CHAPTER 5 INFRARED AND RAMAN SPECTROSCOPY

Transcription

1 CHAPTER 5 INFRARED AND RAMAN SPECTROSCOPY

2 5.1 INTRODUCTION TO INFRARED SPECTROSCOPY Spectroscopy is the study of the interaction of electromagnetic radiation with a chemical substance. The nature of the interaction depends upon the properties of the substance. When radiation passes through a sample (solid, liquid or glass), certain frequencies are absorbed which are unique for each molecule and is the characteristic of a substance. Infrared spectroscopy (IR) is one of the most useful techniques available for structural studies of glasses. For the particular case of glasses modified by metal oxides, IR is a powerful tool because it leads to structural aspects related to both the local units constituting the glass network and the anionic sites hosting the modifying metal cations. Borate glasses provide an ideal case in comparison to other glass forming systems, to demonstrate the effectiveness of infrared spectroscopy in glass science. Infrared reflectance and transmission measurements are the most suitable techniques for glass studies among the various sampling techniques employed in infrared measurements. A key advantage of reflectance spectroscopy is the use of the same sample for data acquisition over a broad and continuous frequency covering both mid and far infrared region, without the need of changing sample form or its thickness, a problem usually encountered in transmission measurements. The spectral profiles obtained from reflectance studies are free of band shape distortions, which are present in transmission spectra. IR method utilizes the optical excitation of the localized vibrational modes of atoms, whose excitation energy is in the infrared region. The transitions involved in Infrared absorption are associated with the vibrational changes within the molecule. Different bonds have different vibrational 125

3 frequencies, and one can detect the presence of bonds in a compound by identifying the characteristic frequency as an absorption band in the infrared spectrum. Infrared spectroscopy is broadly divided into three regions as 1. Near infrared region (13,000 4,000 cm -1 ) 2. Mid infrared region (4, cm -1 ) 3. Far infrared region ( cm -1 ) Infrared spectra are usually plotted as percent of transmittance rather than absorbance the ordinate. This makes absorption bands as dips in the curve rather than as maxima in the case of UV and visible spectra. No two compounds except optical isomers can have identical absorption bands. To interpret the infrared spectra an understanding of the energy levels in the molecule is required. The vibrational and rotational motions are quantized in a molecule between the atoms. By the absorption of suitable energy the molecule undergoes transition to higher quantized energy levels which causes an absorption in the spectrum. Infrared transmittance spectroscopy is the most suitable technique for glass studies. Infrared spectra are usually recorded by measuring the transmittance of light quanta through a continuous distribution of the sample. Interaction of infrared radiation with a vibrating molecule is only possible if the electric vector of the radiation field oscillates with the same frequency as does the molecular dipole moment. A vibration is infrared active only if the molecular dipole moment (µ) is modulated by the normal vibration, µ q o 0 (5.1) 126

4 where q stands for the normal coordinate describing the motion of the atoms during a normal vibration. If this condition is fulfilled, then the vibrations are said to be allowed or active in the infrared spectrum, otherwise they are said to be forbidden or inactive. The frequencies of the absorption bands are proportional to the energy difference between the vibrational ground and excited states. 5.2 FUNDAMENTAL VIBRATIONS OF MOLECULES Each molecule has certain natural vibrational frequencies. When light is incident on the molecule, the frequency which matches the natural vibrational frequency is absorbed by the molecule resulting in molecular vibrations. Modes of vibrations Stretching: Distance between two atoms increases or decreases Bending: Position of the atom changes relative to the original bond axis To interpret the IR spectra an understanding of the energy levels in the molecule is required. The vibrational and rotational motions are quantized in a molecule between the atoms. By the absorption of suitable energy, the molecule undergoes transition to higher quantized energy levels which causes an absorption in the spectrum. IR spectroscopy is the most suitable technique for glass studies. For a particular case of glasses modified by metal oxides, infrared technique is a powerful tool because it gives information about structural aspects related to both the local units constituting the glass network and the anionic sited hosting the modifying metal cations. For a molecule to absorb IR radiation, it has to fulfill certain requirements which are as follows: a) Correct Wavelength of Radiation 127

5 Molecule absorbs radiation only when the natural frequency of vibration of some part of a molecule (i.e., atoms or group of atoms comprising it) is same as the frequency of radiation. After absorbing correct wavelength of radiation, the molecule vibrates at increased amplitude. This occurs at the expense of the energy of IR radiation that has been absorbed. b) Electric dipole This is another condition for a molecule to absorb IR radiation. A molecule can only absorb IR radiation when its absorption causes a change in its electric dipole moment (μ). A molecule is said to be electric dipole when there is a slight positive and a slight negative electric charge on its component atoms. When a molecule having electric dipole is kept in the electric filed (as in the case when the molecule is kept in a beam of IR radiation), the field will exert forces on the electric charges in the molecule. Opposite charges will experience force in opposite directions. This tends to decrease separation. As the electric field of the IR radiation is changing its polarity periodically, it means that the spacing between the charged atoms (electric dipoles) of molecule also changes periodically. When the vibration in the charged atoms is fast, the absorption of radiation is intense and thus, the IR spectrum will have intense absorption bands. On the other hand, when the rate of vibration of charged atoms in atoms in a molecule is slow, there will be weak bands in the IR spectrum. 5.3 INTRODUCTION TO RAMAN SPECTROSCOPY Raman spectroscopy has been successfully used for the structure determination in crystalline materials. Due to the absence of symmetry in glasses, the vibration analysis in these materials is not straightforward. In fact, no agreed theory of 128

6 vibration exists to explain all the aspects of vibrations in disordered materials even though several steps in that direction do exist [1,2]. The method due to Brawer [3] considers disorder to be a perturbation on the vibration of a structural group and hence considers the width of the Raman peak as a measure of the disorder. The intensity of Raman lines is a useful parameter since it is proportional to the number of scattering centers and their scattering efficiency. Raman spectroscopy is a measurement of the wavelength and intensity of in elastically scattered light from molecules. A molecule with no Raman-active modes absorbs a photon with the frequency v o. The excited molecule returns back to the same basic vibrational state and emits light with the same frequency v o as an excitation source. This type of interaction is called an elastic Rayleigh scattering. A Photon with frequency v o is absorbed by Raman-active molecule which at the time of interaction is in the basic vibrational state. Part of the photon s energy is transferred to the Raman-active mode with frequency v m and the resulting frequency of scattered light is reduced to v o - v m. This frequency is called Stokes frequency or stokes. A Photon with frequency v o is absorbed by a Raman-active molecule, which at the time of interaction, is already in the excited vibrational state. Excessive energy of excited Raman-active mode is released, molecule returns to the basic vibrational state and the resulting frequency of scattered light goes up to v o + v m. This frequency is called Anti strokes frequency or Anti-Stokes. When a molecule is exposed to an electric field, electrons and nuclei are forced to move in opposite directions. Thus, a dipole moment, which is proportional to the electric field strength and to the molecular polarizability α, is induced. A molecular vibration can be observed in the Raman spectrum only if there is a modulation of the molecular polarizability by the vibration, 129

7 α q o 0 (5.2) If this condition is fulfilled, then the vibrations are said to be allowed or active in the Raman spectrum, otherwise they are said to be forbidden or inactive. 5.4 REVIEW OF EARLIER WORK A brief review of the earlier work carried out on IR and Raman studies of glasses is presented. Kamitsos et al [4,5] studied the structure of cesium, rubidium and potassium borate glasses by Raman and far infrared spectroscopy. It was demonstrated that the creation of non-bridging oxygens (NBOs) even at very low modification levels of alkali oxide. Konijnendijk [6] used Raman spectroscopy to interpret the molecular structure of glasses in the systems CaO-Na 2 O-B 2 O 3 and MgO-Na 2 O-B 2 O 3 systems. From experimental results it was confirmed that mainly magnesium connected BO 4 tetrahedra and unconnected BO 4 tetrahedra are formed. Krogh-Moe [7] studied the infrared spectra of boron oxide and alkali borate glasses extensively. It was concluded that the crystalline and vitreous system consists of the same structural units and proposed a structural model. The borate glasses mainly consists of boroxol rings, tetra borate and diborate groups. Raman spectroscopy has been used effectively to study the formation of poly borate groups in xr 2 O-(100-x) B 2 O 3 (R = Li, Na, K, Rb & Cs) [8-11] and also in xr 2 O-(100-x) B 2 O 3 (R = Ba, Ca, Sr, Mg, Cd and Pb) [12-14,6]. Meera et al [15] studied the Raman spectra of ZnO-B 2 O 3 glasses. The role of ZnO in network 130

8 modification is not clearly understood, although the Raman data is consistent with the glass forming tendency of ZnO. Bale et al [16-21] reported spectroscopic studies such as EPR, Raman, Infrared and optical absorption of pure Li 2 O Bi 2 O 3 B 2 O 3 ZnO glasses and doped with Cu 2+ ions. The non-linear variation of the spin-hamiltonian parameters with ZnO content was attributed to the mixed former effect. This effect is also evident in the optical absorption spectra. By correlating the EPR and optical absorption data, the bonding parameters α 2, β 2 2 and β 1 have been evaluated and correlated with optical basicity. In addition, the bonding parameters also varied non-linearly suggesting a mixed former effect. IR and Raman spectra shows that these glasses are made up of [BiO 3 ] pyramidal and [BiO 6 ] octahedral units. The formation of Zn 2+ in tetrahedral coordination was observed. The average electronic polarizability of the oxide ion, optical basicity and Yamashita-Kurosawa s interaction parameter were also examined. Vijaya Kumar et al [22] determined theoretical basicity of Bi 2 O 3 -BaO-B 2 O 3, glasses. The infrared studies revealed that theese glasses are made up of [BO 3 ], [BO 4 ] and [BiO 6 ] structural units. Srinivasu et al [23] investigated bismuth based glasses containing LiF, Li 2 O and SrO by different physical and spectroscopic techniques. Infrared and Raman spectroscopic results indicate that the glass network consists of BiO 6 octahedral and BiO 3 pyramidal units. Rao et al [24] prepared and studied FT-IR and Raman spectra of Sm 3+ and Nd 3+ co-doped magnesium lead borosilicate glasses. The FT-IR, Raman spectra reveals the nature of bonding situation and different structural units in glass network. 131

9 Ivascu et al [25,26] reported FT-IR, Raman and thermo luminescence investigations in ternary lithium phosphate glasses. FT-IR and Raman spectroscopy studies revealed a local network structure mainly based on Q 2 and Q 3 tetrahedrons connected by P-O-P linkages. Luminescence investigations show that by adding modifier oxides to phosphate glass dose dependent TL signals results upon irradiation. Thus P 2 O 5 -BaO-Li 2 O glass system is a possible candidate material for dosimetry in the high range (>10 Gy). ESR study proves that the investigated glass system could be successfully used as ESR dosimeter in medicine and industry. Sodium zinc borate and lead bismuth borate glasses containing Nd 3+ were investigated by Karthikeyan et al [27,28] by FTIR, optical absorption and emission studies. From FTIR spectral studies they concluded that the glass contains BO 3, BO 4 and ZnO 4 units and zinc behaves as the network modifier. The formations of ZnO 4 tighten the glass structure and this was confirmed by the nephelauxetic effect. ZnO-K 2 O-B 2 O 3 -P 2 O 5 glasses formulated with Sb 2 O 3 were investigated by Raman and NMR studies. Raman spectra with increasing Sb 2 O 3 content reflect the depolymerization of phosphate chains and NMR spectra reveal a steady transformation of BO 4 into BO 3 units. Potassium zinc borophasphate glasses were prepared and studied by NMR and Raman spectroscopy [29, 30]. FT-IR and EPR investigations have been done on CuO-B 2 O 3 -Bi 2 O 3 glasses by Ardelean et al [31]. They have reported that a part of bismuth ions are incorporated in the glass network as [BiO 6 ] octahedral units depends on CuO content and for all concentration range, the [BO 3 ] units are dominant. Both the structural units [BO 3 ] and [BO 4 ] are depends on the CuO content. 132

10 Properties of unconventional lithium bismuthate glasses were investigated by Hazra et al [32,33] by IR and Raman and optical absorption techniques. From the Raman data they concluded the presence of [BiO 6 ] octahedral units. Increase of optical band gap and the increase of IR cutoff frequency with lithium content is attributed to the decrease in the strength of the glass structure. Structure and crystallization kinetics of Bi 2 O 3 and B 2 O 3 glasses were investigated by Cheng et al [34] using IR and DSC. The composition dependence of IR absorption suggests that addition of Bi 2 O 3 results in a change in the short-range order structure of the borate matrix. The increase of Bi 2 O 3 content causes a progressive conversion of [BO 3 ] to [BO 4 ] units. Bi 2 O 3 exists in the form of [BiO 6 ]. Upender et al [35] carried out infrared, Raman, electron paramagnetic resonance and optical absorption studies on Li 2 O-P 2 O 5 -TeO 2 -CuO glasses. Both Raman and IR studies showed that the present glass system consists of [TeO 3 ], [TeO 4 ], [PO 3 ] and [PO 4 ] units. The spin-hamiltonian parameters have been determined from EPR spectra and it was found that the Cu 2+ ion is present in tetragonal distorted octahedral site with d x2-y2 as the ground state. Bonding parameters and bonding symmetry of Cu 2+ ions have been calculated by correlating EPR and optical data and were found to be composition dependent. Infrared spectra of Na 2 O-B 2 O 3 -SiO 2 and Al 2 O 3 -Na 2 O-B 2 O 3 -SiO 2 glasses have been analyzed to calculate the fraction N 4 of four coordinated borons. A reasonable agreement between the calculated from IR spectra and those determined from NMR spectroscopy could be attained under certain condition. It has been proposed that the absorption bands in the region cm -1 arise from contribution of SiO 2 and B 2 O 3 vibrations. Heat treatment of Na 2 O-B 2 O 3 -SiO 2 glasses does not change the value 133

11 of N 4 and this indicates that the structure of alkali borate phase is the same in the glass obtained from melt and in the heat-treated one [36]. Krishna Kumari et al [37] investigated mixed alkali zinc borate glasses by UV-VIS absorption, EPR and FT-IR spectroscopic studies. FT-IR measurements of all glasses revealed that the network structure of glass system are mainly based on BO 3 and BO 4 units placed in different structural groups in which the BO 3 units being dominant. The EPR spectra of Mn 2+ ions doped glasses exhibited a characteristic hyperfine sextet around g = 2. The spectroscopic analysis of the obtained results confirmed near octahedral site symmetry for the Mn 2+ impurity ions. Crystal field and Racah parameters are evaluated from optical absorption spectra. The optical band gap and Urbach energies are determined the mixed alkali effect. Risen et al [38,39] reported far infrared and Raman spectra of several series of single, mixed alkali meta phosphate and mixed penta silicate glasses in both the annealed and annealed forms. The frequencies of the cation-motion bands in the far infrared and Raman spectra, which correspond to cation site vibrations, do not shift with alkali content, including that the vibrationally significant local geometry and forces associated with a particular cation are unaffected by the introduction of the second cation into the glass structure or by annealing. A simple vibrational model is presented which shows that the cation- dependent shifts are due to small changes in the network bond angles and variation of the cation site forces. Ahamed et al [40,41] prepared and characterized lead containing barium zinc lithium fluoroborate glasses doped with different concentrations of trivalent Dy 3+ and Sm 3+ ions through the XRD, DSC, FTIR, Raman, optical absorption, photoluminescence and decay curve analysis. Coexistence of trigonal BO 3, tetrahedral 134

12 BO 4 units non-bridging oxygen and strong OH bonds was evidenced by IR and Raman spectroscopy. The bonding parameters and the oscillator strengths were determined from the absorption spectra. It was proved that the present glass is more suitable for generation of white light for blue LED chips. Srinivasa Rao et al [42, 43] reported, for the first time the study of mixed alkali effect (MAE) in boroarsenate and borobismuthate glasses through density, DSC, DC electrical conductivity and IR studies. The strength of the MAE in T g. DC electrical conductivity and activation energy has been determined. It was observed that the strength of MAE in DC electrical conductivity is less pronounced with increase in temperature, supporting molecular dynamic results. The IR studies shows that the glass system contains BO 3 and BO 4 units in the disorder manner. Soppe et al [44] observed the mixed alkali effect in Raman spectra of Li 2 O-Cs 2 O-B 2 O 3 glasses. Kamitsos et al [45] employed Infrared and Raman study to investigate the structure of borate glasses. The results presented for single alkali glasses illustrate the strong dependence of the network structure on the nature and content of the oxide modifier. Variations of the cation-motion frequencies in the far-infrared spectra of mixed alkali glasses have been interpreted as suggesting changes in the alkali-oxygen interaction upon alkali mixing. Kistaiah et al [46,47] reported mixed alkali bismuth borate glasses of composition Li 2 O-K 2 O-Bi 2 O 3 -B 2 O 3 :V 2 O 5. The spectroscopic properties of glass samples were studied using infrared and Raman spectroscopic techniques. Acting as complimentary spectroscopic techniques, both types of measurements IR and Raman revealed that the network structure of the studied glasses is mainly based on BO 3 and BO 4 units placed in different structural groups, BO 3 units being dominate and bismuth 135

13 exists as BiO 3 and BiO 6 octahedral units. The observation of disappearance and reappearance of some IR and Raman bands and non-linear variation of the peak positions of some of these bands with alkali content is an important result pertaining to the mixed alkali effect in this glass system. Infrared spectra of mixed-alkali diborate glasses, Li 2 O-Na 2 O-B 2 O 3 and Li 2 O- K 2 O-B 2 O 3, have been investigated by Selvaraj and Rao [48]. B-O stretching and B-O- B bending frequencies exhibit nonlinear shifts which can be described as a mild mixed alkai effect. Both shifts and nonlinear variations of frequencies may be explained on the basis of alteration of the structure of the diborate groups in the presence of Li + ions. Efimov [49] discussed various methods for the quantitative analysis of the IR and Raman spectra of various inorganic glasses to determine physically meaningful optical functions and individual band parameters. Resent data on band intensities or frequencies obtained with these methods for phosphate, borate and germinate glasses were considered. Trends in the IR and Raman band assignments deduced from the current state of vibrational spectroscopy of glasses were analyzed and structural information obtained by different authors for binary phosphate, borate and germinate glasses was compared. Sharada and Suresh babu [50] prepared Li 2 O-B 2 O 3 -Ta 2 O 5 -Bi 2 O 3 glasses via normal melt quenching technique and these glasses were characterized by FTIR, optical absorption and AC conductivity studies. FTIR spectra of the samples recorded in the frequency range cm -1 exhibited characteristic bands corresponding to BO 3, BO 4 stretching vibrations and BO bending vibrations. Tightening of the structure is indicated by increase in the vibration of BO 3 at the cost of BO

14 Mansour [51,52] reported infrared absorption study of Al 2 O 3 -PbO-B 2 O 3 -SiO 2 and Li 2 O-CeO 2 -B 2 O 3 glasses. The IR band located near 700 cm -1 was suggested to be due to the vibrations of bridging oxygens between trigonal boron atoms. The depolymerization of the whole glass skeleton increases with increasing the content of Al 2 O 3. There is a competition between the role of PbO and Al 2 O 3 in changing the value of N 4 and the cross-linking of the glass network. An essential change in the role of PbO (CeO 2 ) in these glasses from glass modifier to glass former was also suggested. Raghavendra Rao et al [53,54] studied and correlated the physical and structural properties of Co 2+ (Ni 2+ ) doped ZnO-Li 2 O-Na 2 O-B 2 O 3 glasses. The structural parameters of all the glasses are evaluated and a non-linear behavior is observed. FT-IR spectra of ZLNB glasses reveal diborate units in borate network. The optical absorption spectra suggest the site symmetry of Co 2+ in the glasses is near octahedral. Crystal field and inter-electronic repulsion parameters are also evaluated. The optical band gap and Urbach energies exhibit the mixed alkali effect. Naresh and Buddhudu [55] carried out optical absorption, FT-IR and Raman spectral studies on transparent and stable glasses in the chemical composition of Li 2 O- LiF-B 2 O 3 -MO (M = Zn and Cd) glasses. The LFB glasses with the presence of ZnO and CdO an extended UV- transmission ability has been achieved. The measured FT- IR and Raman spectra have exhibited the vibrational bands of B-O from [BO 3 ] and [BO 4 ] units and Li-O bonds. Infrared reflectance spectra of B 2 O 3 -Li 2 O-Cs 2 O glasses [56] have been measured in the frequency range of cm -1. The mid-infrared parts of the spectra are discussed in connection with B-O network vibrational modes. The far- 137

15 infrared parts of the spectra yields cation vibrational modes, which distinctly depend on the glass composition. Kashif et al [57] studied the effect of heat treatment and doping the transition metal on LiNbO 3 and LiNb 3 O 8 nano-crystallite phases in lithium borate glass system through XRD and FTIR studies. The FTIR data propose for these glasses and heattreated glass network structures mainly built by: di-, tri-, tetra-, penta- and orthoborate groups. It was found that the quntitative evaluation of these various borate species in the glass structures was influenced by the transition metal. A detailed discussion relating to the N 4 evaluation with the transition metal content was made. FTIR spectra of three MgO-PbO-B 2 O 3 glass series have been analyzed by Doweidar et al [58]. There is a decrease in the fraction of N 4 of four coordinated boron with increasing the MgO content, at the expanse of PbO. A new technique has been presented to make use of the N4 data and follow the change in the modifier and former fractions of PbO and MgO. These fractions change markedly, at different rates, with the glass composition. The ability of the glass to include MgO increases with increasing PbO content. Martino et al [59] reported polarized Raman and infrared absorption studies for GeO 2 -M 2 O (M = Na and Cs) glasses, as a function of the alkali content. Infrared reflectivity measurement confirm the presence of a fraction of higher coordinated Ge atoms, either five-fold or six-fold. Gaafar et al [60] investigated Na 2 O-B 2 O 3 -P 2 O 5 -Fe 2 O 3 glasses prepared by the melt quenching technique. Elastic properties and FT-IR spectroscopic studies have been employed to study the role of P 2 O 5 on the structure of the glass system. Infrared 138

16 spectra of the glasses reveal that the borate network consist of diborate units and is affected by the increase in the concentration of P 2 O 5 content as a second network former. These results are interpreted in term of the replacement of the borate units with B-O-B bridges by phosphate units with non-bridging oxygens (NBOs). Gajanan et al [61] investigated room temperature Raman specra on Oxyfluro Vanadate glass containing various proportions of lithium fluoride and rubidium fluoride to see an effect of mixture of alkali on vanadium-oxygen bond length. The variation in bond length and its distribution about a most probable values was correlated to the alkali environment present in these glasses. They observed that all rubidium environment around the network forming units is more homogenous than all lithium environment. Akagi et al [62] studied the structure of K 2 O-B 2 O 3 glasses and melts by hightemperature Raman spectroscopy. With an increase in the K 2 O content and with increasing temperature, the boroxal rings, which only consist of BO 3 triangular units, were converted into pentaborate groups which consist of BO 4 tetrahedral (O = bridging oxygen atom) and BO 3 units. The fraction of four-coordinated boron atoms, N 4, obtained from deconvolution of the Raman bands was gradually reduced above the glass transition temperature and converged to a constant value over 1400K. Silver oxide doped lead lithium borate (LLB) glasses have been prepared and characterized by XRD, SEM, EDS, FTIR and Raman[63].Results from FTIR and Raman spectra indicate that Ag2O acts as a network modifier even at quantities by converting three coordinated to four coordinated boron atoms. Optical basicity was also evaluated which was affected by the silver oxide composition. 139

17 Spectroscopic technique such as ESR, optical absorption, Raman and IR were synthesized to determine the structural groups and bonding parameters in alkali borotellurite glasses doped with CuO [64]. From ESR spectra, the spin Hamiltonian parameter values indicate that the ground state of Cu 2+ is d x2-y2 and the site symmetry around the Cu 2+ ion is tetragonally distorted octahedral coordination. Bonding parameters calculated from optical absorption and ESR data are found to change with alkali oxide and TeO 2 content. Both Raman and IR results show that glass network consists of TeO 3, TeO 4, BO 3 and BO 4 group as basic structural groups. BO 3 BO 4 transition is also observed, which correlates with the transition of TeO 4 TeO 3 via TeO 3+1. Pure and copper doped multi-component lithium borosilicate glasses [65] have been investigated as a function of Al 2 O 3 concentration by a variety of spectroscopic (optical absorption, IR, Raman and ESR) and dielectric properties (over a range of frequency and temperature). The results of optical absorption and ESR spectral studies have indicated that a part of copper ions do exist in Cu + state in addition to Cu 2+ state especially in the samples containing low concentration of Al 2 O 3. The IR and Raman spectral studies have revealed that there is a decreasing degree of disorder in the glass network with increase in the concentration of Al 2 O 3. Different concentrations of dysprosium doped strontium lithium bismuth borate glasses we synthesized and characterized through Raman, absorption and visible luminescence spectroscopy s [66]. These Dy 3+ doped glasses are studied for their utility for white light emitting diodes. Coexistence of triangle BO 3 and tetrahedral BO 4 units was evidenced by Raman spectroscopy. From the emission spectra, a strong blue emission that corresponds to the transition, 4 F 9/2 6 H 15/2, was 140

18 observed and it also shows combination of blue, yellow and red emission bands for these glasses. In addition to that, white light emission region have been observed from these studies. Maheshvaran et al [67] reported structural and optical behavior of the Er 3+ / Yb 3+ co-doped boro-tellurite glasses through FTIR, Raman, absorption, luminescence, upconversion luminescence and lifetime measurements for green leaser applications. Through the absorption spectra, bonding parameters, oscillator strengths and Judd- Ofelt (JO) parameters were calculated and reported. Different glasses in the system xceo 2 -(1-x)B 2 O 3 were prepared and characterized by thermal expansion and infrared measurements [68]. IR results of CeO 2 -rich glasses (40-60 mol%) confirm that CeO 2 affects the glass network as both a network modifier and a network former. The modifier part of CeO 2 is consumed in transformation of BO 3 units to BO 4 groups. The rest of CeO 2 can participate in the network in the form of CeO 4 units. 5.5 AIM AND SCOPE OF PRESENT WORK The aim of the present study is to obtain specific data regarding the local structure of xli 2 O-(30-x)Na 2 O-10WO 3-60B 2 O 3 (0 x 30) quaternary glass system by means of Raman and infrared spectroscopy. The correlation between spectral assignment and the physical properties of the glasses will be discussed. The presence of mixed alkali oxides in the glass system increases the mixed alkali effect in the present study. 141

19 5.6 RESULTS AND DISCUSSION IR spectra The IR absorption spectra of the present glasses were recorded in the range cm -1. Figure 5.1 shows the normalized FTIR absorption spectra of xli 2 O (30-x)Na 2 O 10WO 3 60B 2 O 3 glasses. The observed infrared spectra of these glasses arise largely from the modified borate networks and are mainly active in the spectral range cm -1 ; therefore the spectra are shown in cm -1 range for better clarity. Each spectrum was deconvelated by using 12 Gaussian functions considering peak assignment as reported earlier [69-71]. An example of the fitting for 5Li 2 O 25Na 2 O 10WO 3 60B 2 O 3 glass composition is shown in figure 5.2. The infrared spectra of the present glasses show absorption peaks. All the glass compositions show absorption peak at 467 cm -1, 540 cm -1, 697 cm -1, 762 cm -1, 874 cm -1, 940 cm -1, 1020 cm -1, 1080 cm -1, cm -1, cm -1, 1438 cm -1 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 5.1. According to the Krogh Moe s the structure of the boron oxide glass consists of a random network of planer BO 3 triangles with a certain fraction of six membered (boroxol) rings [7]. X-ray and neutron diffraction data suggests that glass structure consists of a random network of BO 3 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 142

20 143

21 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 BO 4 units and third region lies between 1200 and 1600 cm -1 and is due to stretching vibrations of B-O in BO 3 triangles [72-74]. Alkali oxides like Li 2 O and Na 2 O are well known glass modifiers and may enter the glass network by transforming sp 2 planar BO 3 units into most stable sp 3 tetrahedral BO 4 units and may also create non bridging oxygens. Both BO 3 and BO 4 units co-exist in these glasses, which is evident from Figure 5.1. The broad IR bands as shown in figure 5.2 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 Li 2 O content is increasing. Padmaja and Kistaiah [75] observed the vibrations of Li cations in mixed alkali zinc borate glasses doped with transition metal ions at around 460 cm -1 in the IR spectra. Moustafa et al [76] observed weak IR band around 438 cm -1 in Li 2 O-B 2 O 3 -Bi 2 O 3 glass system which was attributed to Li-O-Li bonds. The present IR spectra showed non-existence of band at 806 cm -1, which reveals the absence of boroxol rings in glasses and hence it consists of only BO 3 and BO 4 groups [77, 78]. In Li 2 O-B 2 O 3 -Bi 2 O 3 glasses the peak at around 700 cm -1 is assigned to pentaborate units [79]. In alkali boro-tungstate glasses, the IR band around 700 cm -1 stands for B-O-B bond bending vibrations of bridging oxygen atoms [80,81]. In the present study, the IR peak around 697 cm -1 is assigned to bending vibrations of 144

22 pentaborate groups, which are composed of BO 4 and BO 3 units in the ratio 1:4. The intensity of this band increases and then decreases with Li 2 O content. Since WO 3 is a conditional glass former, with the substitution of WO 3 with alkali oxides in borate glass network the intensity of vibrational band due to the BO 3 groups is observed to increase at the expense of BO 4 structural units [82]. From XANES and FTIR studies in TeO 2 -WO 3 glasses it was observed that W 6+ prefers sixcoordination and exhibits an absorption band at 930 cm -1 [83]. In the present IR spectra the peak at around 940 cm -1 is assigned to the stretching vibrations of B-O linkages in BO 4 tetrahedra overlapping with the stretching vibrations of WO 6 units. Boudlich et al [70] reported a mixture of WO 4 and WO 6 units at 880 cm -1 and at 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 penta- borate groups and also due to the starching vibration of non-bridging oxygens of BO 4 groups overlapping with the stretching vibrations of WO 4 units. This peak was observed around 866 cm -1 and at around 850 cm -1 in single alkali boro-tungstate glasses [8,84]. A broad band around 1020 cm -1 is assigned to stretching vibrations of B-O bonds in BO 4 units from tri, tetra and penta borate groups. The weak peak at about 762 cm -1 can be attributed to B-O-B bending vibrations of BO 3 and BO 4 groups with W-O-W vibrations in the borate network [85,86]. This indicates that tungsten enter the glass structure. The IR band around 1080 cm -1 is assigned to penta borate groups [87]. The peak lying in cm -1 is attributed to asymmetric stretching vibrations of the B-O of trigonal (BO 3 ) 3- units in meta-, pyro- and ortho-borate units [85]. The band around 1438 cm -1 is assigned to antisymmetrical stretching vibrations 145

23 with three non-bridging oxygens of B-O-B linkages [88-92]. The weak band observed around 1637 cm -1 indicates a change from BO 3 triangles to BO 4 tetrahedra, and this peak may also be Table 5.1 Infrared absorption band positions of the xli 2 O-(30-x)Na 2 O-10WO 3-60B 2 O 3 glass system Glass IR band positions in wave number (cm -1 ) x= X= X= X= X= X= X= assigned to OH bending mode of vibrations [93,94]. The IR band in the range cm -1 is assigned to B-O stretching vibrations of (BO 3 ) 3- unit in metaborate chains and orthoborates and these groups contain large number of non-bridging oxygens (NBO s) [84]. This suggests the conversation of the BO 4 tetrahydral to the non-bridging oxygen containing BO 3 trangles. The peak at around 540 cm -1 can be attributed to the borate deformation modes such as the in-plane bending of boronoxygen triangles [95]. Figure 5.3(a) and figure 5.3(b) shows the compositional dependence of various peak position of the IR bands in the present study. The above figures depicts a non linear variation in peak positions for IR band centered around 697 cm -1, 874 cm -1, 1020 cm -1 and 1346 cm -1 exhibiting mixed alkali in present glasses. The assignments of IR bands are given in Table

24 5.2 Infrared band assignments of the present glasses. Wavenumber IR band assignments ( cm -1 ) ~ 467 Li cation vibrations ~ 540 Borate deformation mode such as the in-plane bending of B O triangles ~ 697 Bending vibrations of pentaborate groups, which are composed of BO 4 and BO 3 units in the ratio 1:4 ~ 762 B O B bending vibrations of BO 3 and BO 4 groups with W O W vibrations in the borate network ~ 874 Stretching vibration of tri-, tetra- and pentaborate groups also due to the stretching vibration of non-bridging oxygen s of BO 4 groups overlapping with the stretching vibration of WO 4 units ~ 940 Stretching vibration of B O linkages in BO 4 groups tetrahedral overlapping with the stretching vibration of WO 6 units ~1020 Stretching vibration of B O bands in BO 4 units from tri, tetra and penta borate groups ~1080 Pent borate groups ~1266 B-O stretching vibration of (BO 3 ) 3- units in metaborate chains and ortho borates ~ 1346 Asymmetric stretching vibration of the B-O of triangle (BO 3 ) 3- units in meta-, pyro- and ortho-borate units ~ 1438 Anti symmetrical starching vibrations with three non-bridging oxygen s of B-O-B linkages ~1637 OH bending mode of vibration and change from BO 3 triangle to BO 4 tetrahedra 147

25 To quantify the inter-alkali variation effect in the relative population of tetrahedral and triangular borate units we have calculated the fraction of fourcoordinated boron atoms, N 4 and three coordinated boron atoms containing NBOs, N 3 which were estimated as follows [96] N 4 = [A 4 ] / {[A 3 ] + [A 4 ]} and N 3 = 1- N 4 (5.3) where A 3 and A 4 denotes the areas of BO 3 units (the areas of component IR bands from cm -1 ) and BO 4 units (the areas of component bands from cm -1 ), respectively. The amount of four coordinate boron atoms, N 4 and three 148

26 coordinate boron atoms is plotted as a function of inter alkali variation in figure 5.4. It is clear from the above figure that the non-bridging oxygen containing BO 3 units, N 3 varies non linearly. There is some sort of ordering that occurs which leads to a lessening of NBOs at R Li = 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 figure 5.5. There are three regions clearly visible in the Raman spectra : (i) cm -1, (ii) cm -1 and (iii) cm -1. In all the present glasses 149

27 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 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. Deconvoluted Raman spectra of xli 2 O (30-x)Na 2 O 10WO 3 60B 2 O 3 glass system is shown in figure 5.6. All the glass compositions show Raman peak at around 333 cm -1, 554 cm -1,650cm -1, 684 cm -1, 787 cm -1, 825cm -1, 873 cm -1, 957 cm -1 and 1464 cm -1. The Raman band positions of all the glasses under study are given Table 5.3. The vibrational Raman bands at cm -1, cm -1 and 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 WO 4 tetrahedral. The peaks around cm -1 are due to the bending vibrations of W O W in the WO 6 units [97]. In TeO 2 -WO 3 glasses this peak was observed around cm -1 [98]. The Raman band around cm -1 is assigned to stretching vibrations of W O W in the WO 4 or WO 6 units. In the Raman spectra of all the glassy specimens, there is a peak observed around cm -1 which is characteristic of a six membered ring with one or two BO 4 tetrahedra. Earlier, Brill [99] assigned this peak to the formation of six membered rings containing one BO 4 tetrahedron, and the shift of this peak towards lower frequency has been assigned to six memebered rings with two BO 4 tetrahedra. The six membered rings with one BO 4 tetrahedron can be in triborate, tetraborate or pentaborate forms, and rings with two BO 4 tetrahedra can be in diborate, di-triborate or di-pentaborate forms. In alkali borate glasses [100] and also in PbO-B 2 O 3 glasses 150

28 this peak was observed at 806 cm -1 and cm -1, respectively [101]. In the studied glasses, the presence of Raman band in the range at cm -1 has been attributed to the pentaborate groups in the borate glasses. Similar results were observed in mixed alkali zinc borate glasses [62].. 151

29 Table 5.3 Observed Raman band positions of the present glass system. Glass Raman band positions in wave number (cm -1 ) 30Na 2 O-10WO 3-60B 2 O Li 2 O-25Na 2 O-10WO 3-60B 2 O Li 2 O-20Na 2 O-10WO 3-60B 2 O Li 2 O-15Na 2 O-10WO 3-60B 2 O Li 2 O-10Na 2 O-10WO 3-60B 2 O Li 2 O-5Na 2 O-10WO 3-60B 2 O Li 2 O-10WO 3-60B 2 O Table 5.4 Observed Raman band positions in xli 2 O-(30-x)Na 2 O-10WO 3-60B 2 O 3 glass system. Band positions Raman band assignments ( cm -1 ) ~329 Bending vibrations of W-O-W in the WO 6 units ~ 546 In plane bending mode of BO 3-3 units ~ 675 Denotes the existence of BO 2 O 3-2 units ~ 697 Stretching of B-O - bonds attached to large number of borate groups ~ 774 Ring breathing vibration of six membered ring contains both BO 3 triangles and BO 4 tetrahedral ~ 836 Symmetric stretching Vibration of planar orthoborate units (BO 3 ) 3- ~ 873 Starching vibrations of W-O-W in the WO 4 or WO 6 units ~ 951 W-O - stretching vibrations in WO 4 tetrahedra ~1464 Stretching of B O - bonds attached to large number of borate groups 152

30 The Raman bands in the high frequency range cm -1 has been assigned to stretching of B-O - bonds attached to large number of borate groups by Kamitsos [9]. In K 2 O-B 2 O 3 glasses the Raman band around 1490 cm -1 is attributed to BO 2 O - triangles linked to other borate triangular units [62]. In the present study 1464 cm -1 Raman band was assigned to stretching of B-O - bonds attached to large number of borate groups. The Raman band in the range cm -1 is assigned to in planebending mode of BO 3 3- units. In R 2 O-B 2 O 3 (R=Li, Na and K) glasses the Raman band centered at 548 cm -1 is assigned to in plane-bending mode of BO 3-3 units [100]. In all the glasses studied the total alkali content is of 30 mol%. At this concentration of alkali content in borate glasses, the boroxol rings get converted mostly into pentaborate groups. This is observed clearly by the strong presence of a weak peak ~ 650 cm -1 resembling the localized breathing motions of oxygen atoms in the boroxol ring [75]. The Raman band around 825 cm -1 is assigned to W-O single bond stretching vibrations within W O W bonded units [102]. The assignments of Raman bands are given in Table 5.4. Raman spectroscopic studies of alkali borate glasses for different concentrations of R 2 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 R 2 O, leads to the formation of boron in fourfold coordination, i.e. BO 4 units, with the positive alkali ion (R + ) adjacent to the negative BO 4 unit to provide local charge neutrality. The second process is the formation of a non-bridging oxygen (O ) adjacent to the positive alkali ion. 153

31 5.7 CONCLUSIONS Mixed alkali tungsten borate glasses in the form of xli 2 O (30-x)Na 2 O 10WO 3 60B 2 O 3 (0 x 30) were prepared, and their structural properties have been studied. The following conclusions were made: (i) The infrared studies indicate the presence of BO 3, BO 4, WO 3, WO 6 and Li units in the structure of the studied glasses. The intensities and their peak position were affected by the alkali concentrations in each glass. The peak positions of few IR bands showed non-linear variation with alkali content manifesting mixed alkali effect. (ii) The Raman spectra of the investigated glasses exhibits several bands which are attributed to BO 3, BO 4 tetrahedra and pentaborate groups linked to BO 4 tetrahedra. Raman spectra confirms the IR results regarding the presence of tungsten ions mainly as WO 6 groups (iii) The amount of four coordinate boron atoms, N 4 and the non-bridging oxygen containing BO 3 units, N 3 varies non linearly as a function of inter alkali variation. 154

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