New Journal of Glass and Ceramics, 2011, 1, 21-27
doi:10.4236/njgc.2011 .12004 Published Online July 2011 (http://www.SciRP.org/journal/njgc)
21
Structural Inversion and Behavioural Changes as a
Function of Composition in Sr-La-Al-B-Si Based
Glasses
Prasanta Kumar Ojha1,2*, Sangram K. Rath1,2, Tapas K. Chongdar1, Nitin M. Gokhale1,
Ajit R. Kulkarni2
1Naval Materials Research Laboratory, Ambernath, Thane, Maharashtra, India; 2Indian Institute of Technology Bombay, Mumbai,
India.
Received April 29th, 2011; revised May 27th, 2011; accepted June 3rd, 2011.
ABSTRACT
A series of glass sealants for solid oxide fuel cell (SOFC) with compositions SrO (x wt%) La2O3 (15 wt%) Al2O3 (15
wt%) B2O3 (40-x wt%) SiO2 (30 wt%) [x = 10, 15, 20, 25 & 30] [SLABS] have been investigated by quantitative Fourier
Transform Infrared Spectroscopy (FTIR). Structural findings from FTIR reveal that with increasing substitution of B2O3
by SrO, even though the B2O3/SiO2 ratio decreases, however the Si-O-non-bridging bond content in the matrix is in-
creasing and glass structure is getting more inverted. UV-Vis Diffused Reflectance Spectroscopy (UV-Vis-DRS) of the
glass series shows that electrical band gap of glasses decreases in the series from 3.07 eV to 2.97 eV with increasing
substitution from x = 10 to x = 30. Conductivities of the glass samples were measured by AC impedance spectroscopy
and found to be increasing from 2.74 × 10-5 Scm-1 to 1.09 × 10-4 Scm-1 with increasing substitution from x = 10 to x =
30.
Keywords: Glass Cera mics, Mu llite Ceramics Thin Tile, Composites, Bending Strength
1. Introduction
Oxide glasses are amorphous materials showing glass
transition behavior and exhibit composition dependent
properties. Basic studies of glasses reveal that the oxides
used to synthesize glasses can be broadly divided into
three groups. Network formers provide the basic network
of glass, intermediate oxides which, though not able to
form the network by themselves, participate with net-
work formers in the basic structure, and modifier oxides
“invert” the network structure by breaking the network
bonds and generate non bridging oxygens. T he term “in-
vert” was introduced by Trapp and Stevels [1], because
the traditional network forming oxides SiO2, B2O3, and
P2O5 form continuous molecular/ionic networks in nor-
mal conditions; however when the network modifying
oxides are in majorit y on the molar ba sis, the glas ses are
invert ed str uctur all y compare d to convent iona l glasse s as
sho wn in Figur e 1. Composition of the glass and type of
additives decide the nature of different structural units
present in the glass which in turn decides the physical-
chemical properties of glasses. For example, systematic
substitution of PbO by B2O3 in ternary lead borosilicate
glasses [2] decreases the thermal expansion coefficient,
and increases the glass transition temperature. This has
been attributed to the formation of Si-O-B linkages and
increase in concentration of Q4 structural units of silicon
(where Qn represents silicon structural units having
n-number of bridging oxygen atoms). Substitution of
PbO by Bi2O3 in PbO-B2O3-SiO2 glasses results in the
Figure 1. Structure of glass inverte d by modifier ions.

Stru ctural Inversion and Behavioural Changes as a Function of Composition in Sr-La-Al-B-Si Based Glasses
22
increase of thermal expansion coefficient, deformation
and flow temperature [3] due to structural modification.
It has been reported [4-7] that addition of network mod-
ifiers (alkali/alkaline earth metal oxides) to borosilicate
glasses results in the initial conversion of BO3 to BO4
structural units. At higher concentration of modifiers,
BO4 structural units in the glass are replaced by
BO3-structural units (pla nar B O3 s tr uct ur al u nit s wit h o ne
non bridging oxygen atom). Hence the properties of
glasses with different modifier concentrations are differ-
ent. To understand the criticality of glass science it is
pertinent to investigate the structure of glasses and cor-
relate the structure with glass behavior. In this regard
many researchers have tried to elucidate the structure of
glasses using various spectroscopic techniques [8-21].
Extensive studies have been reported on the structural
aspects of boroaluminosilicate glasses using techniques
like FTIR, Raman and Magic Angle Spinning-Nuclear
Magnetic Resonance (MAS-NMR) spectroscopy [11,
22-24]. These studies highlight the existence of various
structural units like trigonally coordinated boron (BO3),
tetrahedrally coordinated boron (BO4), silicon a toms with
3 and 4 bridging oxygen atoms, Qn units with Si-O-B/
Si-O-Al linkages, etc. in the glass matrix. T hese structur-
al units finally govern the properties of t he gla sses.
In this work structure property correlation of a series
of glasses with composition SrO (x wt%), La2O3 (15 wt
%), Al2O3 (15 wt%), B2O3 (40-x wt%) and SiO2 (30 wt
%) have been investigated. Glass compositions were se-
lected for their application as sealant in solid oxide fuel
cell (SOFC) wit h B2O3 and SiO2 as the network formers,
Al2O3 as the intermediate oxide and La2O3 and SrO as
the mod ifier o xides. I n the gl ass co mposit ion, co ncentr a-
tion of SiO2, La2O3 and Al2O3 were kept constant and a
systematic substitution of B2O3 by SrO [x = 10, 15, 20,
25, 30] was introduced. This report includes the investi-
gatio n of struc tural modification as a function of compo-
sition by quantitative FTIR and UV-Vis Diffused Ref-
lectance Spectroscopy (UV-Vis-DRS) and correlated
with the electrical conductivity of glasses measured by
AC Impedance spectroscopy.
2. Experimental
SiO2, AR grade from S. D. Fine-Chem. Ltd., India,
Al2O3, AR grade from CDH, India and La2O3, GR grade
from Loba Chemie, India were used as received for
preparation of batches. Boric acid (H3BO3) (AR grade)
from SRL Pvt. Ltd., India was used as the source for
B2O3 and SrCO3, extra pure grade from Loba Chemie,
India was used as the source for SrO. Batch formulations
for compositions SrO (x wt%), La2O3 (15 wt%), Al2O3
(15 wt%), B2O3 (40-x wt%) and SiO2 (30 wt%) [x = 10,
15, 20, 25, 30] were calculated considering the gravime-
tric factors for H3BO3 and SrCO3. Raw materials in ap-
propriate proportion for 50g batch size were mixed tho-
roughly. To tal mass was melted in a platinum crucib le at
1450˚C for 1hr and quenched in a pre heated brass
mould. Glass samples were characterized by differential
thermal analysis (DTA) for their thermal behaviour.
Subsequen- tly glasses were annealed at temperatures
close their glass transition temperatures for removal of
thermal stresses fro m the glass matrices.
To analyze the phases of the melt quenched sample
X-ray diffractometry (XRD) was carried out using XPert
MPD, PAnalytical. Diffraction studies were carried out
in the range of 20˚ - 80˚ (2
θ ) with step size of 0.005˚
using CuKα radiation. Fourier Transform Infrared Spec-
troscopy (FTIR) was carried out using 1600 Series FTIR
of P erkin-El mer via KBr pellet technique method. Quan-
titative informatio n about the structura l groups in glasses
was obtained from the deconvoluted FTIR spectra. In this
report FTIR data are presented in absorbance mode for
ease of deconvolution. The diffused reflectance spec-
troscopy (DRS) of the glass samples were carried out in
UV-Vis-NIR region using Perkin Elmer precisely,
Lambda 35, UV/VIS Spectroscopy. In the DRS, absor-
bance of the sample has been plotted against energy. The
onset wavelength of the optical absorbance has been
considered for band gap energy calculation using the
standard equation, and is presented in the unit of electron
Volt (eV). Electrical conductivity of samples were meas-
ured from room temperature to 800˚C by Impedance
Spectroscopy using AUTOLAB, ECO CHEMIE, Neth-
erlands. Samples in the form of circular disc were in-
serted between two platinum disks into an alumina hold-
er and positioned (spring-loaded) inside a top loading
furnace. Platinum leads attached to the platinum plates
were connected to the impedance analyzer for collecting,
storing and processing of data. Impedance spectra of the
glasses at different temperatures were recorded in the
frequency range 100 Hz to 1 MHz. From the impedance
data resistance of the sample was used for calcula tin g the
conductivity, taking into account the sample di mensions.
3. Results and Discussion
Different batch compositions used to prepare the glasses
and the code for each such composition is enlisted in
Table 1. 50 g of glass was prepared for each composition
by melt quenching the batch under the conditions men-
tioned earlier. For phase analysis the melt quench
sam-ples werAe analyzed by XRD. Figure 2 shows a
representative XRD plo t of the melt quenched glass wi th
composition SLBS-4. XRD plots of all samples show
absence of high intensity peaks with a broad hump ap-

Structural Inversion and Behavioural Changes as a Function of Composition in Sr-La-Al-B-Si Bas ed Gla sses
23
Table 1. Batches with SrO-La2O3-Al2O3-B2O3-SiO2 compo-
sitions for glass making and their corresponding nomen-
clature.
Glass Code
Glass Composition (wt%)
SrO
La
2
O
3
Al
2
O
3
B
2
O
3
SiO
2
S LABS -3
10
15
15
30
30
S LABS -4
15
15
15
25
30
S LABS -5
20
15
15
20
30
S LABS -6
25
15
15
15
30
S LABS -7
30
15
15
10
30
pearing each case which is a clear indication of the
amo rphous/glassy nature of the sample. For structure
elucidation all the glasses were characterized through
FTIR spectroscopy. Figure 3 shows FTIR spectra for
different glasses. Each spectrum shows four active infra-
red spectral regions. First broad peak appeared in the
range 400 - 600 cm-1 and is assigned to the bending vi-
bration in SiO4 network. Peak in the range of 600 - 850
cm-1 is attributed to the bending vibration of borate seg-
ments. 850 - 1200 cm-1 segment is attributed to stre tching
vibration of structural groups containing BO4 tetrahedral
and overlaps with SiO4 tetrahedral. These structural
groups consist of BO3 and BO4 units without
non-bridging oxygen (NBOs) ions. Peak in the region
1200 to 1500 cm-1 arises from B-O bond vibration of
BO3 units [1 1, 25-27]. This signifies two types of network
structures in the glass: one consisting of BO3 and BO4
units and the other consisting of SiO4 unit. FTIR spectra
were corrected using two- point baseline correction. The
spectra were normalized to eliminate the concentration
effect of the po wder sample in KBr disc. To get quantita-
tive informatio n about structu ral groups, the spectra were
deconvoluted in to Gaussian bands. Only the 400 - 1600
cm-1 range was considered for deconvolution and least
square method was used to analyze the graphs. A repre-
sentative plot is shown in Figure 4 which illustrates de-
convolution of the FTIR spectrum of SLABS-4 glass.
Data generated by deconvolution of FTIR spectra of
samples include peak position, peak height, FWHM of
the peak, and area under the peak. Peaks were assigned
for characteristic bands and relative area under the peak
was calculated with respect to the total area under all the
peaks. FTIR spectra of all glass samples were deconvo-
luted and the generated data were analysed for structural
findings. Tabl e 2 is a representative table enlisting de-
convoluted data generated from FTIR spectra of
SLABS-3 glass. As glasses are having complicated
structures, several peaks obtained on deconvolution of
FTIR spectra could not be assigned for characteristic
vibrations as shown in the Table 2. The table includes
relative area under peak which gives a quantitative idea
of the corresponding structural group in the glass struc-
ture. Although these glasses are having three network
formers BO3, BO4 and SiO4 units, however, glass com-
positions are changing with respect to wt% of B2O3 in the
glass matrix (B2O3 is decreasing from 30 wt% to 10 wt%
from SLABS-3 to SLABS-7) so it will not be technicall y
proper to compare characteristic peaks due to BO3 and
BO4 in different glasses. Therefore the effect of increas-
ing SrO content as network modifier is compared in dif-
ferent glasses relating to the changes in SiO4 network
structure. From deconvolution data the relative area un-
der Si-O– non bridging oxygen peak (~ 929 cm-1) [12]
was calculated for different glass compositions. A
grap hic al pr e sent a ti on o f t he no n b r id ging ox yge n (N B O)
content in the glass with respect to Sr O content is shown
in Figure 5. The relative area which is a representation
of the NBO content in the glass matrix was found to in-
crease linearly with increase in SrO content. This is due
to SrO being a network modifier; it tends to invert the
struct ure by breaking the network bonds in SiO4 tetrahe-
dra l. I n the br oke n net work, the S r+2 io ns o ccup y i nters ti-
tial positions surrounded by non bridging oxygen ions.
Lu et al. [28 ] re por ted, incr ease in gla ss net work c o nnec-
tivity with decr easing B 2O3/SiO 2 i n S LAB S gl a ss s yst e m.
However, in this case it was observed that even if the
ratio decreases from 1 (SLABS-3) to 0.333 (SLABS-7),
the connectivity decreases with formation of more non
bridging oxygens as the SrO content i ncrea se d. T his ma y
be due to network modifiers having more impact over the
glas s formers s uch as S iO2 and B2O3 [29].
Diffused Reflectance Spectroscopy (DRS) of glass
samples were carried out in the UV-Vis region. Figure 6
shows absorbance of glass samples in the wavelength
range of 200 to 800 nm. From the plot it is o bserved tha t
with decrease in wavelength (i.e. with increase in energy)
glass samples start absorbing radiation below a certain
wavelength. Absorbance increases with a different gra-
dient with increase in energy and remain constant at
higher energies. This signifies an indirect type of band
edge in the glasses. Wavelength of the onset point of
absorbance was used to calculate the band gap of glass
and the plot of band gap with respect to SrO content in
the glass matrix is shown inset of Figure 6. The band
gaps of glasses were calculated from the absorption wa-
velength, using the standard wavelength energy conver-
sion formula and values found to be within 2.97 - 3.07
eV. The band gap decreased with increase in SrO co ntent
in the glass. Generally, the optical absorption of glasses
in the UV-Vis region is determined by the oxygen bond
stren gth in t he gla ss for ming ne twork. Any c hange i n the
status of the oxygen bonding, for instance, formation of
non bridging oxygen (NBO) changes the characteristic
absorption edge. In the present study, the position of the
fundamental absorption edge shifts to higher wavelength

Stru ctural Inversion and Behavioural Changes as a Function of Composition in Sr-La-Al-B-Si Based Glasses
24
Figure 2. XRD plot of SLAB S-4.
Figure 3. FTIR plots of SLABS glasses (a. SLABS-3, b.
SLABS -4, c. SLABS-5, d. SLABS-6, e. SLABS-7).
Figure 4. A ty pical deconv oluti on spect ra of SLA BS-4 glass
a. ex perimental c ur ve, b. si mulated c urve a n d the co mput ed
Gaussian bands.
(lo wer ene rgy) wit h incr eas in g SrO c onte nt. T he shifts o f
the absorption band to longer wavelength correspond to
the structural modification with generation of more
NBOs which bound an excited electron less tightly than
the bridging oxygen [30]. UV-Vis DRS result supports
the fi ndings in F TIR deconvoluti on st udy.
All the glasses have been characterized for their ther-
mal behaviour and these glasses show glass transitions
within temperature range of 554˚C to 659˚C and dila
510 15 20 25 30 35
0
4
8
12
16
20
24
28
32
Non bridging Si-O- (Relative area %)
S rO c o n ten t (wt %)
Figure 5. Non br idging Si-O– content i n SLABS glasses as a
functio n of SrO content.
300 400 500 600 700 800
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
10 15 20 25 30
2.96
2.98
3.00
3.02
3.04
3.06
3.08
Band gap (eV)
Sr O c o n te n t (w t%)
Absorbance
W avelength (nm )
SLABS-3
SLABS-4
SLABS-5
SLABS-6
SLABS-7
Figure 6. UV-Vis Diffused Reflectance Spectroscopy (UV-
Vis-DRS) of glasses with band gap versus composition
shown inset.
tometric softening point within 660˚C to 709˚C. More
over, both glass transition temperatures and dilatometric
softening point temperatures show an increasing trend
with i ncrease in substi tution i n the serie s. In thi s work to
elucidate a structure property correlation in the series,
electrical conductivity of the glasses were investigated by
AC impedance spectroscopy from room temperature to
800˚C and conductivity of the samples were calculated
from the resistance value considering the sample dimen-
sions. As the glasses were originally designed for appli-
cation in solid oxide fuel cell (SOFC) sealant so their
conductivity at SOFC operational temperature which is
higher than the glass transition temperatures is of great
concern. Therefore, a plot of conductivity of glass sam-
ples at 800˚C against the SrO content in the sample is
sho wn in Figur e 7. It is ob served that c ond ucti vit y of the
glass samples increased from 2.74 × 10-5 Scm-1 to 1.09 ×
10-4 Scm-1 with increase in SrO content in the glass ma-

Structural Inversion and Behavioural Changes as a Function of Composition in Sr-La-Al-B-Si Bas ed Gla sses
25
Table 2 . Deconvoluti on data of the FTIR spectra of SLABS-3.
Peak Peak Type
Peak Position
(cm–1) Heig ht FWH H Ar e a
Relative Area
(%) Band Assignment
1 Gaussian 431 0.02 42.89 0.78 0.76
2 Gaussian 453 0.03 45.56 1.24 1.20 Si-O-Si and O-Si-O bending vib
3 Gaussian 470 0.01 37.61 0.59 0.57
4 Gaussian 494 0.01 40.80 0.54 0.53
5 Gaussian 556 0.00 37.98 0.01 0.01
6 Gaussian 683 0.13 78.27 10.55 10.28 S tretchi ng vib of B-O-B
7 Gaussian 803 0.00 36.85 0.03 0.03
8 Gaussian 903 0.07 66.05 4.76 4.64 Non bridging Si-O
9 Gaussian 968 0.09 68.05 6.76 6.58 BO4 stretching vib
10 Gaussian 1028 0.14 67.88 9.83 9.58
11 Gaussian 1090 0.15 69.19 10.98 10.70 Si-O-Si antisym stretching vib
12 Gaussian 1143 0.06 51.15 3.15 3.07
13 Gaussian 1276 0.08 81.89 6.63 6.46
14 Gaussian 1370 0.15 106.99 16.76 16.33
B-O bond vibration of borate group
15 Gaussian 1458 0.15 96.57 15.49 15.10
16 Gaussian 1542 0.13 90.62 12.16 11.85
17 Gaussian 1621 0.03 61.07 2.36 2.30
trix from 10 wt% to 30 wt%. In general, conductivity in
oxide glass matrix depends on two factors, temperature
and number of available charge carriers (i.e. oxide ions)
[31]. In this case all the conductivities are reported at a
constant temperature of 800˚C, thus the temperature ef-
fect is constant for all the glasses. Again, the B2O3 con-
tent in the glass matrices which may be contributing to
the total conductivity of the glasses is changing. So in
this case the changing conductivity of glasses is corre-
lated with the structural changes of SiO4 units only. In-
creasing SrO content in the glass leads to increase in
NBO content due to structural inversion and decreasing
10 15 20 25 30
0. 0
5. 0x10
-5
1. 0x10
-4
1. 5x10
-4
Conductivity
(
S.cm
-1
)
S rO (wt%)
Figure 7. Conductivity of SLABS glasses versus SrO con-
tent in the glas s.
band edge. This makes more carriers available in the SrO
content the conductivity of the glas s increases.
4. Conclusions
Glasses with composition SrO (x wt%) La2O3 (15 wt%)
Al2O3 (15 wt%) B2O3 (40-x wt%) SiO2 (30 wt%) were
prepared for x = 10, 15, 20, 25 and 30. FTIR analysis
show two types of network structures in the glass: one
consisting o f BO3 and BO4 units and the other consisting
of SiO4 unit. Deconvolution of FTIR plots highlights the
structural changes with composition. With increase in
SrO content in the glass matrix, non bridging Si -O– con-
tent inc rea ses eve n thou gh the B2O3/SiO2 decreases. This
structural inversion is reflected in the properties of the
glasses. The band edge of the glass samples measured b y
Uv-Vis DRS show a decreasing band gap with increasing
SrO content. Conductivity of glass samples which is
measured by impedance spectroscopy increases with
increasi ng substitution of B2O3 by SrO.
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