9 and 10) prominent bands at  579 cm  1,  637 cm  1,  666 cm  1,  835 cm  1,  1076 cm  1 and  1140 cm  1. The IR spectra of the Ag14 þ ion irradiated samples S1, S2 and S3 is shown in Fig. 9. Whereas, the IR spectra of the samples S4, S5, S6, S7 irradiated with the Li3 þ ion is shown in Fig. 10. The structure of borate glasses is represented in the form of a three dimensional network whose nodes are occupied by three fold or four fold coordinated boron atoms. However, the structural unit in silicate glasses mainly consists of SiO4 tetrahedron and IR absorption spectrum of Si–O stretching region is dominated by a band at 1060 cm  1 whereas the boron structure gives infrared bands around 1000 cm  1due to stretching vibrations of BO4 tetrahedron (McMillan, 1984). The addition of Bi2O3 transform the structure of B2O3 from boroxol groups to the formation of BO4 tetrahedra and is present mainly as tetraborate and diborate groups which are characterized by the appearance of an infrared absorption band at 940 cm  1and a shoulder at 1000 cm  1 (Kamitsos, 2003). The bands at about 1076 cm  1 are attributed to the combined stretching vibrations of Si–O–Si and B–O–B network of tetrahedral structural units consisting of borate and silicate groups

R. Kaur et al. / Radiation Physics and Chemistry 86 (2013) 23–30

13

% Transmittance

3*10

12

5*10

12

1*10

00

00 16

00

15

14

00 13

00

00

0

0

00

12

11

10

90

80

0

0

0

70

60

50

40

0

Pristine

Wavenumber(cm-1) Fig. 9. Showing the FTIR spectra of Ag14 þ irradiated samples.

14

% Transmittance

1*10

13

3*10

5*1012

00

00

16

15

00 14

00 13

00

00

0

00

12

11

10

90

0 80

0 70

0 60

0 50

0

of the bands extending from nearly 2200–4000 cm  1 are due to OH and B–OH vibrations including the characteristic near infrared absorption bands to water (Khalifa, El-Batal, 1997; Dunken and Doremus, 1987; Husung and Doremus, 1990). Radiations can cause displacements of lattice atoms or electron defects, which involve changes in the valence state of lattice or impurity atoms (El-Batal et al., 2001). Irradiating the glass thin film samples with Li and Ag ion within the fluence ranges from 1012 ions/cm2 to 1014 ions/cm2 ion has no significant effect on the peak position shown in Figs. 9 and 10. Also no new bonds appear in the glass structure by irradiating the thin films either with Li or Ag ion. However, there is very small increase in the intensities of the bands representing the atomic displacement within the glass system. During irradiation, the ionization produces electron–hole pairs, providing paths for bond rearrangements, reducing the constraints on the structural relaxation process, releases some of the excess energy stored in the structure, accompanied by decrease of the average bridging bond angle. Since the glass does not have a regular structure, hence, the relaxation process involves the long range effects within the structure participating in it. The increase in intensity of bonds with increasing SHI fluence results in the densification of the glass thin films perhaps due to the increase in the number of non-bridging oxygen’s (NBO’s). The non appearance of sharp bands after irradiation can be related to the possible tendency of the structure to more randomness or amorphicity of the structure. This fact is supported by the increase in the values of Urbach Energy after irradiating the sample. This could also be assigned to changes in the bond angles or bond lengths of the building units that is, Si–O–Si and B–O–B (Primak, 1972).

4. Conclusion

13

1*10

Pristine

40

29

Wavelength (cm-1) Fig. 10. Showing the FTIR spectra of Li3 þ irradiated samples.

(Kim and Bray, 1968). The existence of band at about 666 cm  1 is attributed to the presence of some super structural borate units (Gattef et al., 1996). Also the absorption bands due to AlO4 or AlO6 groups are located at same position of borate groups. Thus the absorption band near 680 cm  1 is assumed to be combined vibration of AlO6 groups and the vibrations of bridging oxygens between trigonal boron atoms. The band at 1140 cm  1 apppears to be superposition of many small adjacent bonds which are observed between 1200–1400 cm  1. These bands arise from asymmetric stretching relaxation of B–O bond of trigonal BO3 units with non-bridging oxygens (NBOs) such as metaborate chains and rings, pyroborate and orthoborate groups. The weak band observed at 579 cm  1 is due to vibrations of BO4 tetrahedral and the weak bands below 500 cm  1 are due to Bi–O vibrations (Saritha et al., 2008; Singh, 1997). The strong band at 637 cm  1 is due to the bending vibration of B–O–B linkages of BO3 units (Shaaban et al., 2008). Another small band at around 835 cm  1 can be related to the symmetrical stretching vibration of the Bi–O bonds in the [BiO3] groups (Sharma et al.,2006). The rest

The decrease in the optical band gap (Eopt) and increase in the refractive index from optical absorption analysis indicates the change in the glass network. The increase in Urbach energy Eu signifies that degree of disorder increases with increase in the fluence rate of SHI. This fact has been supported by IR studies in which the number of non-bridging oxygen’s (NBOs) increases. The high energy Ag ion has resulted in significant changes in the glass network as shown in the XRD patterns of the glass. Thus the high energy heavy ions Li and Ag by undergoing inelastic collisions with the glass matrix were capable of inducing displacements in the glass structure. Since the structure of this glass corresponds to that of radiation hard glass thus the use of this glass can be predicted for High Level Nuclear Waste immobilization.

Acknowledgment The authors are thankful to IUAC, New Delhi for providing financial assistance and necessary facilities for accomplishment of the present work. References Avasthi, D.K., 2000. Some interesting aspects of swift heavy ions in materials science. Current Sci. 78, 11. Baia, L., Stefan, R., Popp, J., Simon, S., Keifer, W., 2003. Vibrational spectroscopy of highly iron doped B2O3–Bi2O3 glass systems. J. Non-Cryst. Solids 324, 109. Bale, H., Rao, N.S., Rahman, S., 2008. Spectroscopic studies of Bi2O3–Li2O–ZnO– B2O3 glasses. Solid State Sci. 10, 326. Bishay, A., 1970. Radiation induced color centers in multicomponent glasses. J. Non-Cryst. Solids 3, 54. Cheng, Y., Xiao, H., Guo, W., Guo, W., 2006. Structure and crystallization kinetics of Bi2O3–B2O3 glasses. Thermochim. Acta 444, 173. Choudhury, P., Pal, S.K., Raya, H.S., 2006. On the prediction of viscosity of glasses from optical basicity. J. Appl. Phys. 100, 113506.

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