Optical properties of the Na2O–B2O3–Bi2O3–MoO3 glasses

Journal of Alloys and Compounds 494 (2010) 210–213
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Optical properties of the Na 2 O–B 2 O 3 –Bi 2 O 3 –MoO 3 glasses
Yasser B. Saddeek a , K.A. Aly a,∗ , A. Dahshan b , I.M.El. Kashef c
a Physics Department, Faculty of Science, Al-Azhar University, P.O. 71452, Assiut, Egypt
b Department of Physics, Faculty of Science, Suez Canal University, Port Said, Egypt
c Department of Physics, Faculty of Education, Suez Canal University, Al Arish, Egypt
article
info
abstract
Article history:
Received 29 July 2009
Received in revised form
14 November 2009
Accepted 17 November 2009
Available online 24 November 2009
PACS:
61.43.Fs
78.20.Ci,78.30.−j
Keywords:
Borate glasses
Bismuthate glasses
Optical properties
Glasses with compositions (100 − x)Na 2 B 4 O 7 –0.5Bi 2 O 3 –0.5MoO 3 , with 0 ≤ x ≤ 40 mol% have been prepared
using the melt quenching technique. The optical transmittance and reflectance spectrum of the
glasses have been recorded in the wavelength range 300–1100 nm. The values of the optical band gap
E opt
g
for indirect transition and refractive index have been determined for 0 ≤ x ≤ 40 mol%. The average
electronic polarizability of the oxide ion ˛O2− and the optical basicity have been estimated from the calculated
values of the refractive indices. The variations in the different physical parameters such as the
optical band gap, the refractive index, the average electronic polarizability of the oxide ion and the optical
basicity with Bi 2 O 3 and MoO 3 content have been analyzed and discussed in terms of the changes in
the glass structure. The results are interpreted in terms of the increase in the number of non-bridging
oxygen atoms, substitution of longer bond-lengths of Bi–O, and Mo–O in place of shorter B–O bond and
the change in Na + ion concentration.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
The most common applications of glasses are based on their
optical transparency in the visible region in a designed range of
wavelengths. Application of glasses for making optical instruments
and lenses is based on properties like refractive index and its dispersion,
which can be varied by varying chemical composition. One
of the more important applications based on optical properties of
glass today is in the field of information technology through the use
of glass fibers for transatlantic communication cables, telecoms and
cable TV [1].
Bi 2 O 3 exists in a monoclinic form, and has an irregular octahedral
arrangement of six oxygen atoms located at distances from
2.14 to 2.8 Å. Three of these are appreciably closer to (2.14–2.29 Å)
than the other three (2.48–2.8 Å). Thus, pure Bi 2 O 3 glass due to its
low field strength and its high polarizability cannot be obtained
compared with pure B 2 O 3 glass. However, in the presence of very
small additions of conventional glass formers, it may build a glass
network of BiO 3 pyramids. With larger additions (e.g., 9–75 mol%
B 2 O 3 ), widespread areas of glass formation have been found [2,3].
Also, the bismuth borate glass systems have nowadays-wide applications
for optoelectronic materials such as laser host fibers for
∗ Corresponding author. Tel.: +20 126418273/882325647; fax: +20 882325436.
E-mail addresses: kamalaly2001@gmail.com (K.A. Aly),
adahshan73@gmail.com (A. Dahshan).
communications and photonic switches. Also, these glasses exhibit
a high refractive index, widely adjustable in the range from 1.8 to
2.5 by the appropriate glass composition and have high energy gap
attributable to the high polarizability of Bi 2 O 3 which allows using
them in wide spectra range from UV up to infrared [4–6].
On the other hand, classical alkali borate glasses have been
studied extensively because of their important advantages as solid
electrolytes in storage batteries. In this type of glass, Na 2 O converts
firstly, the three oxygen-coordinated trigonal borons (BO 3 ) to four
oxygen-coordinated tetrahedral borons (BO 4 ). This procedure continues
until the concentrations of BO 3 and BO 4 are nearly equal in
the glass structure. Boron atoms in these glasses are characterized
by its high strength covalent B–O bonds, and with the modifierdependence
B 4 /B 3 ratio which is affected in its turn by the nature
and the concentration of the modifier oxides [7–16].
In view of the aforementioned aspects, (100 − x)Na 2 B 4 O 7 –
0.5Bi 2 O 3 –0.5MoO 3 , with 0 ≤ x ≤ 40 mol%, glasses have been synthesized
and the studied to explore the relationship between the
structure of the glass and its macroscopic behaviour for the design
of materials suitable for specific applications. Infrared and UV spectroscopy
have been used for the present investigations.
2. Experimental procedures
The glass samples having the general chemical formula (100 − x)Na 2B 4O 7–
0.5Bi 2O 3–0.5MoO 3, with 0 ≤ x ≤ 40 mol%, have been prepared by the melt quenching
technique. Required quantities of Analar grade Bi 2O 3,MoO 3,Na 2CO 3, and H 3BO 3
were mixed together by grinding the mixture repeatedly to obtain a fine powder. The
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
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Y.B. Saddeek et al. / Journal of Alloys and Compounds 494 (2010) 210–213 211
Fig. 1. X-ray diffraction patterns of the glass system (100 − x)Na 2B 4O 7–
0.5Bi 2O 3–0.5MoO 3, with 0 ≤ x ≤ 40 mol%.
Fig. 3. Tauc’s plots for the glass system (100 − x)Na 2B 4O 7–0.5Bi 2O 3–0.5MoO 3, with
0 ≤ x ≤ 40 mol%.
is given by
˛h = A 0 (h − E opt
g ) p (2)
Fig. 2. Spectral behaviour of (a) transmittance and (b) reflectance of the glass system
(100 − x)Na 2B 4O 7–0.5Bi 2O 3–0.5MoO 3, with 0 ≤ x ≤ 40 mol%.
mixture was melted in a Porcelain crucible in an electrically heated furnace under
ordinary atmospheric conditions at temperature about 1373 K for 2 h to homogenize
the melt. The obtained glass samples were quenched into preheated stainless-steel
mold preheated at a temperature of about 573 K for 2 h to remove any internal
stresses. The samples were then grounded and optically polished to have suitable
dimensions (2 cm × 1cm× 0.15 cm) for the present optical measurements.
X-ray measurements were performed to check the non-crystallinity of the glass
samples using a Philips X-ray diffractometer PW/1710 with Ni-filtered, Cu K radiation
( = 1.542 Å) powered at 40 kV and 30 mA. The XRD patterns of the glasses as
shown in Fig. 1 did not reveal discrete or any sharp peaks, but the characteristic
broad humps of the amorphous materials.
The transmittance (T), and the reflectance (R) optical spectra of the glasses
(Fig. 2) were recorded at room temperature in the wavelength range 300–1100 nm
using a computerized double beam spectrophotometer, type SHIMADZU UV-2100.
where ˛0 is an energy-independent constant (band edge steepness
parameter in Tauc’s picture [18]). For amorphous materials indirect
transitions are valid according to Tauc [18], i.e. power part p = 2; so,
the values of indirect optical band-gap energy (E opt
g ) can be obtained
from Eq. (2) by extrapolating the absorption coefficient to zero
absorption in the (˛h) 1/2 –h plot as shown in Fig. 3. The respective
values of E opt
g are obtained by extrapolating to (˛h) 1/2 = 0 for the
indirect transitions [18,19]. The compositional dependence of E opt
g
shows a decrease trend as the Bi 2 O 3 and MoO 3 content as shown
in Fig. 4. The decreasing values of E opt
g can be understood in terms
of the structural changes that are taking place in the studied glass
system. According to the IR analysis [20], the stoichiometry of the
sodium diborate glass, by considering the association of Na 2 O with
both MoO 3 and B 2 O 3 . Therefore, the BO 4 units will be destroyed and
is converted into asymmetric BO 3 units with non-bridging oxygens.
Also, the structure of the investigated glasses has distorted [BiO 6 ]
octahedral units which shift the structural unit [BO 3 ] to lower wave
numbers, and shifted the band of [BO 4 ] unit to higher wave number.
This behaviour can be explained as to be due to the formation
of new bridging bond of Bi–O–B. This new bond is formed due to the
inducement of the electrostatic field of the strongly polarizing Bi 3+
ions. The stretching force constant of this bond tends to decrease,
since the stretching force constant of Bi–O bonding is lower than
that of the B–O bonding. Accordingly, the bond-length of Bi–O
3. Results and discussion
3.1. Determination of optical band gap
The optical absorption coefficient, ˛, of a material can be evaluated
from the optical transmittance, reflectance and the thickness
of the sample (d) using the relation:
˛ = 1 d ln 1 − R
T
(1)
The absorption coefficient ˛() as a function of photon energy (h)
for direct and indirect optical transitions, according to Pankove [17]
Fig. 4. Dependence of the indirect optical band gaps of the glass system
(100 − x)Na 2B 4O 7–0.5Bi 2O 3–0.5MoO 3, with 0 ≤ x ≤ 40 mol% on the parameter x.

212 Y.B. Saddeek et al. / Journal of Alloys and Compounds 494 (2010) 210–213
dependence of the refractive indices of the studied glasses can be
explained as follows. According to the Lorentz–Lorenz equation,
the density of the material affects the refractive index in a direct
proportion. Also, there are some factors influence on the refractive
index such as; the coordination number, Z, of the studied glasses
can explain its increase as the introducing of the structural units
BiO 6 , and MoO 6 as stated previously, on the expense of either BO 4
or BO 3 will increase the coordination number of the glasses, and
creating more NBOs, as noted in IR spectra [20]. This in turn leads
to an increase in the average coordination number of the studied
glasses, with respect to the base glass sample, which is increasing
their index values. Also, the creation of NBOs creates more ionic
bonds, which manifest themselves in a larger polarizability over
the mostly covalent bonds of bridging oxygen providing a higher
index value.
Fig. 5. The refractive index of (100 − x)Na 2B 4O 7–0.5Bi 2O 3–0.5MoO 3, with
0 ≤ x ≤ 40 mol% glasses as a function of wavelength in a wavelength range from 400
to 1100 nm.
bonding increases, which in turn expand the glass network and
replaces B–O linkage by the weaker Bi–O and Mo–O linkages. Thus,
the available oxygen environment around the bismuth cations will
increase, and this will decrease the values of E opt
g .
3.2. Determination of the refractive index
According to the theory of reflectivity of light, the refractive
index (n) as a function of the reflectance (R), and the extinction
coefficient (k) is given by the quadratic equation:
R = (1 − n)2 + k 2
(1 + n) 2 (3)
+ k 2
The extinction coefficient can be determined from the wavelength
() and the calculated values of ˛ according to the relation:
˛ = 4k
(4)
The refractive index can be well described by a modified Sellmeier
dispersion relation (solid lines in Fig. 5):
n() = A − B + C 2 − D 3 + E 4 (5)
where A, B, C, D and E are the Sellmeier coefficients are averaged
and given in Table 1 for all glasses.
As shown in Eq. (5), the changes in the refractive index are
affected by wavelength changes. Fig. 5 shows the plots of the refractive
index as a function of the wavelength for the studied glasses.
From this figure the refractive decrease with increase the wavelength
and increase as the Bi 2 O 3 and MoO 3 content increases in
the wavelength range of 200–2000 nm. Thus, the compositional
3.3. Polarizability and optical basicity of the glasses
The average electronic polarizability of ions and the optical
basicity are considered to be from the most important properties of
the materials, which is closely related to their applicability in the
field of optics and electronics. The optical basicity proposed was
used as a measurement of acid–base properties of the oxide glasses,
and is expressed in terms of the electron density carried by oxygen.
It was found that, optical non-linearity is caused by the electronic
polarization of a material upon its exposure to intense light beams,
so, the non-linear response of the material is governed by the electronic
polarizability. For this purpose, materials with higher optical
non-linearity have to be found or designed on the basis of the correlation
between the optical non-linearity and with some other easily
understandable and accessible electronic properties [21–24].
For isotropic substance such as glasses, the average molar refraction
(R m ) was given by the Lorentz–Lorenz equation:
[ ] [ ]
n 2 − 1 M n 2 R m =
n 2 + 2 = − 1
n 2 V m (6)
+ 2
where the quantity (n 2 − 1)/(n 2 + 2) is called the reflection loss
[24,25]. The molar refraction is related to the structure of the
glass and it is proportional to the molar electronic polarizability
of the material (˛m) (in ×10 −24 cm 3 ) through the following
Clasius–Mosotti relation:
( 3
)
˛m = R m (7)
4N
where N is Avogadro’s number. For various ternary oxide glasses
with the general formula y 1 A p O q –y 2 B r O s –y 3 C n O m , where y’s
denote the molar fraction of each oxide, the electronic polarizability
of oxide ion can be calculated on the basis of refractive indices
using the following equation:
[ ∑ ]
(Vm ˛O2−(n) /2.52)((n 2 − 1)/(n 2 + 2)) − ˛cat
=
(8)
N O
2−
Table 1
∑
The chemical composition, the refractive index parameters, the molar cation polarizability ˛cat and the number of oxide ions in the chemical formula (N O 2− ).
∑
Composition (mol%)
Refractive index parameters
˛cat
N O 2−
A B C (×10 6 ) D (×10 9 ) E (×10 11 )
0 2.59 2203.46 4.19 2.64 5.72 0.06 2.34
5 2.60 2145.27 4.35 2.84 6.41 0.14 2.37
10 2.99 3627.31 6.72 4.36 9.86 0.21 2.41
15 3.33 4815.91 8.58 5.50 12.36 0.29 2.44
20 3.10 3698.12 7.37 5.04 12.1 0.37 2.47
25 2.48 1232.81 3.96 3.08 8.15 0.44 2.51
30 2.87 2802.81 7.24 5.74 16.01 0.52 2.54
35 2.38 737.817 3.59 3.09 8.96 0.60 2.57
40 3.61 6072.79 12.9 9.80 26.59 0.67 2.60

Y.B. Saddeek et al. / Journal of Alloys and Compounds 494 (2010) 210–213 213
A = 1.67
(
1 − 1
˛O2−
)
(9)
Fig. 6. The oxide ion polarizability ˛O2− of (100 − x)Na 2B 4O 7–0.5Bi 2O 3–0.5MoO 3,
with 0 ≤ x ≤ 40 mol% glasses as a function of wavelength in a wavelength range from
400 to 1100 nm.
The A values were estimated according to Eq. (8). Fig. 7 shows a
plot of optical basicity versus the wavelength for different glassy
compositions. According to Fig. 7, the values of A decrease with an
increase in the wavelength, and increase with an increase in the
Bi 2 O 3 and MoO 3 content (at any value of the wavelength). Zhao et
al. [28] reported that B 2 O 3 is a strong acidic oxide with low optical
basicity (0.42), while Bi 2 O 3 is an oxide with a significant basicity
(1.19). The increased optical basicity of the glasses with large Bi 2 O 3
and MoO 3 content indicates that the acidic oxide of Bi 2 O 3 has a
significant effect. The low optical basicity means a reduced ability
of oxide ions to transfer electrons to the surrounding cations. On
the other hand, borate glasses with a large amount of Bi 2 O 3 content
possess high optical basicity. High optical basicity means low
electron donor ability of the oxide ions.
4. Conclusions
Increasing the Bi 2 O 3 and MoO 3 content on the expense of
Na 2 B 4 O 7 in the studied glass system, reveals some remarkable features;
as the Bi 2 O 3 and MoO 3 content increase, more NBOs were
created and the bond-length of the borate structural units increase
which in its turn increase the refractive indices, the polarizability,
and the optical basicity of the studied glasses. Also, the observed
decrease in the optical band gap was related to the weaker bond
strength of the Bi–O compared with that of B–O.
Acknowledgements
The authors thank Dr. Shaban I Hussien, Egyptian Atomic Energy
Authority for his help in preparing the samples.
References
Fig. 7. Optical basicity for the (100 − x)Na 2B 4O 7–0.5Bi 2O 3–0.5MoO 3, with
0 ≤ x ≤ 40 mol% glasses.
where ˛O2−(n) the ∑ electronic polarizability of oxide ion based on
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