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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