CHAPTER-2
HEAVY METAL OXIDE GLASSES CONTAINING
BISMUTH AND LEAD
There is an increasing interest in the scientific community regarding heavy metal oxide
(HMO) glasses containing bismuth and lead oxide. Since lead and bismuth has high
atomic weight, these are placed together in the periodic table. The atomic number of lead
is 82 and bismuth is 83. The electronic configuration of both elements is same for s orbit
i.e. 6s2. Therefore, they have many similar properties. HMO glasses have high density,
low transformation temperature, high refractive index, large thermal expansion, good
gamma-ray shielding properties and transparency to the visible light. HMO glasses also
have good chemical durability and excellent infrared transmission properties as compared
to other glasses.
2.1 Heavy metal oxide glasses containing lead
Lead based glasses are used for several applications. For example, these glasses can be
used for industrial applications such as optical glass, crystal glasses, glass ceramics and
low melting glasses. These glasses have high refractive index and high density. The
excellent property of lead oxide glass is in terms of phase diagram which indicate that it
is possible to make a glass containing 90 mol% of lead oxide. These glasses are softer in
nature. Therefore, cutting, grinding and polishing is easy. Lead oxide glasses also have
good chemical durability.
2.1.1 Lead borate glasses
Lead oxide can behave as network former as well as network modifier. It acts as glass
former, at higher values of PbO and behaves as network modifier at lower concentration
of PbO. The PbO-B2O3 glasses have very wide range of glass formation (20-80 mol%
PbO). These glasses have high density and excellent transmittance values. The lead
9
borate glasses containing 40-90 mol% PbO was studied by infrared spectroscopy by
Brekhoviskikh and Cheremisinov in 1960. All the reported glasses have shown spectra
containing bands. These bands are very strong by nature. This indicates that the
molecular structure of B2O3 was retained in high lead glasses. These bands produced low
softening point and viscosities of the glasses. Later Bray et al. 1963 studied the lead
borate glasses by nuclear magnetic resonance (NMR) technique. It was concluded that
the boron atoms are situated in BO3 triangles. At low content of PbO, the Pb-O bond
becomes ionic and Pb2+ ions are considered as network modifiers whereas the formation
of BO4 units proceeds at the rate of two tetrahedral for each oxygen atom. The formation
rate of tetrahedral is reduced above 20 mol% of PbO because some lead atoms behave as
pyramids. These pyramids contain BO3 units rather than BO4 units. The change in BO3
units was discovered at 30 mol% PbO. This was speculated due to change in electronic
distribution in B3-O units which results from replacement of B3-O-B4 by B3-O-Pb bonds.
When the content of PbO and B2O3 are same, the fractional content of boron B4 units
attains its maximum value (~0.5). Laird and Bergeron in 1970 studied the structure of
lead borate glasses and melts. They stated that BO3 units were formed as a result of
destruction of boron-oxygen network. De Luca and Bergeron in 1971 provided the
structure of PbO-2B2O3 glass which contains 1/3 boron atoms in four co-ordination
(B4O7)-2 states with one Pb2+ ion linked with each state. Figure 2.1 has shown that each
lead atom co-ordinates with four oxygen atoms. The lead borate glasses have higher
values of densities (Saini et al. 2009) as shown in figure 2.2. George et al. 1999 suggested
that the variations occur in the density values of PbO-B2O3 glasses due to the use of
different sample preparation techniques and crucible materials. Binary and ternary lead
borate glasses have been studied by Zahra et al. 1993 and Witke et al. 1995 by using
Raman spectroscopy technique. It was found that PbO is placed into four co-ordinated
positions in the network. Lead borate glasses are known to form homogeneous and single
phase glasses in 20-80 mol% PbO. Bray, 1985 has pointed out that the glass transition
temperature is highest at 27 mol% PbO.
10
Fig. 2.1: Structure of PbO-2B2O3 glass.
11
Fig. 2.2: Density of lead borate glasses (Saini et al. 2009).
12
2.1.2 Lead silicate glasses
Lead silicate glasses have been studied in wide range of composition. The melting
temperature of these glasses has been achieved below 10000C. These glasses have many
commercial applications in technical areas of electronic multipliers, glass to melt seals,
colored TV tubes etc.. These glasses have also excellent physical properties such as good
values of micro-hardness as well as electrical and thermal conductivity. The X-ray
diffraction of lead silicate glasses was undertaken by Bair in 1936. It was found that SiO-Pb angle is smaller than 1800 and bond length of Pb-Si is 3.80A0. Bair stated that bond
angles are separated from each other by silicon oxygen tetrahedral. He suggested that the
Pb-Pb distance is variable (4 ~ 6.50A0). It was concluded that the exact value depends
upon the composition of PbO . Pb-Pb interaction was neglected by him.
Krogh-Moe in
1958 proved the accuracy of Bair’s results. PbO-SiO2 glass system was studied. The
random network theory could not describe the structure of lead silicate glasses with high
content of lead. Bagdyk’yants and Alekseev in 1960 studied the lead silicate glasses by
electron diffraction method. It was found that in high silicate glasses, lead atoms are
randomly distributed at three dimensional Si-O network and each lead atom is attached to
two oxygen atoms. Rabinovich 1976 and Pirious and Arash 1980 have studied the
different structural units which have SiO4 tetrahedra and PbO4 pyramids. Bessada et al.
1994 has given the structure of PbO-SiO2 glasses at high content of PbO. He stated that
PbO4 are linked with each other to form polymeric chains. Toropov et al. 1969 has
established the results of PbO-SiO2, 2PbO-SiO2, 3PbO-2SiO2 and 4PbO-SiO2 glass
systems. Brosset 1963 has shown the position of Pb-Pb peak which describes that the
lead groups have a definite structure. In alkali metal cations, the vitreous lead
orthosilicate and high lead glasses are completely stable commercial products. The lead
ion has capability to form glass because it has high polarizablity . Bobovich and Tulub in
1958 have provided the Raman spectra of lead silicate glasses. It was concluded that the
lead containing glasses gives the excellent intense spectrum. This indicates that Pb-O
bond is covalent as per Pauling- electro-negativity scale. This covalency occurs due to
strong mutual polarizablity of Pb2+ and O2- ions and their participation in glass forming
network. The density values increase with increasing mole fraction of lead oxide content
as shown in table 2.1 by Singh et al. 2008.
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2.2 Heavy metal oxide glasses containing bismuth
Glasses containing bismuth have special applications in the field of industry such as low
loss fibers, infrared transmitting materials and active medium of Raman fiber optical
amplifiers and oscillators ( Donald et al. 1978, Dumbaugh 1986, Vogal et al. 1991 and
Pan et al. 1994). These glasses can be used for production of electron multipliers after
reduction of hydrogen atmosphere. Batal 2007 stated that reducing glasses show very
high surface conductivity (up to 8-10 orders of magnitude higher than unmodified
glasses). These glasses cannot vitrify individually but addition of other oxides can
vitrify them. Miyaji et al. 1994 reported that bismuth ions have five or six fold coordination state. Bismuth ions always play the role of network former. Bismuth
containing glasses have high refractive index and chemical durability. The electronic
configuration of bismuth is 6s2. Bishay and Maghrabi 1969 concluded that BiO3 groups
are formed in bismuth glasses. But some authors (Dimitriev et al. 1992, Gattaf 1997
Milankov et al. 2005 and Stehle 1998) claimed that Bi ions can participate as network
former and network modifier ( ratio of these two participants depends upon the type of
other glass constituents). Milankov et al. 2005 provided the theory that Bi ions are
present in the network forming units as BO6 groups but other authors (Trzeblatowski et
al. 2000, Witkowska et al. 2002 and Stentz et al. 2000) stated that BiO3 are network
former groups and BiO6 are network modifier groups. But some other authors, for
example, Egili et al 1998, Dimitriev et al. 2001, Baia et al. 2002 and Culea et al. 2004
suggested that BiO3 and BiO6 units are present and their ratio changes with the other
oxide glass components.
2.2.1 Bismuth borate glasses
Bismuth borate glasses have shown large number of applications due to their many
extraordinary properties such as wide range of glass formation, high density and high
refractive index. The glass formation range is possible from 20-80 mol% of bismuth
14
Table 2.1: Chemical composition and density of PbO-SiO2 glass system (Singh et al.
2008).
Glass samples
Composition (Mole Fraction)
PbO
SiO2
G1
0.45
0.55
G2
0.50
0.50
G3
0.55
0.45
G4
0.60
0.40
G5
0.65
0.35
G6
0.70
0.30
15
Density (g/cm3)
(±0.04)
5.60
5.99
6.22
6.45
6.65
6.82
oxide. Levin and Daniet 1962 gave the phase diagram of bismuth borate glasses.
Egoryshwa et al. 2005 have prepared the single crystals of all bismuth borate phases and
detected all of them by mid infrared absorption spectroscopy. George et al. 1999 gave the
experimental procedure of formation of bismuth borate glasses which contain 88 mol%
of bismuth oxide. This process was popularly known as
roller quenching method.
Glasses which contain high concentration of Bi2O3 have very high density value
(~9 gcm-3). The glass transition temperature, Tg values found were very high at 23 mol%
of bismuth oxide. Bajaj et al. 2009 reported that density and molar volume increases
with the increasing concentration of bismuth oxide. These values are provided in table
2.2. Murata and Mouri 2007 have obtained the UV-Visible absorption of bismuth borate
glasses which show the optical absorption band around 440nm. Further, Sanz et al. 2006
stated that highest melting temperature of bismuth borate glasses effects the optical
properties of these glasses. The optical absorption spectra of bismuth borate glasses is
shown in figure 2.3. Becker 2003 has measured the glass transition temperature, liquidus
temperature and crystalline onset values of bismuth borate glasses. Liebertz 1983 proved
that borate glassy melts are recognized by high viscosity. Therefore, their rate of
nucleation and crystal growth have very small values.
2.2.2 Bismuth silicate glasses
Bismuth silicate glasses can be prepared by the reaction and melting of the components.
The bismuth silicate glasses after reduction in hydrogen atmosphere possess high
secondary emission and therefore, they can be used in electron multiple devices. In
bismuth silicate glasses, the Bi ions are present in trivalent state. The crystal structure of
Bi2SiO5 glass is provided in figure 2.4. The structural similarities in local structure
around bismuth and oxide ions have been confirmed by X-ray radial distribution and 29Si
NMR studies. In 50 mol% Bi2O3 and 50 mol% SiO2 glass, bismuth ions are substituted at
BiO6 sites and
BiO6 units are occupied by eight BiO6 units, the edges are joined with
16
Table 2.2: Chemical composition, density and molar volume in bismuth borate glasses
(Bajaj et al. 2009).
Composition
(mol %)
Sample
No.
Density
(gm /cm3)
Molar Volume
(cm3 mol-1)
Bi2O3
B 2 O3
1
20
80
4.457±0.002
33.40
2
25
75
4.815±0.002
35.04
3
30
70
5.470±0.003
34.46
4
33
67
5.636±0.003
35.56
5
37.5
62.5
6.006±0.003
36.34
6
40
60
6.246±0.001
36.51
7
41
59
6.359±0.006
36.50
8
42
58
6.389±0.003
36.95
9
47
53
6.737±0.004
37.98
10
50
50
6.874±0.005
38.96
11
53
47
7.074±0.005
39.54
12
55
45
7.214±0.004
39.85
13
60
40
7.550±0.011
40.72
14
66
34
7.765±0.012
42.65
17
18
Fig. 2.4: Crystal structure of Bi2SiO5.
19
each other. The oxygen sites are presented into three groups; Bi-O-Bi, Si-O-Bi and Si-O-Si,
whose populations are defined as 20%, 40% and 40% in 50 mol% of bismuth and 50 mol%
of silicate glass. Qn distributions represent the amounts of oxide ions present in the structural
groups which was obtained from 29Si-NMR analysis. The Bi-O-Bi bridges form layers; Si-OSi bridges form chains of Q2 units and the bismuthate layers are joined with silicate chains.
IR and Raman spectra of Į-Bi2O3 and bismuthate glasses contain the selenite spectra
(Witkowska et al. 2005). But Witkowska et al. 2002 stated that no single commonly accepted
diagram for bismuth silicate glasses has been reported. Later, Witkowska in 2005 have
undertaken extended X-ray absorption fine structure (EXAFS) and molecular dynamics (MD)
studies of bismuth silicate glasses with bismuth oxide in the composition range of 3-5 mol
% . It was concluded that BiO5 units are present in all glasses and one or two Bi-O distances
are much larger than others within BiO5 groups. The density and molar volume of bismuth
silicate glasses increases with the increase of bismuth oxide content ( Batal 2007). Their
values are provided in table 2.3.
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Table 2.3: Chemical composition, density and molar volume in bismuth silicate glasses
(Batal 2007).
Sample
No.
Glass Composition
(mol %)
Density
(gm /cm3)
Molar Volume
Vm(cm3 mol-1)
Bi2O3
SiO2
1
90
10
7.284
58.39
2
85
15
7.210
56.18
3
80
20
7.155
53.77
4
75
25
7.071
51.54
5
70
30
6.921
49.72
6
65
35
6.813
47.56
7
60
40
6.716
45.20
8
55
45
6.657
42.55
21