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Chapter 5
Thermogravimetric and High Temperature XRD Studies
THERMOGRAVIMETRIC AND HIGH TEMPERATURE XRD STUDIES
5.1 Introduction
Phase transitions in nanoparti,:les are expected to be much different from those of
bulk crystals.'" Hayashi et all reported the phase transition studies of gas evaporated
WO, microcrystals and observed a n':w phase at low temperature. Chang et a13 reported
the phase transformation studies in nanostructured yttrlum oxide. According to these
workers, a high pressure monoclinic modification is stabilized in yttria particles smaller
than 8 nm. Cziraki et a14 reported the differential scanning calorimetric studies of
nanostructured Ni foils and observec an additional exothermic peak at 600K which was
explained on the basis of grain grow h process. It has been reported that the free energy
of nanoparticles is higher than that of a conventional polycrystalline counterpart. As a
result, their microstructure and ato nic configuration changes when exposed to high
temperature. These changes can be a-companied by an exotherm andlor end~therm.~
It has been realised that X-ray diffraction can yield useful information on
nanosized particles. Many authors"-l2 have reported the X-ray diffraction studies of
nanostructured materials. Charact1:risation of the structure of grain boundaries in
nanostructured materials has been controversial. Zhu et a16 studied nanocrystalline a-Fe
using large-angle X-ray diffraction techniques and concluded that the grain boundary
regions in nanocrystalline materials lack both short-range and long-range order.
Fitsimmons et all1 reported the structural characterisation of nanometer-sized crystalline
Pd using the X-ray diffraction techique and the results did not support the structure
proposed by Zhu et aL6 voge17 has reported the use of Debye Function Analysis (DFA)
as a tool for numerical simulation of the diffracted intensity of polydispersed
nanocrystalline systems. It was f:lt that the study of the high temperature X-ray
diffraction pattern of nanoparticles would yield interesting results.
In this chapter, the study of phase transition temperature and the crystal structure
of the nanoparticles of AgI, CuI and AgzHgI4 at different temperatures are described.
Phase transitions were investigated using thermogravimetry. The crystal structure at high
temperatures was studied using high kmperature X-ray diffraction.
5.2 Experimental
The nanoparticles of AgI, Agz HgI4 and CuI were prepared by chemical routes as
described in section 3.2.1 of chapter 3. Thermogravimetric analyses were performed on a
STA 15+ TGAIDTA as described in sc:ction 2.3.2 of chapter 2. XRD measurements were
recorded using a Bruker AXS D5035 X-ray diffractometer with Ni filtered Cu k,
radiation and with the X-ray generator operating at 40kV and 30mA. High temperature
X-ray diffraction were carried out using high temperature camera HTK 16 (Anton Paar,
Austria) as described in section 2.3.1 cf chapter 2.
5.3 Results and Discussion
5.3.1 AgI Nanoparticles
Fig. 5.1 shows the thermogrsvimetric analysis (TGA) and differential thermal
analysis (DTA) traces of the nanopanicles of AgI. One exothermic peak is observed at
-150°C in the DTA curve which shou's the transition to the cubic (a) phase. The second
exothermic peak at -55S°C corresponds to the melting point of silver iodide. The phase
transition temperature of AgI has been reported to be 147°C by many workers."-" The
present study of the nanoparticles of ,\gI reveals that the phase transition is taking place
at a slightly higher temperature.
A large fraction of atoms in very small particles are surface atoms and these
atoms have significant influence on the thermal properties of nanostructured materials.
Structural changes can be expected in small particles because of the large amount of free
energy associated with their grain bomdaries." The substantial energy associated with
the interface regions becomes substartial when the size of the crystallites is in nanosize
regimeIg and it may influence phase transitions in nanophase materials. Other types of
lattice imperfections may also exist due to the small size of the particles. Hence the
phase transitions in nanoparticles are expected to exhibit modified behaviour from that of
the bulk materials.
, , , , , 1::: 0 100 200 300 100 SW 6W
Temperature I.0
Fig. 5.1. TG 4 and DTA traces of nanoparticles of AgI
13.14.17 A number of articles dealing with powder X-ray diffraction studies of
polycrystalline as well as single crystalline Agl have already been published. Three
forms of silver iodide are reported to exist: i) A high temperature cubic form and ii) a 13.14.17 wurtzite type hexagonal and iii) zinc blende type cubic. I~Iowever, there are
contradictory reports on the polym~rphism of AgI. According to the early work of
isle^."' precipirates of AgI at room temperature usually consist of a mixture of
hexagonal and cubic structures. ~ u r e l ~ , ' ~ and Majumdar and ~ 0 ~ ' ~ reported that the
cubic phase is only metastable and converts to hexagonal phase when the temperature is
kept above 100°C but below 147°C. Takahashi et a12' reported that y-AgI is metastable
and is-Agl transforms to y-Agl when ground or strongly pressed. Cochrane and
~ l e t c h e r ~ ~ have reported that nearly all powder specimens of Agl at om temperature
and at atmospheric pressure consist of a mixture of cubic and hexagonal phases in
varying proportions depending on th: method of preparation. It has been reported that
the three peaks between 22" and 26' of the XRD pattem of Agl are useful in examining
the hexagonal and cubic structure.13. All the three lines occur in hexagonal structure,
but only the central line is present with cubic material.
The onset of the superionic cclnduction in AgI is associated with a true first order
phase transition accompanied by changes in structure and discontinuities in the specific
heat. The entropy-increase at this solid-solid transitions is often half the entropy increase
on melting and this fact has lead to t'le concept of 'sublattice melting' at the tran~ition.~'
At all temperatures, the iodine anions in AgI are closely bound to their lattice sites and
the cations are relatively mobile. In ihe room temperature phase, the cations are situated
at well defined lattice sites and have lrery low mobility. They become much more mobile
as the phase transition temperature is approached. According to the concept of sublattice
melting of superionic conductors, th,: transition to the superionic phase is viewed as a
"melting" of the mobile-ion sublattice. This ionic liquid can flow readily through the
rigid but relatively open immobile-ion sublattice.
High temperature XRD studies of polycrystalline AgI were reported earlier by
many authors. 13.14.21 Takahashi et a12 reported high temperature X-ray diffraction studies
of pellet and powder AgI samples in order to explain the conductivity changes in y-AgI.
When the sample was heated to 12OoC, some additional peaks were observed in the
diffraction pattem and these were identified as the peaks of P-AgI. At 170°C, which is
above the phase transition, four different peaks were observed and were identified as
those of body centered cubic lattice. ~ l u r e l y ' ~ had reported the transformation rates for the
y cubic to hexagonal irreversible phase change of AgI using the high temperature X-ray
diffraction technique. According to 13urley, this reaction obeys first order kinetics and is
interpreted in terms of a mechanism where the silver atoms initiate the transition by the
movement to interstitial sites and the iodine lattice then moves to attain the lowest lattice
energy.
Fig. 5.2 shows the diffraction pattern of the nanoparticles of AgI recorded at room
temperature. In order to see whether the pattern is compatible with hexagonal phase, the
ICDD-PDF data of hexagonal silver iodide24 is presented as vertical bars on the lower x-
axis. The height of the bars represents the diffraction intensity. It is seen from the XRD
pattern that nano AgI contains both !? (hexagonal) and y (cubic-zincblende) phases. The
relative composition of 0 and y phlses in AgI powder was obtained by the method
reported by err^'' (section 3.4.1 of chapter 3). An estimation of the relative percentage
of and y phases21 as discussed in cliapter 3 of this thesis shows that the present sample
of AgI nanoparticles contains 55% of hexagonal phase and 45% of cubic phase.
50
2 theta
Fig. 5.2. X-ray diffraction patt-m of nanoparticles of AgI. ICDD-PDF pattern is also shown (vertical lines) for comparison
In order to understand the phase transition behaviour, the temperature dependence
of the XRD pattern of AgI nanopmicles at four different temperatures, 300,418,428 and
473K. are shown in Fig. 5.3. It was observed that the pattern showed not much changes
up to 418K. When the temperatlre was raised to 418K, signs of additional peaks
appeared at three positions includirg one in the region of the diffraction triplet between
22" and 26". The additional peaks observed at 28 values of 24.5, 35.2 and 43.5" were
found to grow in intensity with incr:ase in temperature. It is important to note that the
high temperature and low temperatule phases coexist even at 473K which is much above
the normal phase transition temperatlre of 420K. The intensities of the diffraction peaks
observed at room temperature were ound to decrease with increase in temperature, as is
usually the case. Variation in the intensities of the diffraction peaks at different
temperatures is shown in table 5.1 imd those of the additional peaks in table 5.2. The
decrease in the intensities of peaks with temperature might be due to thermal agitation
which has the effect of smearing out the lattice planes.25
40
2 theta
Fig. 5.3. X-ray diffraction pattc:rn of nanoparticles of AgI nanoparticles at a) 300K b) 418K c) 428K and d) 473K
The peaks at 24.5, 35.2 and 43.5" can be identified with the help of the ICDD-
PDF data of high temperature cubic phase of ~ ~ 1 . ~ ' Fig. 5.4 shows the pattern recorded
at 428K. The ICDD-PDF data of cubic Agl is also marked as vertical bars in the pattern.
As seen from the Fig., the additiona peaks are found to match with the ICDD-PDF data
for a AgI (within -0.5' of 20). The small decrease in the d values (as indicated by a
small increase in 20 values of -0.5") compared to the d values of the bulk sample (ICDD-
PDF value) may be attributed to the $light contraction of the lattice in small particles27
The intensities of these peaks shorn unusual increase in intensity with increase in
temperature (table 5.2). Temperature characteristics of the X-ray diffraction intensity
from zinc blende structures and wurtzite structures were studied by Miyakc and
~osh ino . " They had reported the temperature effects on X-ray diffraction intensities of
a-CuI and y-Cul, both having zinc blende structures and had observed an intensity
increase for (200), (222) and (420) lines. This was explained on the basis of a large
Debye factor15 for the copper atoms compared to that of iodine atoms. These high values
were interpreted as due to the vigo~ous isotropic vibrations of metal atoms. Such a
theoretical explanatioli is difficult for crystals like a-AgI having non-zincblendc
structure
50
2 theta
Fig. 5.4. X-ray diffraction pattern of nanoparticles ofAg1 at 428K. ICDD-PIIF pattern of a-AgI (20- 1058) is also shown (vertical lines)
Table 5.2 Variation of intensities of X-ray diffraction lines (see text)
(in arbitrary scale)
Table 5.3 Variation of intensities of X-ray diffraction lines (see text)
(in arbitrary scale)
The present study of nanoparlicles of AgI revealed that the phase transition to a
phase is not taking place at a sharp ..emperature. Further, complete transition from the
lower phase of hexagonal and cubic t a a phase does not take place at the expected phase
transition temperature, as evidenced from the high temperature XRD pattern. The
occurrence of gradual phase transitions in the case of W 0 3 microcrystals were explained
as due to the size distribution of micr~cr~stals . ' The surface atoms of very small crystals
are expected to influence strongly t ~ e phase transition properties. The coexistence of
stable monoclinic and metastable telragonal structure in pure zirconia particles at room
temperature has been observed by Inany worker^.^',^^ The results were interpreted in
terms of grain size effect according lo which the smaller the particle radius, the higher is
the surface tension which results in .m increased internal pressure in the particles for the
phase stabi~isation.~ The coexisten3:e of difference phases in the nanoparticles of AgI
may be attributed to the grain size effect.
5.3.2 CuI nanoparticles
Fig. 5.5 shows X-ray diffraction patterns of copper (I) iodide recorded at room
temperature. The ICDD-PDF data of the cubic phase of copper iodide3" is also shown as
vertical lines. The small decrease in the d values of nanoparticles of CuI compared to the
ICDD-PDF data might be due to the s ight contraction of the lattice usually exhibited by
sn~all particles.27
At ambient pressure, CuI exists in three stable phases between roo111 temperature
and its melting point. At -300K, Cut has the cubic zinc blende structure (y-Cul) and at
'f-643K. it transforms from a face-ce,~tered cubic close packed 1 sublatticc to a slightly
distorted hexagonal close packed one (P phase). At T-673K, the anion sublattice reverts
to fcc with the cations statistically distributed over all the tetrahedral interstices and
displaying large anharmonic thermal .iibrations?'
51
2 theta
Fig. 5.5. X-ray diffraction pattern of nanoparticles of Cut. ICDD-PDF pattern of CuI (06-0246) is also shown (vertical lines)
Fig. 5.6 shows the TGA and DTA traces of the nanoparticles of Cul. The
transition from the y to p phase is shown by the exotherm peak at -65 1 K. 'The exotherm
peak at 672K corresponds to the transition of phase from to a. Thus, in the present
study it is found that the transition from y to P phase takes place at a higher temperature
than that of the bulk CUI." The P to o transition takes place at 672K which agrees with
the reported value for bulk Cul. In a thermally equilibrated polycrystalline material, the
grain size is sufficiently large to reduce the substantial energy associated with the
interface regions between the randomly oriented crystallites and in such a material the
presence of grain boundaries has no in luence on the macroscopic properties. I f the grain
size is reduced to nanometer range, the influence of grain boundaries can no longer be
neglected for many physical properties.32 The changes observed in the phase transition
temperature of nanoparticles of AgI was explained in the previous section on the basis of
the large amount of free energy associated with the grain boundaries of nanoparticle~.'~
The same reason is valid for the nanoparticles of Cul.
0 100 300 100 5MJ 690 2W i e rnpe ia ture (.ti
Fig. 5.6. TGA and DTA traces of nanoparticles of Cul
The XRD patterns of the na~oparticles of CuI recorded at four temperatures are
shown in Fig. 5.7. Ir can be seen from the pattern that the peaks show changes in the d
values with increase in temperature When the temperature was increased to 653K, the
peaks were slightly shifted to 1owc:r 20 angles. No additional peaks appeared at this
temperature. Copper iodide is expected to change to the hexagonal (P) phase at this
temperature.'' So the pattern at this ternperature was matched with the ICDD-PDF data
of hexagonal copper iodide," (Fig. 5.8) but the observed peaks were found not to
correlate with the ICDD-PDF data of hexagonal phase. When the temperature was
further increased to 678K. marked diffizrcnce in d values were found. The sample was
again heated to 683K and the peaks were found to come back closer to the original
positions. This is the a phase of copper iodide."
51
2 theta
Fig. 5.7. X-ray diffraction pattein of nanoparticles of CuI at a) 300K b) 653K c) 678K and d) 683K
Temperature effect on the intensity of the X-ray reflections from crystals is one of
the interesting subject in the X-ray CI ystallography. The intensity anomaly observed in
the case of crystals of zinc blende anc wurtzite types can be used to explain the vigorous
isotropic vibrations of metal atoms at high temperatures.I5 Since the y and a fbrms of
CuI are the zinc blende types, the int(:nsity of X-ray reflections at different temperatures
can be analysed to get an idea of the structure of nanoparticles. Table 5.3 shows the
variation in the observed intensity of the diffraction lines of nanoparticles of CuI with
temperature. It is seen that the peaks (1 1 I), (220) and (31 1) decrease in intensity with
increase in temperature. The intensity of peak (200) initially increased and then was
found to decrease with temperature, while that of the pcak (222) showed the reverse
trend.
50
2 theta
Fig. 5.8. X-ray diffraction pattt:rn of nanoparticles of Cul at 653K along with the ICDD-PDF pattern of hexagonal Cul
Table 5.3 Variation of intensities of X-ray reflections of nanoparticles of Cul
(in arbitrary scale)
Temperature dependence of intensity of X-ray reflections from crystals having
zinc blende and wurtzite type structtres were studied in detail by Miyake and ~ o s h i n o . ' ~
They had explained the intensity anomaly found in the case of Cul on the basis of the
Debye factors2' of Cu and I atoms. According to these workers, the temperature effect
observed in the case of CuI is characrerised by large values of Debye factors for the metal
atom at higher temperatures and a rapid increase in its value with temperature. This
procedure was adopted by the present author to study the changes observed in the
intensity of X-ray reflections from C JI nanoparticles.
The effect of temperature on the intensity is usually formulated by replacing the
atomic scattering factorf; for the ith species of atoms byf; exp (-Mi) where exp (-Mi) is
the temperature factor. The quaniity M depends on both the amplitude of thermal
vibration and the scattering angle 2 0 The peak (220) shows a decrease in intensity while
the peak (200) shows an increase in intensity. For (220), the structure factor is given by25
For (200), since the structure factor is given by
lF12=16 k exp (-Md - f~, exp ( - M - ) ] ~ , (5.2)
the observed intensity increase can be explained if the value of the Debye factor for the
copper atom decrease with rise in tenperature much more rapidly than that for the iodine
atom." The value of M was calculated from these two equations using the observed
intensity value of the peaks. Tabl~: 5.4 shows the variation of the Debye factors with
temperature, where B is defined by M = B (sin01h)~. Since the factor B is regarded as a
quantity independent of the scattering angle, the use of atomic scattering factors
associated with Debye factors is equivalent to assuming isotropic statistical
displacements of atoms from their mean positions.'5 It can be seen from the table 5.4 that
the increase of Bc. is rapid compart:d to that of BI. Thus the intensity anomaly observed
can be explained by isotropic vibrations of Cu atoms at high temperatures. According to
the work of Miyake and ~oshino, ' ' BI increased linearly with temperature, while the
increase in the value of Bc. was anomalously rapid. In the present study, the variation of
B, with temperature {[BT - B30c]l) was not a linear one. This showed that the
nanoparticles of CuI possessed a slightly different structure compared to the bulk Cul.
But the general trend of variation of'the B value was identical to that of the bulk sample.
Table 5.4 Changes in the values of [BT(K) - B3WI:] for copper and iodine atoms with temperature T (K)
5.3.3 Ag2HgI4 Nanoparticles
Fig. 5.9 shows the TGA and DTA traces of the nanoparticles of AgzHgI4. One
endotherm peak is observed at 49.6"C in the DTA curve which shows the transition from
tetragonal ( P ) to cubic (a) phase. The second peak in the DTA curve shows the melting
of the sample. The phase transition temperature of the polycrystalline AgzHgI4 has been
reported to be in the range 50-52°C by many a ~ t h o r s . ~ ~ , ~ ~ The slight lowering of the
transition temperature may be attrit~uted to the substantial energy associated with the
interface regions which becomes pi.edominant when the size of the crystallites is in
nanosize regime.19
X-ray diffraction pattern c~f nanoparticles of Ag2Hg14 recorded at room
temperature is shown in Fig. 3.5 of chapter 3. Careful examination of the d values with
the standard ICDD-PDF data revea ed that the pattern contain new peaks besides the
peaks of the tetragonal structure, a!; explained in the section 3.4.3. These additional
peaks were identified as those of the cubic structure of AgzHgI4. The X-ray diffraction
patterns of nanoparticles of Ag2Hg14 recorded at various temperatures, covering the
stability range of P phase of Ag2HgI, and well beyond the P to a transition temperature,
are shown in Fig. 5.10. It is observe11 from the Fig. that the P to a phase transition is not
taking place at a sharp transition terlperature. The peaks corresponding to the a phase
appears to grow from a temperature of -40°C. A complete transition is found to take
place only at temperatures >60°C.
Fig. 5.9. TGA and WL'A traces of nanoparticles of Agzl-igI~
100.0 l 2
- 99 0 - U 99.6 CL - - 99.1 z 5-
9 9 2 -
99.0-
98.8-
98.6-
98.1-
50
2 theta
Fig. 5.10. X-ray diffraction pattern of nanoparticles of /IgzI~lgld at different t,:mperatures
Temperature ('C)
:::0.5
I I 20 1 0 60 80 100 120 110 160 180
0 0
1. 0
- - 1.5 > z - ,- - 2.0 <I
- 2.5
- 3.0
- 3 . 5
- 4 .0
- 1.5
200
5.4 Conclusion
High temperature structural behaviour of nanoparticles of Agl, CuI and Ag2Hg14
were studied using thermogravimetric md X-ray diffraction analyses. High temperature
XRD studies of AgI nanoparticles reve.~led the presence of P and y phases in addition to
the a phase even at 473K which is higher than the phase transition temperature of AgI.
The high temperature X-ray analysis of Cul nanoparticles revealed a different crystal
structure for the CuI nanoparticles. The results obtained from these studies were
analysed based on factors which originate from the finite size of the particles.
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