infrared and raman spectroscopy -...
TRANSCRIPT
CHAPTER 5
INFRARED AND RAMAN SPECTROSCOPY
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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
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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 (4,000 – 200 cm-1)
3. Far infrared region (200 – 10 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,
0≠
∂∂
oqµ (5.1)
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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
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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
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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 vo. The excited molecule returns back to the
same basic vibrational state and emits light with the same frequency vo as an
excitation source. This type of interaction is called an elastic Rayleigh scattering. A
Photon with frequency vo 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 vm and the resulting frequency of scattered
light is reduced to vo- vm. This frequency is called Stokes frequency or “stokes”. A
Photon with frequency vo 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 vo+ vm. 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,
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0≠
∂∂
oqα (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-Na2O-B2O3 and MgO-Na2O-B2O3 systems.
From experimental results it was confirmed that mainly magnesium connected BO4
tetrahedra and unconnected BO4 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 xR2O-(100-x) B2O3 (R = Li, Na, K, Rb & Cs) [8-11] and also in
xR2O-(100-x) B2O3 (R = Ba, Ca, Sr, Mg, Cd and Pb) [12-14,6]. Meera et al [15]
studied the Raman spectra of ZnO-B2O3 glasses. The role of ZnO in network
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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 Li2O–Bi2O3–B2O3–ZnO glasses and doped
with Cu2+ 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 α2, β2 and β12 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
[BiO3] pyramidal and [BiO6] octahedral units. The formation of Zn2+ 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 Bi2O3-BaO-B2O3,
glasses. The infrared studies revealed that theese glasses are made up of [BO3], [BO4]
and [BiO6] structural units. Srinivasu et al [23] investigated bismuth based glasses
containing LiF, Li2O and SrO by different physical and spectroscopic techniques.
Infrared and Raman spectroscopic results indicate that the glass network consists of
BiO6 octahedral and BiO3 pyramidal units.
Rao et al [24] prepared and studied FT-IR and Raman spectra of Sm3+ and
Nd3+ co-doped magnesium lead borosilicate glasses. The FT-IR, Raman spectra
reveals the nature of bonding situation and different structural units in glass network.
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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 Q2 and Q3 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 P2O5-BaO-Li2O 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 Nd3+ 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 BO3, BO4
and ZnO4 units and zinc behaves as the network modifier. The formations of ZnO4
tighten the glass structure and this was confirmed by the nephelauxetic effect.
ZnO-K2O-B2O3-P2O5 glasses formulated with Sb2O3 were investigated by
Raman and NMR studies. Raman spectra with increasing Sb2O3 content reflect the
depolymerization of phosphate chains and NMR spectra reveal a steady
transformation of BO4 into BO3 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-B2O3-Bi2O3 glasses by
Ardelean et al [31]. They have reported that a part of bismuth ions are incorporated in
the glass network as [BiO6] octahedral units depends on CuO content and for all
concentration range, the [BO3] units are dominant. Both the structural units [BO3] and
[BO4] are depends on the CuO content.
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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 [BiO6] 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 Bi2O3 and B2O3 glasses were
investigated by Cheng et al [34] using IR and DSC. The composition dependence of
IR absorption suggests that addition of Bi2O3 results in a change in the short-range
order structure of the borate matrix. The increase of Bi2O3 content causes a
progressive conversion of [BO3] to [BO4] units. Bi2O3 exists in the form of [BiO6].
Upender et al [35] carried out infrared, Raman, electron paramagnetic resonance and
optical absorption studies on Li2O-P2O5-TeO2-CuO glasses. Both Raman and IR
studies showed that the present glass system consists of [TeO3], [TeO4], [PO3] and
[PO4] units. The spin-Hamiltonian parameters have been determined from EPR
spectra and it was found that the Cu2+ ion is present in tetragonal distorted octahedral
site with dx2-y2 as the ground state. Bonding parameters and bonding symmetry of Cu2+
ions have been calculated by correlating EPR and optical data and were found to be
composition dependent.
Infrared spectra of Na2O-B2O3-SiO2 and Al2O3-Na2O-B2O3-SiO2 glasses have
been analyzed to calculate the fraction N4 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 1000-1120 cm-1 arise from contribution of SiO2 and
B2O3 vibrations. Heat treatment of Na2O-B2O3-SiO2 glasses does not change the value
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of N4 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
BO3 and BO4 units placed in different structural groups in which the BO3 units being
dominant. The EPR spectra of Mn2+ 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 Mn2+ 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 Dy3+ and
Sm3+ ions through the XRD, DSC, FTIR, Raman, optical absorption,
photoluminescence and decay curve analysis. Coexistence of trigonal BO3, tetrahedral
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BO4 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 Tg. 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 BO3 and BO4 units in the disorder manner. Soppe et al
[44] observed the mixed alkali effect in Raman spectra of Li2O-Cs2O-B2O3 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 Li2O-K2O-Bi2O3-B2O3:V2O5. 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 BO3 and
BO4 units placed in different structural groups, BO3 units being dominate and bismuth
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exists as BiO3 and BiO6 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, Li2O-Na2O-B2O3 and Li2O-
K2O-B2O3, 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 Li2O-B2O3-Ta2O5-Bi2O3 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 400-1500cm-1 exhibited characteristic bands corresponding to
BO3, BO4 stretching vibrations and BO bending vibrations. Tightening of the
structure is indicated by increase in the vibration of BO3 at the cost of BO4.
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Mansour [51,52] reported infrared absorption study of Al2O3-PbO-B2O3-SiO2
and Li2O-CeO2-B2O3 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
Al2O3. There is a competition between the role of PbO and Al2O3 in changing the
value of N4 and the cross-linking of the glass network. An essential change in the role
of PbO (CeO2) 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 Co2+ (Ni2+) doped ZnO-Li2O-Na2O-B2O3 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 Co2+ 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 Li2O-
LiF-B2O3-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 [BO3] and
[BO4] units and Li-O bonds.
Infrared reflectance spectra of B2O3-Li2O-Cs2O glasses [56] have been
measured in the frequency range of 10-5000 cm-1. The mid-infrared parts of the
spectra are discussed in connection with B-O network vibrational modes. The far-
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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 LiNbO3 and LiNb3O8 nano-crystallite phases in lithium borate glass system
through XRD and FTIR studies. The FTIR data propose for these glasses and heat-
treated glass network structures mainly built by: di-, tri-, tetra-, penta- and ortho-
borate 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 N4 evaluation with the transition metal content was made.
FTIR spectra of three MgO-PbO-B2O3 glass series have been analyzed by
Doweidar et al [58]. There is a decrease in the fraction of N4 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 GeO2-M2O (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 Na2O-B2O3-P2O5-Fe2O3 glasses prepared by the
melt quenching technique. Elastic properties and FT-IR spectroscopic studies have
been employed to study the role of P2O5 on the structure of the glass system. Infrared
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spectra of the glasses reveal that the borate network consist of diborate units and is
affected by the increase in the concentration of P2O5 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 K2O-B2O3 glasses and melts by high-
temperature Raman spectroscopy. With an increase in the K2O content and with
increasing temperature, the boroxal rings, which only consist of BO3 triangular units,
were converted into pentaborate groups which consist of BO4 tetrahedral (O =
bridging oxygen atom) and BO3 units. The fraction of four-coordinated boron atoms,
N4, 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.
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Spectroscopic technique such as ESR, optical absorption, Raman and IR were
synthesized to determine the structural groups and bonding parameters in alkali boro-
tellurite glasses doped with CuO [64]. From ESR spectra, the spin Hamiltonian
parameter values indicate that the ground state of Cu2+ is dx2-y2 and the site symmetry
around the Cu2+ ion is tetragonally distorted octahedral coordination. Bonding
parameters calculated from optical absorption and ESR data are found to change with
alkali oxide and TeO2 content. Both Raman and IR results show that glass network
consists of TeO3, TeO4, BO3 and BO4 group as basic structural groups. BO3→ BO4
transition is also observed, which correlates with the transition of TeO4 → TeO3 via
TeO3+1 .
Pure and copper doped multi-component lithium borosilicate glasses [65] have
been investigated as a function of Al2O3 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
Cu2+ state especially in the samples containing low concentration of Al2O3. 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 Al2O3.
Different concentrations of dysprosium doped strontium lithium bismuth
borate glasses we synthesized and characterized through Raman, absorption and
visible luminescence spectroscopy’s [66]. These Dy3+ doped glasses are studied for
their utility for white light emitting diodes. Coexistence of triangle BO3 and
tetrahedral BO4 units was evidenced by Raman spectroscopy. From the emission
spectra, a strong blue emission that corresponds to the transition, 4F9/2→6H15/2, was
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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 Er3+/
Yb3+ 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 xCeO2-(1-x)B2O3 were prepared and
characterized by thermal expansion and infrared measurements [68]. IR results of
CeO2-rich glasses (40-60 mol%) confirm that CeO2 affects the glass network as both a
network modifier and a network former. The modifier part of CeO2 is consumed in
transformation of BO3 units to BO4 groups. The rest of CeO2 can participate in the
network in the form of CeO4 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 xLi2O-(30-x)Na2O-10WO3-60B2O3 (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.
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5.6 RESULTS AND DISCUSSION
5.6.1 IR spectra
The IR absorption spectra of the present glasses were recorded in the range
300- 2000 cm-1. Figure 5.1 shows the normalized FTIR absorption spectra of xLi2O–
(30-x)Na2O–10WO3–60B2O3 glasses. The observed infrared spectra of these glasses
arise largely from the modified borate networks and are mainly active in the spectral
range 400-1600 cm-1; therefore the spectra are shown in 350-1800 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 5Li2O–
25Na2O–10WO3–60B2O3 glass composition is shown in figure 5.2. The infrared spectra
of the present glasses show 11-12 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, 1228-1266 cm-1, 1346-1380 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 BO3 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 BO3 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
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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 BO4 units and third region lies between
1200 and 1600 cm-1 and is due to stretching vibrations of B-O in BO3 triangles [72-
74]. Alkali oxides like Li2O and Na2O are well known glass modifiers and may enter
the glass network by transforming sp2 planar BO3 units into most stable sp3
tetrahedral BO4 units and may also create non–bridging oxygens. Both BO3 and BO4
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 Li2O 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 Li2O-B2O3-Bi2O3 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 BO3 and BO4 groups
[77, 78]. In Li2O-B2O3-Bi2O3 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
145
pentaborate groups, which are composed of BO4 and BO3 units in the ratio 1:4. The
intensity of this band increases and then decreases with Li2O content.
Since WO3 is a conditional glass former, with the substitution of WO3 with
alkali oxides in borate glass network the intensity of vibrational band due to the BO3
groups is observed to increase at the expense of BO4 structural units [82]. From
XANES and FTIR studies in TeO2-WO3 glasses it was observed that W6+ prefers six-
coordination 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 BO4 tetrahedra overlapping with the stretching vibrations of WO6 units.
Boudlich et al [70] reported a mixture of WO4 and WO6 units at 880 cm-1 and
at 900-950 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 BO4 groups overlapping with the stretching vibrations of WO4 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 BO4 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 BO3 and BO4 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 1346-1380 cm-1 is attributed to asymmetric stretching
vibrations of the B-O of trigonal (BO3)3- units in meta-, pyro- and ortho-borate units
[85]. The band around 1438 cm-1 is assigned to antisymmetrical stretching vibrations
146
with three non-bridging oxygens of B-O-B linkages [88-92]. The weak band
observed around 1637 cm-1 indicates a change from BO3 triangles to BO4
tetrahedra, and this peak may also be
Table 5.1 Infrared absorption band positions of the xLi2O-(30-x)Na2O-10WO3-60B2O3
glass system
Glass IR band positions in wave number (cm-1)
x=0 465 539 697 762 874 934 1020 1074 1266 1377 1407 1637
X=5 464 532 697 764 874 938 1015 1050 1266 1363 1460 1629
X=10 456 533 694 761 868 945 1010 1075 1228 1346 1433 1638
X=15 467 535 695 762 877 944 1015 1081 1234 1350 1438
1636
X=20
467 539 697 764 873 940 1017 1080 1261 1371 1442 1632
X=25
467 537 698 764 876 940 1023 1075 1254 1360 1448 1632
X=30 444 540 695 759 879 946 1021 1076 1246 1381 1452 1641
assigned to OH bending mode of vibrations [93,94]. The IR band in the range 1228-
1266 cm-1 is assigned to B-O stretching vibrations of (BO3)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 BO4 tetrahydral to the
non-bridging oxygen containing BO3 trangles. The peak at around 540 cm-1 can be
attributed to the borate deformation modes such as the in-plane bending of boron-
oxygen 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-1and 1346 cm-1 exhibiting mixed alkali in present
glasses. The assignments of IR bands are given in Table 5.2.
147
5.2 Infrared band assignments of the present glasses.
Wavenumber
( cm-1 )
IR band assignments
~ 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 BO4
and BO3 units in the ratio 1:4
~ 762 B–O–B bending vibrations of BO3 and BO4 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 BO4 groups overlapping
with the stretching vibration of WO4 units
~ 940 Stretching vibration of B–O linkages in BO4 groups tetrahedral
overlapping with the stretching vibration of WO6 units
~1020 Stretching vibration of B–O bands in BO4 units from tri, tetra and penta
borate groups
~1080 Pent borate groups
~1266 B-O stretching vibration of (BO3)3- units in metaborate chains and ortho
borates
~ 1346 Asymmetric stretching vibration of the B-O of triangle (BO3)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 BO3 triangle to BO4
tetrahedra
148
To quantify the inter-alkali variation effect in the relative population of
tetrahedral and triangular borate units we have calculated the fraction of four-
coordinated boron atoms, N4 and three coordinated boron atoms containing NBOs, N3
which were estimated as follows [96]
N4 = [A4] / {[A3] + [A4]} and N3 = 1- N4 (5.3)
where A3 and A4 denotes the areas of BO3 units (the areas of component IR bands
from 1200- 1600 cm-1) and BO4 units (the areas of component bands from 800- 1200
cm-1), respectively. The amount of four coordinate boron atoms, N4 and three
149
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 BO3 units, N3
varies non linearly. There is some sort of ordering that occurs which leads to a
lessening of NBOs at RLi=0.5.
5.6.2 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) 250 –
500 cm-1, (ii) 500 – 1100 cm-1 and (iii) 1250 – 2000 cm-1. In all the present glasses
150
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
xLi2O–(30-x)Na2O–10WO3–60B2O3 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 329 -340 cm-1, 873-904 cm-1 and 951- 960 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 WO4 tetrahedral. The peaks around 329-340 cm-1 are due to the
bending vibrations of W–O–W in the WO6 units [97]. In TeO2-WO3 glasses this peak was
observed around 340-360 cm-1 [98]. The Raman band around 873-904 cm-1 is assigned to
stretching vibrations of W–O–W in the WO4 or WO6 units.
In the Raman spectra of all the glassy specimens, there is a peak observed
around 774-793 cm-1 which is characteristic of a six membered ring with one or
two BO4 tetrahedra. Earlier, Brill [99] assigned this peak to the formation of six
membered rings containing one BO4 tetrahedron, and the shift of this peak towards
lower frequency has been assigned to six memebered rings with two BO4 tetrahedra.
The six membered rings with one BO4 tetrahedron can be in triborate, tetraborate or
pentaborate forms, and rings with two BO4 tetrahedra can be in diborate, di-triborate
or di-pentaborate forms. In alkali borate glasses [100] and also in PbO-B2O3 glasses
151
this peak was observed at 806 cm-1 and 760-780 cm-1, respectively [101]. In the
studied glasses, the presence of Raman band in the range at 675-692 cm-1 has been
attributed to the pentaborate groups in the borate glasses. Similar results were
observed in mixed alkali zinc borate glasses [62].
.
152
Table 5.3 Observed Raman band positions of the present glass system.
Glass Raman band positions in wave number (cm-1)
30Na2O-10WO3-60B2O3 339 546 652 677 788 812 874 934 1407
5Li2O-25Na2O-10WO3-60B2O3 334 564 664 675 764 808 882 938 1460
10Li2O-20Na2O-10WO3-60B2O3 339 555 649 691 776 810 874 946 1433
15Li2O-15Na2O-10WO3-60B2O3 340 554 638 692 779 815 867 944 1438
20Li2O-10Na2O-10WO3-60B2O3 329 559 649 698 792 820 873 940 1442
25Li2O-5Na2O-10WO3-60B2O3 337 564 644 684 764 817 886 940 1448
30Li2O-10WO3-60B2O3 333 561 650 686 776 825 879 942 1452
Table 5.4 Observed Raman band positions in xLi2O-(30-x)Na2O-10WO3-60B2O3 glass system.
Band positions
( cm-1 )
Raman band assignments
~329 Bending vibrations of W-O-W in the WO6 units
~ 546 In plane bending mode of BO33- units
~ 675 Denotes the existence of BO2O23- units
~ 697 Stretching of B-O- bonds attached to large number of borate groups
~ 774 Ring breathing vibration of six membered ring contains both BO3
triangles and BO4 tetrahedral
~ 836 Symmetric stretching Vibration of planar orthoborate units (BO3)3-
~ 873 Starching vibrations of W-O-W in the WO4 or WO6 units
~ 951 W-O- stretching vibrations in WO4 tetrahedra
~1464 Stretching of B–O- bonds attached to large number of borate groups
153
The Raman bands in the high frequency range 1429-1547 cm-1 has been
assigned to stretching of B-O- bonds attached to large number of borate groups by
Kamitsos [9]. In K2O-B2O3 glasses the Raman band around 1490 cm-1 is attributed to
BO2O- 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 546-564 cm-1 is assigned to in plane-
bending mode of BO33- units. In R2O-B2O3 (R=Li, Na and K) glasses the Raman
band centered at 548 cm-1 is assigned to in plane-bending mode of BO33- 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 R2O 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 R2O, leads to the formation of boron in fourfold coordination, i.e.
BO4– units, with the positive alkali ion (R+) adjacent to the negative BO4– unit to
provide local charge neutrality. The second process is the formation of a non-bridging
oxygen (O– ) adjacent to the positive alkali ion.
154
5.7 CONCLUSIONS
Mixed alkali tungsten borate glasses in the form of xLi2O–(30-x)Na2O–
10WO3–60B2O3 (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 BO3, BO4, WO3, WO6 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 BO3 , BO4 tetrahedra and pentaborate groups linked to BO4
tetrahedra. Raman spectra confirms the IR results regarding the presence of
tungsten ions mainly as WO6 groups
(iii) The amount of four coordinate boron atoms, N4 and the non-bridging oxygen
containing BO3 units, N3 varies non linearly as a function of inter alkali variation.
155
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