chapter describes the results on the glass formation range of alumino-tellurite
system and the Te-O and Al-O coordinations in the samples using Raman and 27Al
MAS-NMR techniques respectively. The thermal properties are studied by
differential scanning calorimetry.
4.2 Glass preparation
Aluminum tellurite glasses of the
– (100-x) TeO2; (x = 1, 2, 3, 5, 7 and 20-mol%)
synthesized by splat and ice quenching. Appropriate amounts of the starting
materials Al2O3 (Aldrich India, 99.9%) and TeO2
(Aldrich India, 99%) were mixed and melted in a Pt crucible at ~ 950oC
in muffle furnace for 1 h. Samples containing 1 and 2-mol% of Al2O3
were prepared by ice quenching due to the higher content of TeO2
which required high quenching rate. In ice quenching the bottom of the platinum
crucible containing the melt was immersed in an ice-water bath and transparent
thin flakes of the sample were formed. Although other samples (5-20-mol% Al2O3)
were prepared by splat quenching in which the small quantity of the melt was
pressed between two massive steel plates. Transparent and bubble free aluminum
tellurite samples were formed. Multiple attempts were carried out to synthesize
sufficient amounts of samples for analysis. At higher concentration of Al2O3
(20-mol%), semi-transparent sample was obtained.
patterns of all alumino tellurite samples exhibit broad hump in the range: 20o
to 35o without any sharp peaks, which confirms the amorphous
structure of these samples (Fig. 4.1).
While all samples were clear and transparent, the sample: 20AlTe was
semi-transparent although it was found to be X-ray amorphous. This sample
contained highest concentration of Al2O3 (20-mol%) and
its semi-transparency is due to amorphous-amorphous phase separation. As
reported in an earlier studies by Kaur et.al.
it is found that even in Al2O3-B2O3-TeO2
glass system, higher concentration of Al2O3 (30-mol%)
shows the phase separation and formed inclusions of ?-Al2O3 crystals (Kaur & Khanna, 2014). XRD pattern of sample 1AlTe
confirms that it is possible to prepare TeO2 glass containing very
small amount of alumina (1-mol%) content at normal quenching rate (~103
Ks?1). The short-range structure and thermal properties of 1AlTe
glass are expected to be quite similar to that of pure TeO2 glass (Barney et al., 2013; Tagiara
et al., 2017).
XRD patterns of aluminum tellurite glasses.
4.4 Thermal analysis
thermographs of Al2O3-TeO2 glasses are shown
in Fig. 4.2 and their thermal
properties are summarized in Table 4.1.
The Tg value increases
steadily from 311oC to 392oC on increasing Al2O3
concentration from 1 to 20-mol%. Tg
for alumino-tellurite glasses are in excellent agreement with the results from
the earlier study, for example Tg
for 1AlTe and 3AlTe glasses prepared in the present work are 311oC
and 323oC respectively and Taigara et al. (Tagiara, et al., 2017) reported Tg of 311.9oC
and 322.7oC for glasses of
the same compositions. Taigara et. al.
and Lombson et.al. also reported that
the pure TeO2 glass prepared in alumina crucible shows Tg at 385oC and
380oC respectively while TeO2 glass prepared in Platinum
(Pt) crucible shows Tg at
303oC (Tagiara, et al., 2017; Lambson, 1984
#385). However in the present work, the
addition of 1-mol% Al2O3 in TeO2 glass
prepared in Pt crucible shows Tg
at 311oC, which is lower than the pure tellurite glass formed by
using alumina crucible. This indicates that the pure tellurite glass formed in
alumina crucible has high content of alumina impurity as compare to tellurite
glass containing 1-mol% of Al2O3. The high content of Al2O3
forms stronger Al-O-Te linkages over Te-O-Te linkages because of the higher
bond dissociation energy of Al-O (512 kJ mol-1) bonds over Te-O (391
kJ mol-1) bonds and results the high in Tg values. The increase in Tg of the alumino-tellurite glasses directly correlate
with the bond dissociation energy of the glasses which increases from 392 kJ
mol-1 to 415 kJ mol-1 with increasing Al2O3
DSC patterns of aluminum tellurite glasses.
The average single bond enthalpy, EB
is calculated by the following formula (Kaur et al., 2015):
ETe-O and EAl-O are the single bond
dissociation energies of Te-O (391 kJ mol-1) and Al-O bonds (512 kJ mol-1)
respectively (Cottrell, 1958; Darwent,
et al., 2015). The increase in EB corresponds to the
replacement of weaker Te-O-Te linkages over stronger Al-O-Te/Al-O-Al linkages. Fig. 4.3 shows the linear variation of Tg with EB according to the following linear equation:
Variation of Tg with bond dissociation energy of aluminum
Table 4.1: Compositional and DSC data of
Tc ±1 oC
Tm ±1 oC
The addition of Al2O3
beyond 1-mol% in tellurite glasses shows number of exothermic peaks
(crystallization peak) and single endothermic peak (melting peak). These
multiple crystallization peaks reveals the existence of several crystalline
phases of Al2O3-TeO2 system which can be
formed by devitrification of the samples after annealing them at certain
temperature (> Tg).
Incorporation of Al2O3 in Al2O3-TeO2
glasses shifts the Tc1
from 347 oC to higher temperature, at 530 oC. The
crystallization temperatures Tc2
and Tc3 also get prominent
with increasing Al2O3 concentration from 1 to 20-mol%.
This increase in crystallization temperature reveals that the tendency of Al2O3-TeO2
glasses towards devitrification decreases with increasing Al2O3
tellurite glass with 20-mol% of Al2O3 shows two glass
transitions phenomena, first a strong transition at 393 oC and the
second weak glass transition at 440 oC. This multiple transitions
phenomena indicate the presence of amorphous-amorphous phase separation in
alumino-tellurite glasses, which was also found by Kaur et. al. in case of alumino-borotellurite glasses containing 20-mol%
of Al2O3 (Kaur & Khanna, 2014). This implies that the higher
concentration of Al2O3 in tellurite and as well as in
borotellurite leads to the phase separation. The thermal stability
of Al2O3-TeO2 glasses increases from 36
oC to 138 oC on increasing Al2O3
concentration from 1 to 20-mol% (Table
4.5 Raman Spectroscopy
spectra of alumino-tellurite glasses are shown in Fig 4.4. Raman patterns show a strong peak at ~59 cm-1,
a shoulder at 106 cm-1 and two broad bands in the range: 300 to 550 cm-1 and 550 to 970 cm-1.
The spectra are baseline corrected and deconvoluted with peaks centered at 619,
662, 719, 779 and 821 cm-1. The peak at 59 cm-1 is the
boson peak which is the characteristic peak of vitreous solids in low frequency
region. The intensity of this peak depends upon the chemical composition and
correlation functions of the sample and it is growing with increasing the Al2O3
content from 1-20-mol% (Malinovsky & Sokolov, 1986). The shoulder at 106 cm-1
is due to the longitudinal optical mode vibrations of TeO4 units
around the bridging oxygens (Pine & Dresselhaus, 1972). The broad band in the Raman shift
range: 300 to 550 cm-1 is due to symmetric stretching vibrations of
Te-O-Te and O-Te-O linkages. The intensity of this band increases with
increasing Al2O3 content from 1 to 20-mol %. This is due
to the formation of stronger Al-O-Al/Te-O-Al (512 kJ mol-1) linkages
over Te-O-Te (391 kJ mol-1) linkages on
Raman spectra of aluminum tellurite glasses.
Al2O3 content. Raman spectra in the range 550 to 970 cm-1
corresponds to the tellurite structural units (Table 4.2). Raman spectra of all the alumino-tellurite glasses in
this range were deconvoluted by Gaussian peaks and centered at 619, 662, 724,
775 and 806 cm-1 as shown in Fig.
4.5. The peak at 619 corresponds to the antisymmetric vibrations of TeO4
subunits. The peak at 662 cm-1 is due to the antisymmetric
vibrations of Te-O linkages mainly in TeO4 units
Due to boson peak
longitudinal optical mode vibrations of TeO4
units around the bridging oxygens
Stretching vibrations of Te-O-Te, O-Te-O linkages
Stretching vibrations of TeO4 tetrahedra
Stretching vibrations of TeO3+1 and TeO3
Due to the AlO4 units having three bridging and
one non bridging oxygen
4.2: Raman bands assignments of alumino-tellurite
is also due to small vibrations of Te-O units produced by TeO3+1 and
TeO3 subunits. Peaks at 724 and 775 cm-1 are assigned to
stretching vibrations of Te-O- and Te=O bands having NBOs, formed by
TeO3+1 and TeO3. (A. Kaur et al., 2016; Manning,
et al., 1995). The peak at 806 cm-1
is due to the Al-O linkages in AlO4 units having three bridging and
one non-bridging oxygen (Yadav & Singh, 2015). The intensity of peaks at 724 and
775 cm-1 increases with increasing Al2O3
content in Al2O3-TeO2 glasses as shown in Fig. 4.5, which indicates the increase in concentration of TeO3/TeO3+1
units. Even the peak at 806 cm-1 is shifted to the higher
wavenumber, 862 cm-1 on increasing Al2O3
amount from 1 to 20-mol% which indicates the formation of stronger AlO4
Deconvoluted Raman spectra of aluminum tellurite glasses.
areas under these deconvoluted peaks were used to calculate the coordination of
Te with oxygen, NTe-O from following formula:
4.3: Te-O speciation calculated from Raman
NTe-O decreases significantly from 3.66
to 3.33 (Table 4.3) with increase in
Al2O3 concentration from 1 to 20-mol% which reveals the
structural transformation: TeO4 à TeO3+1
TeO3 in Al2O3-TeO2 glasses. Earlier
studies on tellurite glasses by Barney et.
al. (Barney, et al., 2013) and Gulenko et. al. (Gulenko et al., 2014) it is found that the value of NTe-O
for pure tellurite glass from neutron diffraction studies to be 3.68 and 3.73
respectively. Similarly, Pietrucci et.
al. (Pietrucci et al., 2008) reported from ab initio calculations that NTe-O = 3.73. These values
are consistent with NTe-O value of 3.66 for the sample: 1AlTe
characterized in the present work. Hence at low Al2O3
content of 1-mol%, the short-range structure of alumino-tellurite glass is
quite similar to that of pure tellurite glass.
4.6 27Al MAS-NMR
spectra of alumino-tellurite glasses are shown in Fig. 4.6 and it has three resonance peaks at ~ 6 ppm, 31 ppm and 52
ppm. The resonance peak at 6 ppm corresponds to hexa-coordinated AlO6
(Al6), the peak at 31 ppm is due to the five-fold coordinated AlO5
(Al5) and finally the peak at 52 ppm is due to the tetrahedral AlO4
(Al4) units (Table 4.3) (Clayden et al., 1999). At low concentration of Al2O3
(3-mol %) in Al2O3-TeO2 glasses, the amount of
Al6 is higher than that of Al4 which implies that at low Al2O3
content mainly hexa-coordinated Al6 exist in Al2O3-TeO2
glasses. High concentration of Al2O3 (20-mol%) in Al2O3-TeO2
glasses shows the additional peak at 13 ppm which corresponds to Al6
units of ?-Al2O3 phase (N. Kaur et al., 2016). On increasing the amount of Al2O3
in Al2O3-TeO2 glasses from 3 to 20-mol% the
intensity of peak at 52 ppm gets prominent which indicates the formation of AlO4
over AlO6 units.
Al27MAS-NMR spectra of aluminum tellurite glasses.
4.4: Al27 MAS-NMR assignments of Al2O3-TeO2
Due to AlO6 units
Due to AlO5 units
Due to AlO4 units
find out the concentration of existing A4, A5 and A6
units in Al2O3-TeO2 glasses, their NMR spectra
were normalized and fitted with Gaussian peaks centered at 6 ppm, 31 ppm and 52
ppm (Fig. 4.7). The areas (A) under these peaks were used to
calculate the concentration of Al4, Al5 and Al6
structural units by using following formulas:
The addition of Al2O3
upto 20-mol% increases the concentration of Al4 from 16% to 33%
while Al6 decreases from 54% to 35%. The concentration of Al5 increases by a small amount from 30% to 33%.
This decrease in Al6 illustrates the structural transformations Al6
Al5 à Al4 on adding Al2O3
in Al2O3-TeO2 glasses. These results are also
consistent with the DSC and Raman analysis which find an enhancement of Tg
and increase in the intensity of Raman peak at ~850 cm-1 due to
increase in the concentration of Al4 with increase in Al2O3
mol%. Fig. 4.8 shows the relative
concentration of Te4, Te3, Al6, Al5
and Al4 units with increasing Al2O3 content
in Al2O3-TeO2 glasses.
One of the deconvoluted Al27MAS-NMR spectra of aluminum
tellurite glasses containing 3-mol% of Al2O3.
4.5: Al-O speciation calculated from Al27
Variation of Te4, Te3, Al6, Al5
and Al4 units with Al2O3 mol% in Al2O3-TeO2
glasses containing varying concentration of Al2O3 were
prepared and characterized by XRD, DSC, Raman and Al27 MAS-NMR
studies. The absence of sharp peaks in XRD patterns confirms the amorphous
structure of all samples and transparent glasses can be formed with Al2O3
concentration in the range: 1 to 7-mol% while higher concentration of Al2O3
(20-mol%) in tellurite glasses form phase separated samples.
analysis found that the Tg of alumino-tellurite glasses increases
from a value of 311oC to 393oC on increasing Al2O3
concentration from 3 to 20-mol%. The Tg values show linear
correlation with the average bond dissociation energy of the glasses, the
latter increases with Al2O3 concentration. The average
bond dissociation energy of glasses increase due to the higher bond enthalpy of
Al-O bonds (512 kJ mol-1) than that of Te-O bonds (391 kJ mol-1).
The addition of Al2O3 in tellurite glasses form stronger
Te-O-Al linkages over weaker Te-O-Te linkages and enhance the thermal stability
of the alumino-tellurite glasses. Aluminum tellurite glass containing 20-mol%
of Al2O3 shows amorphous-amorphous phase separation.
study shows that on increasing Al2O3 content from 1 to
20-mol%, the concentration of TeO4 units decreases drastically. The
decrease in the concentration of TeO4 units confirms the structural
transformation: TeO4àTeO3. Raman studies also
found that the Te-O co-ordination in alumino-tellurite glass containing 1-mol%
Al2O3 (NTe-O = 3.64) is close to that of pure
tellurite glass (NTe-O = 3.68) as reported by neutron diffraction
MAS-NMR spectra of aluminum tellurite glasses show that at low concentration of
Al2O3 in tellurite glasses, mostly hexa-coordinated, AlO6
units exist along with small amount of AlO5 and AlO4
units. On increasing Al2O3 concentration from 3 to
20-mol% the fraction of AlO5 and AlO4 increase at the
expense of AlO6. At higher concentration of aluminum oxide (20-mol%)
sharp peak due to AlO6 units broaden and splits into two peaks.
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