Indian Journal of Pure & Applied Physics Vol. 53, January 2015, pp. 42-48 Structural and optical studies of Fe2O3 doped barium strontium titanate borosilicate glasses Avadhesh Kumar Yadav*2 & C R Gautam1 1 2 Department of Physics, University of Lucknow, Lucknow 226 007, India Department of Physics, Indian Institute of Technology (BHU), Varanasi 221 005 *E-mail: yadav.av11@gmail.com Received 21 August 2013; revised 8 October 2014 ;accepted 24 November 2014 Perovskite (Ba,Sr)TiO3 borosilicate glasses in the system 64[(Ba1−xSrx).TiO3]-30[2SiO2.B2O3]-5[K2O]-1[Fe2O3] (0.4 ”x•1.0) were prepared by conventional melt-quench method. Synthesized glass samples were characterized by infrared and Raman spectroscopic techniques for their structural investigations while optical properties of these glasses have been studied by using UV-Vis NIR spectroscopy. These investigations confirm the absence of boroxol ring in glassy matrix. Optical band gap of barium strontium titanate borosilicate glasses lies in the range 2.48-2.87 eV and its density has been found to be in the range 2.70-3.12 g/cc. Keywords: Infrared spectroscopy, Raman spectroscopy, UV-Vis NIR spectroscopy, Barium strontium titanate 1 Introduction Glasses are inorganic product of fusion which are cooled to a rigid condition without crystallization and long range order of atoms are completely absent1. The structure of multi-component glasses can be described in a less straight forward way and the choice of binary systems seems advantageous for undertaking systematic studies2,3. The vibrational band’s shape, intensities and their positions would be affected by different vibrations by composition variation. At fixed composition, the shape of the former band is found to be dependent on the substrate temperature during deposition. Insight into the "mixing" nature of these vapour-deposited glassy solids has been obtained from an analysis of the spectral distribution of Si-OSi, B-O-B and B-O-Si bonds as a function of composition4. Borates and borosilicate glasses containing boron oxide have been widely used for optical lenses due to high refractive index and low dispersion characteristics5. Raman spectroscopy is a very useful tool to provide valuable information about impurities, internal stress, crystal symmetry and bond nature6−9. IR and Raman studies on lithiumpotassium-borate glasses exhibited due to vibrational stretching of borate network of the BO3 and BO4 units placed in different structural groups10. Recently, the IR and Raman spectroscopic studies on glass system [(BaxSr1−x)O.TiO2]-[2SiO2.B2O3][K2O].[La2O3], have been reported. The different absorption bands such as molecular water, hydrogen bonding, B-O-B bonding, B-O-Si linkage etc. occurred in different wavenumber regions. These absorption bands depend on variation of Ba/Sr ratio as well as doping concentration of La2O3. Here, La2O3 acts as network modifier for glasses while the borosilicate works as glass former. The metallic cations were observed in low wavenumber regions.. The spectroscopic studies of barium strontium titanate borosilicate glasses11−15 with addition of Fe2O3, have already been investigated. In the present paper, the IR, Raman and UV-Vis NIR spectroscopy on BST borosilicate glass system 64[(Ba1−xSrx)TiO3]30[2SiO2-B2O3]-5[K2O]-1[Fe2O3], has been studied. 2 Experimental Details High purity analytical reagent grade chemicals BaCO3 (Himedia 99%), SrCO3 (Himedia 99%), TiO2 (Himedia 99%), SiO2 (Himedia 99.5%), H3BO3 (Himedia 99.8%), K2CO3 (Himedia 99.9%), and Fe2O3 (Himedia 99.9%) were used for the preparation of various glasses and their glass ceramic samples. The batch of 20 g in appropriate amount of AR grade chemicals has been prepared. Appropriate amount of different reagents chemicals of raw materials, as per the composition of glasses, was properly calculated, weighed and mixed in an agate mortar using acetone as mixing medium and dried. The glass batches weighing of 20 g were melted in a high-grade alumina YADAV & GAUTAM: STRUCTURAL AND OPTICAL STUDIES OF GLASSES crucible in open air atmosphere using a programmable electric furnace (Metrex Scientific Instruments (P) Ltd. New Delhi). The melting temperatures for different compositions were in the range 10501450°C. The melt was maintained at the melting temperature in the furnace for 10 min for refining and homogenization. The melt was poured into an aluminum mould and pressed by a thick aluminium plate and then immediately transferred into a preheated programmable muffle furnace for annealing at temperature 450°C for 3 h to remove the residual stresses due to temperature gradient, which is produced by rapid cooling. The glasses were cooled within the furnace after annealing. Infrared spectroscopy (IR) is an important tool for understanding the structure and dynamics of amorphous materials. It is also used to assign the observed absorption peaks to the proper vibration of the atoms in geometric grouping. The spectra of solids, many variables can affect the absorption peaks position as well as absorbance and the assignment of vibrational peaks of the atoms are very difficult. Usually, the method of repeated occurrence is followed in analyzing the IR spectrum of solid materials. Infrared absorption/transmission spectra of the powdered glass samples mixed with KBr powder and pressed as pellets were recorded using JASCO FT/IR-5300 and Brucker FTIR Tensor-27 in the wavenumber range 400-4000 cm−1 at room temperature. IR spectra of the prepared glass samples show the various absorption bands in different wavenumber regions. These bands are the characteristics of various vibrational modes of borate network as well as metallic cations. Raman spectroscopy is one of the powerful technique to investigate the structure of a material. In IR spectroscopy, the nature of the light matter interaction is not the same as in Raman spectroscopy and the fundamental differences between the two processes determine the selection rules, which control Raman or IR activity of normal mode of vibration. Interaction of IR radiation with a normal mode of vibration occurs only when the electric field of radiation oscillates with the same frequency as instant dipoles caused by atomic vibrations. A normal vibration is, therefore, IR active only if a change in the dipole moment of the vibration occurs and is one photon process, as only photon is absorbed. Therefore, IR spectra give additional information than Raman spectra by which the symmetries of normal 43 modes of vibration of molecules and crystal lattices are determined. Micro Raman set-up, Renishaw, UK, equipped with a grating of 1800 lines/mm and Olymapus (model MX-50) A/T was attached with spectrometer which focuses laser light into sample and collect the scattered light at 180° by scattering geometry. The 15.4 nm Ar+ laser was used as an excitation source and GRAM-32 software for data collection. Raman spectra (RS) of powdered glass samples were also recorded in the wavenumber range 200-3500 cm−1. UV-visible absorption spectroscopy is a very useful technique to characterize the optical and electronic properties of different materials such as thin films, filters, pigments and glasses. Measurement of the optical band gap of glass sample is carried out using the data within range 200-1200 nm obtained by spectrophotometer. There is a sharp increase in absorption at energies close to the band gap that manifests itself as an absorption edge in the UV-visible absorption spectra. UV-vis spectroscopy of the sample was carried out using UV-visible spectrophotometer (Varian, Carry-50Bio). It measures the percentage of radiation in the different regions such as ultra-violet (200-400 nm), visible (400-800 nm) and near infrared (800-1200 nm) regions that is absorbed at each wavelength within ultra-violet, visible regions and near infrared. Optical transitions are basically two types, direct and indirect transitions. In these transitions, the electromagnetic radiation interacts with the electrons in the valence band which reaches to conduction band by gaining fundamental band gap. These transitions occur in both crystalline and amorphous semiconductor materials. These transitions are related with Mott and Davis relation. For photon energies just above fundamental edge, the relation between absorption coefficient (Į) and photon energy is given below16,17: α = (hν − Eg ) n / hν …(1) where A is a constant related to the extent of the band tailing, n = 1/2 for allowed direct transition, = 2 for allowed indirect transition, hȣ is the photon energy and Eg is the optical band gap energy of the material. The absorption coefficient, Į, was calculated at different photon energies by using the relation, Į=A/d, where A is the absorbance and d is the thickness of the samples. The optical band gap, Eg, was calculated by extrapolating the linear parts of the curves to (Įhȣ)1/2 =0 of curve (Įhȣ)1/2 versus hȣ and extrapolating for INDIAN J PURE & APPL PHYS, VOL 53, JANUARY 2015 44 the glass samples. In general, density strongly depends on the compositions and structure of the glass and glass ceramic samples. Density of the glass and glass ceramic samples was determined using Archimedes principle18. Distilled water was used as the liquid medium. Density of glass and glass ceramic samples were calculated using the formula: ρ= (W2 − W1 ) ρw (W4 − W1 ) − (W3 − W2 ) …(2) where ȡ is the density (g/cc), ȡw the density of distilled water 1 g/cc, W1 the weight of empty specific gravity bottle (g), W2 the weight of specific gravity bottle with sample (g), W3 the weight of specific gravity bottle with sample and distilled water (g) and W4 is the weight of specific gravity bottle with distilled water (g). 3 Results and Discussion 3.1 Infrared Spectroscopy IR spectra of glass samples BST5K1F0.4, BST5K1F0.6, BST5K1F0.8 and ST5K1F1.0 are shown in the Fig. 1(a-d) and the peak positions have Fig. 1 — Infrared spectra of glass samples (a) BST5K1F0.4, (b) BST5K1F0.6, (c) BST5K1F0.8 and (d) ST5K1F1.0 been listed in Table 1. IR spectra of all glass samples consist of different absorption bands in the wavenumber range 400-5000 cm–1. These bands are affected by doping of Fe2O3 as well as the variation of Ba/Sr ratio. The positions of these bands are shifted with compositions. A broad absorption band in the wavenumber range 3451-3456 cm–1 is observed in IR spectra for all glass samples but the position of this band slightly varies with composition. This absorption band occurs due to molecular water content inside the glassy network19. The absorption band near 2927 cm–1 occurs due to the hydrogen bonding20-22. The sharpness of this band decreases with increasing the content SrO. The absorption bands in the wavenumber range 1227-1652 cm−1 are occurred due to the asymmetric stretching relaxation of the B-O bonds of trigonal BO3 units. Such types of vibrational modes were observed within wavenumbers 12001750 cm−1 range23. The doublet splitting was observed in IR pattern of glass sample BST5K1F0.4 in two bands positions at 1652 and 1635 cm−1, but the band at 1652 cm−1 was not found for other glass samples except IR spectrum of BST5K1F0.4. The absorption band near 1635 cm−1 was shifted to some higher wavenumber side by increasing the content SrO. A broad absorption band at about 1364 cm−1 in all glass samples and a shoulder peak at 1227 cm−1 have also been observed in glass BST5K1F0.4. The band in the range 1020-955 cm–1 is attributed to a stretching vibration of B-O-Si linkage4. This band was found very broad and its broadness follows linear trend except IR pattern of glass sample BST5K1F0.6. A weak absorption band is observed near 715 cm–1 in IR spectra of all glass samples. This weak band was present due to the diborate linkage, B-O-B in the borate glassy network. In this linkage, both boron atoms are tetrahedrally coordinated with triborate super structural units24,25. This band has shifted towards lower wavenumber side with increasing SrO content. In these glass samples, no absorption band at 806 cm−1 was observed and it confirms the absence of boroxol ring in glassy network. The bands near 444 cm−1 were occurred due to vibrations of metallic cations in glassy matrix in the IR spectra of all glass samples. The similar bands were also present in IR spectra of PbO-B2O3 glass as well as barium-borate oxide glasses and attributed to the vibrations of Pb2+, Ba2+ and Mn2+ cations26-29. Hence, network-modifying behaviour was observed in which these ions entered the interstices of the network. This supports our YADAV & GAUTAM: STRUCTURAL AND OPTICAL STUDIES OF GLASSES 45 Table 1 — Peak positions in IR spectra of glass samples in the system 64[(Ba1−xSrx)TiO3]-30[2SiO2.B2O3]-5[K2O]-1[Fe2O3] Glass sample code 1 BST5K1F0.4 BST5K1F0.6 BST5K1F0.8 ST5K1F1.0 3451 3456 3453 3456 2 2927 2927 2928 2927 Wavenumber of different absorption peaks (cm−1) 3 4 5 6 7 I II 1652 - 1635 1635 1636 1637 1368 1364 1367 1360 1227 - 1020 984 955 984 716 715 712 712 8 I II 452 - 444 443 444 441 Table 2 — Peak positions in Raman spectra of glass system 64[(Ba1−xSrx).TiO3]-30[2SiO2.B2O3]-5[K2O]-1[Fe2O3] Glass sample code 1 BST5K1F0.4 BST5K1F0.6 BST5K1F0.8 ST5K1F1.0 Fig. 2 — Raman spectra of glass samples (a) BST5K1F0.4, (b) BST5K1F0.6, (c) BST5K1F0.8 and (d) ST5K1F1.0 results and network-modifying behaviour of BaO and SrO was observed. 3.2 Raman Spectroscopy Raman spectra of glass samples BST5K1F0.4, BST5K1F0.6, BST5K1F0.8 and ST5K1F1.0 are shown in Fig. 2(a-d) and peak positions of Raman spectra for these glass samples have been listed in Table 2. Raman spectra of all glass samples show four bands in wavenumber regions 1342-1322, 819-830, 712-722 and 279-288 cm−1. The first observed Raman band in these glasses is due to the B–O− vibrations from various borate groups. Such types of bands were also found in IR spectra of BST borosilicate glass samples. The second band occurs due to symmetric breathing vibrations of six-member rings with one or two BO3 triangles replaced by BO4 tetrahedra and third observed Raman band was attributed to metaborate groups30-34. The spectral bands near 3450 Raman band positions (cm−1) 2 3 4 1342 1332 1329 1322 823 819 830 825 722 718 712 716 288 282 284 279 and 2927 cm–1 are absent in Raman spectra due to the asymmetric change in dipole moment, thus IR active, but polarizability remains the same, Raman inactive. The band near 1342 cm−1 shifted lower wavenumber side with increasing the content of SrO. The positions of bands are observed with variation of Ba/Sr ratio. Raman shift from bands 823 cm−1 to 819 cm−1 increases with increasing the content of SrO from 40% to 60% while this band again shifted towards higher wavenumber with further in increasing the content of SrO from 60% to 80%. The position of Raman band at 722 cm−1 shifted towards lower wavenumber side with increasing the concentration of SrO up to 80 mole% but for pure Sr content glass sample, its position shifted towards higher wavenumber side. The low frequency band near 288 cm−1 gives the similar behaviour as first Raman band. The assignment of IR and Raman bands in the spectra of different glass samples are summarized in Table 3. 3.3 UV-Vis NIR Spectroscopy Figure 3 shows the UV-visible transmission spectra of glass samples BST5K1F0.4, BST5K1F0.6, BST5K1F0.8 and ST5K1F1.0. The absorption spectra of these glass samples do not show sharp absorption edge close to band gap. This characteristic of spectra is showing the amorphous nature of glass samples. Fig. 4 shows the Davis and Mott plot between (Įhȣ)1/2 and photon energy (hȣ) for all glass samples. The band gap of BST borosilicate glass samples have been determined by extrapolation of linear part in Davis INDIAN J PURE & APPL PHYS, VOL 53, JANUARY 2015 46 Table 3 — Assignment of infrared and Raman bands in the spectra of various glass samples in glass system 64[(Ba1-xSrx)TiO3]-30[2SiO2.B2O3]-5[K2O]-1[Fe2O3] Wavenumber (cm−1) Raman IR IR assignments Raman assignments Vibrations of metallic Cations Symmetric breathing vibrations BO3 triangles replaced by BO4 tetrahedra Symmetric breathing vibrations of six-member rings with one or two BO3 triangles replaced by BO4 tetrahedra 441-452 712-716 279-288 712-722 Vibrations of metal cations such as Ba2+, Sr2+ Bonding of B–O–B linkages (diborate linkage) 955-1020 819-830 Stretching vibration of B–O–Si linkage 1227-1652 1322-1342 2927-2928 3451-3456 - Asymmetric stretching relaxation of the B–O bond of trigonal BO3 units Hydrogen bonding Molecular water B–O− vibrations - Table 4 — Optical band gap and density of glass samples in the glass system 64[(Ba1-xSrx).TiO3]-30[2SiO2.B2O3]-5[K2O]-1[Fe2O3] Glass sample code BST5K1F0.4 BST5K1F0.6 BST5K1F0.8 ST5K1F1.0 Fig. 3 — UV-Vis spectra of glass samples (a) BST5K1F0.4, (b) BST5K1F0.6, (c) BST5K1F0.8 and (d) ST5K1F1.0 Optical band gap (eV) Density (g/cc) 2.87 2.77 2.66 2.48 3.12 2.93 2.85 2.70 and Mott plots whereas the direct band gap was determined by extrapolation of linear part in Tauc plots as listed in Table 4. The optical band gap Eg of these glasses varies with Ba/Sr ratio in the glass composition. The value of Eg was found to decrease with increasing the concentration of SrO. These results show the compositional dependence of optical band gap35,36. Such compositional dependence of optical band gap was followed by linear trend line as shown in Fig. 5. The optical band gap follows linear equation yd=3.143-0.64x. The band gap varies from 2.48 to 2.87 eV. The similar results of band gap of BaTiO3 were reported by Suzuki et al., Piskunov et al.and Wemple et al.37,38. 3.4 Density Studies Fig. 4 — Davis and Mott plots of glass samples (a) BST5K1F0.4, (b) BST5K1F0.6, (c) BST5K1F0.8 and (d) ST5K1F1.0 The density of glass and glass ceramics was determined by the liquid displacement method in the system 64[(Ba1-xSrx).TiO3]-30[2SiO2.B2O3]-5[K2O]-1[Fe2O3] and listed in Table 4. It was observed that the density of glass and glass ceramic samples was affected by various factors such as chemical constituents, internal structure and heat treatment process. The density of BST borosilicate glasses was YADAV & GAUTAM: STRUCTURAL AND OPTICAL STUDIES OF GLASSES Fig. 5 — Variation of optical band gap with change of Ba/Sr ratio 47 4 Conclusions On the basis of above results on the study of structural and optical properties of barium strontium titanate glass ceramics in the glass system 64[(Ba1-xSrx).TiO3]-30[2SiO2.B2O3]-5[K2O]-1[Fe2O3] (0.4 ” x ≤ 1.0), it can be concluded as: (i) The BST borosilicate glasses were formed by various structural groups and confirmed by the combined study of infrared and Raman spectroscopy. Such types of structural groups are dominant in different wavenumber regions. The metal cations Ba2+, Sr2+ are effective in low wavenumber side in IR spectra of these glasses. Here, the ferric oxide acts as the network modifier of these BST borosilicate glasses. (ii) The optical band gap of BST borosilicate glasses was in the range 2.48-2.87 eV. It is interesting that the Fe2O3 plays an important role that it decreases the optical band gap of barium strontium titanate borosilicate glasses. (iii) The density of BST borosilicate glasses lies in the range 2.70-3.12 gm/cc. Its value decreases with increasing the content of SrO. Acknowledgement The authors are gratefully acknowledged to the University Grant Commission (UGC), New Delhi, India for financial support under Major Research Project F. No. 37-439/2009 (SR). References Fig. 6 — Variation of density of glasses with change in Ba/Sr ratio found in the range 2.70-3.12 g/cc. The density of these glass samples is strongly influenced by Ba/Sr ratio. With increasing the content of SrO in glassy matrix, the density of BST borosilicate glasses decreases. This may be due to high density of Ba (3.51 g/cm3) in comparison to Sr (2.61 g/cm3). The density in Fe2O3 doping was slightly greater than La2O3 glass samples for the same composition, which may be due to high density of Fe (7.86 g/cm3) to the La (6.51 g/cm3). The variation of density with respect to composition change was shown in Fig. 6. 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