Thermal and spectroscopic investigations of glasses in the system PbO–PbCl2–PbBr2

Abstract

Glass formation has been examined in the system 15PbO.xPbCl2.(85–x)PbBr2 (where 0≤x≤25) where the PbO concentration is kept constant while PbCl2 and PbBr2 concentrations are varied. The glasses have been examined using thermal and spectroscopic techniques. Tg, ΔCp, refractive index, optical basicity have been found to remain unaffected by the composition which is a curious feature of these glasses. It is found that there is a wide infrared window available for use in the investigated glasses. The IR window extends from 1000 to 1500 cm–1 and in glasses where PbCl2 is less than 20 mol%, the window extends up to 2260 cm–1. X-ray photoelectron spectra (XPS) revealed that the 4f5/2 and 4f7/2 peaks due to f-level transitions have a constant difference in energies, but with energy and FWHM values that varying sensitively and systematically with composition. The observations are discussed and suitable explanations are provided.

Full text

Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 56 Number 3 June 2015 115
Phys. Chem. Glasses: Eur. J. Glass Sci. Technol. B, June 2015, 56 (3), 115–120
1. Introduction
Halide glasses became an interesting eld of study
when heavy metal uoride glasses were shown to
have potential for making optical communication
bres. Heavy metal uoride glasses have wide appli-
cations as infrared transmission windows suitable for
use with laser sources. Light emiing diodes (LEDs)
requiring this window region are already in use.(1–3)
Light aenuation in uoride glasses was shown to
be orders of magnitude lower than in silicate glass
bres and this feature reduces the requirement for
repeaters. The infrared transmission capability of
halide glasses has many other applications in inte-
grated optics. Mixed oxyhalide glasses also exhibit
a broad infrared transmiance range and low values
of optical losses.(4–10)
Lead halides are known to form glass in combina-
tion with lead oxide. Oxide (O2−) and uoride (F−)
ions are known to behave chemically in a similar
way because they have very similar nephelauxetic
behaviour.(11,12) Therefore the presence of PbO in-
stead of PbF2 may be expected to bestow similar
physical property advantages in halide glasses.
Several aspects of PbO–PbF2 and PbO–PbCl2 glasses
have been reported by us previously.(13–21) Against
this background we have now investigated ternary
PbO–PbCl
2–PbBr2 glasses. The presence of PbO was
previously found(9,11,16) to increase the glass formation
capability. In this communication we report and
discuss physical properties of several glasses with
a constant molar percentage of PbO and varying
concentrations of the lead halides, PbCl2 and PbBr2.
2. Experimental
Glasses with the general formula 15PbO.xPbCl2.
(85−x)PbBr2 (where 0≤x≤25) were prepared by using
a microwave heating technique. Analar grade lead
oxide, lead chloride and lead bromide were used as
starting materials. Appropriate quantities of chemi-
cals were mixed thoroughly by grinding in order to
homogenize the mixture. Mixtures were melted for 3
to 6 min in a silica crucible in a domestic microwave
oven operating at 2·45 GHz and 850 W. Melting
in the microwave oven was rapid and the melts
aained temperature of ~500°C (measured using a
thermocouple kept very close to, but not dipping
into the melt). None of the components used here
decompose at this temperature, since the duration of
the sample preparation is very short in microwave
synthesis relative to conventional melting. Therefore
the composition of the glass remains very close to the
nominal composition. An evaporative loss of lead
is extremely unlikely(22) and so no further analysis
of composition was aempted. Glass pieces were
obtained by rapidly quenching the melt between two
brass blocks preheated to 100°C (preheating to 100°C
prevents cracking of the samples). The glass pieces
were annealed in a mue furnace for 2 h at 200°C
to remove thermal strains which generally develop
during quenching and the glasses were kept in a dry
atmosphere over CaCl2.
Glass pieces were crushed to a ne powder in an
agate mortar and subjected to x-ray diraction using
an x-ray diractometer (Model: Rigaku DMAC-1C)
employing Cu Kα radiation. The XRD paern did not
show any sharp peaks, and there were only broad and
diuse hump like features at low angles characteristic
Thermal and spectroscopic investigations of
glasses in the system PbO–PbCl2–PbBr2
B. K. Chethana,1 R. Viswanatha,1 C. Narayana Reddy2 & K. J. Rao1*
1 Solid State Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India
2 Sree Siddaganga College of Arts, Science and Commerce, Tumkur 572103, India
Manuscript received 11 November 2014
Revised version received 16 January 2105
Accepted 17 January 2015
Glass formation has been examined in the system 15PbO.xPbCl2.(85−x)PbBr2 (where 0≤x≤25) where the PbO concentra-
tion is kept constant while PbCl2 and PbBr2 concentrations are varied. The glasses have been examined using thermal
and spectroscopic techniques. Tg, ΔCp, refractive index, optical basicity have been found to remain unaected by the
composition which is a curious feature of these glasses. It is found that there is a wide infrared window available for use
in the investigated glasses. The IR window extends from 1000 to 1500 cm−1 and in glasses where PbCl2 is less than 20
mol%, the window extends up to 2260 cm−1. X-ray photoelectron spectra (XPS) revealed that the 4f5/2 and 4f7/2 peaks due
to f-level transitions have a constant dierence in energies, but with energy and FWHM values that varying sensitively
and systematically with composition. The observations are discussed and suitable explanations are provided.
* Corresponding author. Email kalyajrao@yahoo.co.in

116 Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 56 Number 3 June 2015
of amorphous materials (see Figure 1). These features
have not been investigated further to derive any
structural information. However the two humps cor-
respond respectively to distances of 4·00 Å and 2·83
Å. The laer is most likely arises from a contribution
of Pb–Cl (2·86 Å) and Pb–Br (2·967Å) bond lengths.(23)
The 4·00 Å feature is likely a Pb–Pb bond length
of √2×2·83 assuming an octahedral coordination of
halogens around lead atoms.
The densities (ρ) of the glasses were determined
using glass pieces free from air bubbles by the Archi-
medes principle using toluene (density=0·860 g/cm3)
as the immersion liquid. The molar volume (Mv)
was calculated using the relation Mv=M/ρ, where M
is the composition weighted molecular weight. The
relative error in the measurement of density was
±0·005 g/cm3. Glass transition temperature (Tg) and
the dierence in heat capacities, ∆Cp=(Cp
l−Cp
g) were
determined from the thermograms (see Figure 2)
obtained using a dierential scanning calorimeter
(Meler-Toledo DSC1) at a heating rate of 2°C/min;
Cp
l, Cp
g are the specic heat capacities at the liquidus
temperature and the glass transition temperature,
respectively. Glass pieces weighing 40–50 mg were
hermetically sealed in aluminium pans and were
annealed at a temperature near, but lower than, Tg
for 30 min and cooled to 100 K below Tg before the
nal runs were made for accurate determination of
thermal parameters. Infrared spectroscopic measure-
ments of the freshly prepared glass samples were
recorded on a Perkin-Elmer PE-580 double beam IR
spectrometer in the range 400–2300 cm−1 using a KBr
pellet at room temperature.
Ultraviolet and visible optical absorption spectra
were measured for powdered glass samples on a Cin-
tra 40 UV-Visible spectrometer, covering the spectral
range from 200 nm to 1000 nm. X-ray photoelectron
spectra (XPS) were recorded on a Thermoscientic
Multilab 2000 equipment employing Al Kα x-rays.
3. Results and discussion
The glass compositions and their codes are listed in
Table. 1, along with their physical properties namely
density, molar volume, refractive index (calculated),
glass transition temperature and change in molar
specic heat capacity at Tg. The mole fraction of lead
oxide is held constant at 15 mol%. An observation
of signicance is that pure binaries PbCl2–PbBr
2 do
not form glasses and the addition of PbO is found to
be essential for glass formation. We also noted that
PbCl2 severely restricts glass formation range and
more than 25 mol% PbCl2 in the composition leads to
partial crystallization even at high quenching rates of
nearly (≈1000°C/min). It may be noted that the Tg and
densities vary lile (less than 2% variation in densi-
ties and less than 2% in Tg). Substitution of PbBr2 by
PbCl2 is expected to decrease the molar volume. Such
substitution of PbBr2 by PbCl2 (Table 1, Pb1 to Pb5)
should have resulted in volume decrease of 6·73%
10 20 30 40 50 6
0
0
75
150
225
300
Intensity (arb. units)
2
θ
Pb1
Pb3
Pb5
36038040042044046048050
0
120
140
160
180
200
∆Cp
Cp
g
Cp
l
Tl
Tg
C
p
J/mol/K
Temperature (K)
Pb1
Pb2
Pb3
Pb4
Pb5
Table 1. Glass code, composition, density (ρ), molar volume (Mv), glass transition temperature (Tg), dierence in heat
capacities (∆Cp), congurational entropy (Sc) and glass formation index (G) for 15PbO.xPbCl2.(85−x)PbBr2 glasses
Glass code Composition (mol %) ρ MV Tg ∆Cp Sc Gi Gm G=Gi+ Gi
PbO PbCl2 PbBr2 (gmcc–1) (cc) (K) (J mol–1K–1) (J mol–1K–1)
Pb1 15 0 85 7·06 48·92 418 15·77 3·514 11·59 1·132 12·722
Pb2 15 5 80 7·01 48·38 417 15·92 5·09 16·815 1·127 17·42
Pb3 15 10 75 6·98 47·71 415 16·72 6·074 20·04 1·122 21·162
Pb4 15 15 70 6·94 47·08 413 16·34 6·81 22·46 1·116 23·576
Pb5 15 20 65 6·89 46·48 413 16·42 7·37 24·32 1·111 25·431
Figure 1. XRD spectra of the 15PbO.xPbCl2.(85−x)PbBr2
glass system [Colour available online]
Figure 2. DSC thermograms of the 15PbO.xPbCl2.(85−x)
PbBr2 glass system [Colour available online]
B. K. CHETHANA ET AL: THERMAL AND SPECTROSCOPIC INVESTIGATIONS OF PbO–PbC l2–PbB r2 GLASSES

Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 56 Number 3 June 2015 117
purely on the basis of ionic volume contributions.
But the observed molar volume decrease is even more
being 7·13%, suggesting that the glass structure is
more eciently packed in chloride ion rich glasses.
Substituting Br− ions by Cl− ions appears to have
increased the packing eciency. This is as expected
and appears to be a general feature of simple ionic
and metallic glasses.
Besides Tg the variation in ∆Cp with composition
is also very minimal in these glasses. This is a curi-
ous observation and can be analyzed as follows: The
glass forming compositions examined here can be
treated as pseudo-binaries (a cut in the ternary PbO.
PbCl2.PbBr2 phase diagram, corresponding to 15%
PbO). If pure PbBr2–PbCl2 melts exhibited near ideal
behaviour, a linear increase would be seen in their
melting temperatures. Correspondingly Tg should
have increased linearly in line with the Tg/Tm≈2/3
rule. But the observed behaviour is that the variation
is essentially at. This behaviour can arise under two
conditions. In the rst case liquidus temperatures are
depressed by the 15 mol% lead oxide present in the
melt, but the depression itself is non-uniform because
ionization of PbO into Pb2+ and O2− ions increases,
as PbCl2 concentration increases in the melt. This
aects the number of particles and hence colligative
properties such as the depression of melting points,
etc. This may result in the observed nearly constant
Tl and therefore nearly constant Tg (this may not be
the situation as we will see later). The other possi-
bility is that there is an inherent non-ideality of the
molten salt mixture PbBr2+PbCl2 and the normally
concave depression in liquidus temperatures is for-
tuitously at in the region of 0 to 25 mol% of PbCl2.
Unfortunately, the phase diagram of PbBr2–PbCl2
has not been reported in the literature to the best of
our knowledge and we cannot conrm either of the
above possibilities.
Near constancy of both Tg and ∆Cp has another
implication. That is the entropy loss between Tl and
Tg is a constant for all the compositions. Therefore the
enthalpy changes at liquidus temperatures should be
dependent on Tl only.
This is because,
( )
l
lp
l
pl g
pl pg
dln
l
T
T
H
S CT
T
CT T
CT CT
D
D= = D
ò
=D -
=D -D
(1)
where the subscript ‘l’ represents liquidus, therefore
∆Sl is dependent only on Tl. Addition of PbO to a
PbBr2+PbCl2 mixture is essential to ensure glass
formation. In order to understand the critical role of
PbO, we have used a general model of glass forma-
tion. We have proposed elsewhere(24) that a glass
formation index G is given by
( )
( )
2
cm 42
a
c
c
1
1 0·1 16
exp 2·45 exp
GK
r
Nr
cc
cc
éù
êú
= +D
êú
D -D +
ëû
éù
æö
÷
éù
ç
êú
÷
´ -- ç÷
êú
êú
ç
ëû
÷
ç
èø
ëû
(2)
where Kc is a constant, χm is the mean electronegativ-
ity, ∆χ is the dierence in electronegativity of the
bonded pair of atoms, Nc is the coordination number,
ra and rc are the anion and cation radii, respectively.
The terms in the square brackets represent in order,
the contributions of the bond ionicity, structural con-
straints and the size ratio of the atomic constituents.
The exponent is expressed as a modulus because a
departure of Nc from the ideal value of 2·45 leads
to the formation of rigid (>2·45) or oppy (<2·45)
regions, both of which enhance the crystallization
tendency.(25) For a complex system consisting of
several components another term, Gm is included
which arises from the mixing entropy, Gm=b∆Sc=b(−
R∑xilnxi) where b is a model related constant (b=3·3
molJ−1), which makes Gm non-dimensional. Xi is the
mole fraction of i-th component and R is the gas
constant. Thus, G for complex systems is given by
G=∑iGi+Gm, where Gi is the glass formation index for
i-th component of the system.(24) For multicomponent
systems, Gm arises in the form of mixing entropy, the
basis of this expression is discussed in Ref. 24. The
calculated values of glass formation indices for our
glasses are listed in Table 1. It is satisfactory to note
that the glass formation indices fall in the right range
(G≥20 for glass formers, 10≤G≤20 for conditional
glass formers and G<10 for glass modiers) for the
compositions studied here.
From the optical spectra of Figure 3 the absorption
edge or the Urbach edge, was identied and the data
was utilized to calculate the optical band gaps. Use
was made of Urbach’s law α(ω)=α0exp(hω/Eu), where
Eu is the Urbach energy.(26) The optical band gap is the
energy which corresponds to the intercept of linear
extrapolation of the Urbach edge. Both the optical
300400500600
2· 4
3·2
4·0
4·8
5·6
E
g
=3.83 eV
Pb5
2345
8·0×10
3
1·0×10
4
1·2×10
4
1·4×10
4
1·6×10
4
Eg=3·83 eV
(
α
h
ν
)
1/2
hν (eV)
Pb5
Absorbance (a.u)
Wavelength (nm)
Pb1
Pb2
Pb3
Pb4
Pb5
Figure 3. UV-VIS absorption spectra of the 15PbO.xPbCl2.
(85−x)PbBr2 glass system. Inset: variation of (αhυ)1/2 ver-
sus hυ for Pb5 glass [Colour available online]
B. K. CHETHANA ET AL: THERMAL AND SPECTROSCOPIC INVESTIGATIONS OF PbO–PbC l2–PbB r2 GLASSES

118 Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 56 Number 3 June 2015
edge and the observed intercepts on the wavelength
axis are listed in Table 2.
Noting that the edge frequency corresponds to
the optical excitation of Pb2+ ions, we have calculated
the optical basicity values of these glasses using the
method of Duy & Ingram.(27) The optical basicity
f
f CaO
uu
uu
-
L= - (3)
where υf is the absorption frequency of free Pb2+ ion,
υ is the absorption frequency of Pb2+ in the medium
and υCaO is the absorption frequency of Pb2+ in the
reference compound CaO.
It is seen from Table 2, that optical band gaps and
optical basicities also remain essentially insensitive
to inter-anionic substitution (Cl−´Br−) in the glasses
and do not exhibit any profound inuence on these
properties. We also note that the band gaps in these
glasses are notably higher than those reported for
PbO and similar to the values reported for crystalline
PbCl2. However the value is also higher for PbCl2-
free glass (Pb1, which contains only PbO–PbBr2).
We feel that this may be a consequence of the higher
electronegativity of the halogens (both Cl and Br)
which results in an increased Slater charge in Pb2+
acting on its peripheral electrons which gives rise
to increased band gap energies. The relatively small
broad peak around 400 nm present in all glasses is
most probably due to (inter-band) defect states pre-
sent in these glasses. These have not been examined
further here. PbO seems to be completely ionized
in PbCl2+PbBr2 melts contrary to the possibilities of
partial ionization of PbO as thought previously to
account for the constancy of Tl. PbO dissociates into
Pb2+ and O2− completely which helps to increase the
congurational entropy. The proportion of O2− ions to
halide ions in our glasses is 15 to 170 (15PbO.xPbCl2.
(85−x)PbBr2). Therefore, entropy considerations make
it unlikely that entropy reducing –Pb–O–Pb–O–Pb–
type chains (with signicant Pb–O covalent bonding)
would be present in the melts. It is therefore curious
as to how and why PbO helps PbCl2–PbBr2 mixtures
to form glasses.
The refractive indices of these glasses have been
calculated using semi-empirical equations. The three
semi-empirical equations we examined in this context
were,(28–31)
2g
2
11
20
1
E
n
n
-=-
+
(4)
2
g
1A
n
EB
æö
÷
ç÷
ç
=+ ÷
ç÷
ç÷
+
èø
(5)
n=α+βEg (6)
where Eg is the optical band gap, A=13·6 eV, B=3·4
eV, α=4·08 eV, β=−0·62 eV−1. We found that these
expressions yield nearly identical values of refractive
indices with the use of appropriate constants. We
have used Equation (4) to determine refractive indi-
ces and their values are listed in Table 2. Here again
the refractive indices of the investigated glasses are
found to be nearly constant in spite of the inter-halide
substitution and in spite of dierent polarizabilities
of Cl− and Br− ions.
The x-ray photoelectron Spectra (XPS) of the
glasses are presented in Figure 4. Energies cor-
responding to the peaks in the spectra, full width
at half maximum (FWHM) of the peaks and the
energy dierences between the twin peaks are listed
in Table 2. The peaks arise from excitations of 4f5/2
and 4f7/2 inner levels of Pb. The following relevant
observations may be made from Figure 3; (i) 4f7/2
and 4f5/2 ionization peaks shift slightly towards
higher energies in chloride rich compositions. The
FWHM of the 4f5/2 are less aected but those of 4f7/2
peaks increase systematically. These are the larger
angular momentum spectral peaks. (ii) The energy
dierence E4f5/2–E4f7/2 remains unaltered as expected
Table 2. Glass code, wavelength (λ), optical basicity (Λ), refractive index (n), f-level transitions (4f7/2 and 4f5/2) and
FWHM, dierence in f-level energies (∆) for 15PbO.xPbCl2.(85−x)PbBr2 glasses
Glass code λ Eg Λ n 4f7/2 FWHM (4f7/2 peak) 4f5/2 FWHM (4f5/2peak) ∆=4f7/2 –4f5/2
(nm) (eV) (eV) (eV) (eV) (eV) (eV)
Pb1 388·4 3·77 1·127 2·144 140·44 1·272 145·28 1·6 4·84
Pb2 390·6 3·78 1·32 2·141 140·53 1·422 145·38 1·68 4·85
Pb3 392·6 3·80 1·136 2·137 140·57 1·45 145·44 1·7 4·86
Pb4 394·10 3·8 1·137 2·13 140·79 1·54 145·64 1·8 4·85
Pb5 396·2 3·83 1·139 2·12 140·94 1·70 145·82 2·1 4·88
135140145150
8·0×104
1·6×105
2·4×105
3·2×105
4·0×1054f5/2
Binding energy (eV)
Intensity (a.u)
Pb1
Pb2
Pb3
Pb4
Pb5
4f7/2
Figure 4. XPS spectra of the 15PbO.xPbCl2.(85−x)PbBr2
glass system [Colour available online]
B. K. CHETHANA ET AL: THERMAL AND SPECTROSCOPIC INVESTIGATIONS OF PbO–PbC l2–PbB r2 GLASSES

Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 56 Number 3 June 2015 119
because these correspond to the inner energy levels.
Therefore it appears XPS spectra reveal a lile more
sensitively the eect of inter-halogen substitution in
the glasses. Another feature is that, the peak energy
also increases slightly to higher values when Cl− ions
substitute for Br− ions. It may be noted that Cl− ions
have higher ionic potential e/r, compared to Br– ions.
Chlorine with its greater electronegativity than that of
bromine pulls away more eectively electron density
from the outermost orbital of lead, particularly the
s electron cloud, so that inner higher j-value orbitals
are less screened from the nucleus.(11) Therefore we
observe slightly increased binding energies.
The infrared spectra obtained using the KBr pellet
method (10% concentration and 2 mm thick) of the
glasses studied here are presented in Figure 5. It is
clear that the spectra are at up to about 1000 cm−1
beyond which absorption peaks appear. It is only in
PbCl2-rich (Pb5) glass that there is another absorption
feature around 1600 cm−1 (possibly due to adsorbed
water). An absorption peak begins to emerge below
900 cm−1 in all glasses and in Pb5 there is a clear peak
centred around 825 cm−1. This spectroscopic feature
in a glass that contains only Br− and Cl− and O2− ions
aached to Pb2+ ions is rather interesting from the
fundamental point of view. It is tempting to associ-
ate this with –Pb–O–Pb– stretching in short linear
chain fragments. In many PbO containing glasses we
have earlier reported evidence for the formation of
[PbO4/2]2− type of structural units integrated into the
glass structure. But in any such situation IR spectral
features which can be associated with lead–oxygen
bonding occur at still lower frequencies (<600 cm−1).(32)
The highest frequency associated with Pb–O stretch-
ing has been reported to be 730 cm−1.(9) Therefore the
observed IR feature can only arise if the Pb–O bond is
strengthened and blue shifted by about 100 cm−1. We
are therefore led to speculate that there is probably a
re-hybridization of valence orbitals on, at least a small
proportion of, lead atoms so that the electrons in such
orbitals are polarized and pulled away far more ef-
fectively by the chlorine atoms. Therefore the formal
charge on Pb atoms becomes greater than +2 such
as Pb(2+δ)+. We feel the formation of Pb4+ is unlikely
because the melts were not exposed for long enough
to atmospheric oxygen in the microwave preparation
route employed in the present studies.
4. Conclusions
PbO–PbCl2–PbBr2 glasses represent an interesting
ionic glass system. Within the large composition
range arising from inter-halide variation, several
properties of the resulting glasses do not exhibit any
notable variation. It is found that only photoelectron
spectroscopy is sensitive to the small changes in the
bonding of lead ions in these glasses. It is also found
that these glasses provide a wide infrared transmis-
sion window.
Acknowledgement
K. J. Rao is thankful to DST for supporting this work
through the award of a (Senior) Ramanna Fellowship.
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2100180015001200900600
5·6 µm
Wavenumber (cm
-1
)
Transmittance (%)
Pb 5
Pb 4
Pb 2
Pb 3
Pb 1
20
30
40
50
Figure 5. IR spectra of the 15PbO.xPbCl2.(85−x)PbBr2
glass system [Colour available online]
B. K. CHETHANA ET AL: THERMAL AND SPECTROSCOPIC INVESTIGATIONS OF PbO–P bC l2–PbB r2 GLASSES

120 Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 56 Number 3 June 2015
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