Pyrochlore to perovskite phase transformation in solgel derived leadzirconate
titanate thin films
Chi Kong Kwok and Seshu B. Desu
Citation: Applied Physics Letters 60, 1430 (1992); doi: 10.1063/1.107312
View online: http://dx.doi.org/10.1063/1.107312
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Copyright by the AIP Publishing. Kwok, CK; Desu, SB, "Pyrochlore to Perovskite phase-transformation in sol-gel derived
lead-zirconate-titanate thin-films," Appl. Phys. Lett. 60, 1430 (1992); http://dx.doi.org/10.1063/1.107312
Pyrochlore to perovskite phase transformation in sol-gel derived
lead-zirconate-titanate thin films
Chi Kong Kwok and Seshu B. Desu
Department of Materials Engineering, Virginia Polytechnic Institute and State University,
Blacksburg, Virginia 24061
(Received 15 November 1991; accepted for publication 6 January 1992)
Pyrochlore to perovskite phase transformation in sol-gel derived lead-zirconate-titanate (PZT)
films was studied by x-ray diffraction and transmission-electron microscopy (TEM). X-ray
diffraction studies of PZT films on sapphire substrates indicated that the pyrochlore to
perovskite phase transformation was completed at 650 “C. In contrast, TEM investigations of
free-standing PZT films showed that the phase transformation was completed at much higher
temperatures. This discrepancy in the behavior of free-standing films versus films on substrate
_ _
can be related to the size effect.
There is considerable interest in ferroelectric thin films,
particularly in the lead-zirconate-titanate (PZT) system
for their potential applications in high-density dynamic
random-access memory (DRAM) capacitors and nonvol-
atile memories. Sol-gel process, sputtering, and electron-
beam evaporation’-3
are some of the thin-& processing
techniques used to produce the PZT films. In general, as-
deposited films are amorphous and post-deposition anneal-
ing is needed to transform the film from the amorphous
structure to the desirable ferroelectric-perovskite phase.
The amorphous structure will first transform into an inter-
mediate pyrochlore phase4 and then the pyrochlore phase
will transform into the perovskite phase at a higher tem-
perature.
Although the presence of this intermediate pyrochlore
phase is commonly observed,’ few efforts have been made
to study the pyrochlore to perovskite transformation of the
PZT thin films. This letter reports our findings on the mi-
crostructural changes throughout the transformation by
transmission-electron microscopy (TEM). A simple and
unconventional technique for the TEM sample preparation
was used ifi this study.
In this study, the PZT films were fabricated from a
metallo-organic solution (0.25 M) of lead acetate, zirco-
nium n-propoxide, and titanium isopropoxide dissolved in
acetic acid and n-propanol. This solution was then hydro-
lyzed to form the precursor. Details of this precursor prep-
aration are similar to that suggested by Yi, Wu, and Sayer.*
This precursor was then spin casted onto the single-crystal
sapphire substrates and the TEM grids. The films were air
dried for 5 min and then annealed at different annealing
temperatures for 15 min in air.
Conventional TEM sample preparation for ceramic
materials usually requires mechanical polishing, dimpling,
and ion milling. This process is tedious, time consuming,
and prone to introduce defects and artifacts. A novel TEM
sample-preparation method, which is particularly suitable
for the sol-gel process, was used in this experiment. In this
study, the sol was spin coated at 3000 rpm for 30 s directly
onto 400 mesh platinized-nickel TEM grids. The surface
tension held the liquid film in place and the film was air
dried and then annealed in the furnace. The final film
thickness is estimated to be around 700 A. This method is
expeditious and does not need any mechanical polishing or
ion-beam bombardment. In general, it is difficult to prepare
good ceramic TEM samples by the conventional method,
but almost all of the TEM samples in this study prepared
by the spin-cast method are successful. Using the new
preparation method, the film is fabricated without the sup-
port of a substrate. The behavior of this free-standing film
may be quite different from that of the film deposited on a
substrate and this difference has to be taken into consider-
ation when the results are interpreted.
The presence of pyrochlore is commonly recognized
when the amorphous PZT films are annealed to form the
perovskite phase;’ however, there is little consensus about
exact peak positions and
(hkl)
identification of the pyro-
chlore structure because a standard diffraction file’ of the
PZT pyrochlore phase has not been established. Okada’
observed few pyrochlore peaks located at
d
spacings of
2.95, 2.60, and 1.84 8, using Cu
Ka
x-ray radiation. In this
study, a computer program developed by Weidemann,6
was used to calculate the theoretical peak positions and the
intensities of the PZT perovskite and pyrochlore phases.
The comparison of the peak positions and peak intensities
of the perovskite phase and the pyrochlore phase from the
theoretical calculation and from the standard diffraction
file’ is tabulated in Table I. The comparison of the PZT
perovskite phase shows excellent agreement between the
experimental results and the computed results. In this
study, the x-ray diffraction of the pyrochlore phase shows
only two very broad peaks at d spacings of 3.02 and 2.62 A.
The comparison of the computed results of the pyrochlore
phase and the results obtained in this experiment by x-ray
diffraction and TEM electron diffraction also show very
good agreement. The advantage of the electron diffraction
over the x-ray diffraction is that many more peaks can be
located such that positive identification of the pyrochlore
phase is possible.
In this study, x-ray diffraction was used to study the
phase transition as a function of the annealing tempera-
ture. The PZT films were deposited on single-crystal sap-
phire disks and annealings were performed in a quartz-tube
furnace at predetermined temperatures for 15 min in air.
The final film thickness is about 1700 A. X-ray diffraction
results show that when the as-deposited film was annealed
1430 Appl. Phys. Lett. 60 (12), 23 March 1992 0003-6951/92/l 21430-03$03.00
@I 1992 American Institute of Physics 1430
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TABLE I. X-ray diffraction data of perovskite and pyrochlore phases.
hkl
Perovskiteb
db
d
calcC
z/I,”
z4 calcC
001
4.15
4.15 Q
8
100 4.04
4.04 12
16
101
2.89
110
2.85
100
100
111
2.35
2.35
15
8
002 2.0-l
2.91 9
7
200 2.02
2.02 16
12
102 1.84
1.84 5
4
201 1.81
1.81
4
210 1.81
1.80
16
4
112 1.68
1.68 12
14
211 1.66
1.66 25
26
022
1.44
1.44
9
8
Pyrochlored
hkl
d
CdlCC
Z/4, ca,c@ Gay 4em
111
6.04
31 . . .
I..
311 3.16
14 . . .
. . .
222 3.02
100
3.02
3.03
400 2.62
31 2.62
2.63
331 2.40
10 . . .
. .
511
2.02
6 . . .
. . .
440 1.85
52 . . .
1.85
531 1.77
8 . . .
. . .
622 1.58
44 . . .
1.58
444 1.51
9 . . .
. . .
800 1.31
8 . . .
1.31
662 1.20
18 . . .
1.20
=Any peak with Z/Z0 < 5% is not tabulated.
bJCPDS card No. 33-784.
Valculated from the “powder” computer program.
dZr/Ti = 53/47. x ray: observed by x-ray diffraction in this study. tern:
observed by TEM electron diffraction in this study.
at 45O”C, no definitive peak was observed and the film
remained amorphous. As annealing temperature increased
to 550 “C, two very broad pyrochlore peaks were found at
28 of 29.5” and 34”. At 600 ‘C, both perovskite and pyro-
chlore phases were presented. Formation of a single-per-
ovskite phase was observed at or above 650 “C.
From the x-ray diffraction of the PZT sample annealed
at 550 “C, the pyrochlore phase is the only phase present
and has a very broad (222) peak. The average diameter of
the crystal size can be related to the peak broadening* and
calculated to be around 6 nm. Figure 1 is a bright-field/
dark-field pair of micrographs showing a homogeneous,
fine-grain phase where the dark field was taken from a
small area of the most intense ring. The crystal size as
measured from the dark-field micrograph is around 8 nm,
which agrees very well with the result from the x-ray dif-
fraction. The diffraction-ring pattern is indexed according
to the d spacings for the pyrochlore phase listed in Table I.
Hence, we can identify this fine-grain structure as pure-
pyrochlore phase. Figure 2 shows two particles that are
found on a homogeneous-pyrochlore background. The par-
ticle in Fig. 2(a) can be identified as the perovskite phase
by the spot pattern. The
d
spacings measured from the spot
pattern match very well with the
d
spacings listed in Table
I. The pyrochlore-perovskite transformation involved a
1431
Appl. Phys. Lett., Vol. 60, No. 12, 23 March 1992
C. K. Kwok and S.B.Desu
1431
650°C 15 min.
FIG. I.(a) Bright field, (b) dark field, and (c) selected area diffraction of
the pyrochlore phase showing very fine-grain structure.
volume change (5.7 ~01% ) and this volume change will
induce strain in the perovskite phase. For a very thin, free-
standing film, as in this case, the strain can be partially
relieved by bending the thin perovskite particles, and all
the perovskite particles found at temperatures from 650 to
700 “C had bend contours. At higher annealing tempera-
tures, all the pyrochlore phase transformed to the perov-
650°C
15 min.
FIG. 2. TEM micrographs of the perovskite particles growing on the
pyrochlore phase: (a) bright field image, (b) perovskite diffraction pat-
tern, and (c) pyrochlore diffraction pattern.
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PIG. 3. The microstructural de-
velopment of the PZT film as a
function of annealing tempera-
ture.
b
d
h
skite phase, the strain was thermally relieved completely,
and the bend contours disappeared, as can be seen in Fig.
3(i). In Fig. 2(b), in addition to the single [l lo] spot
pattern from the perovskite phase, a faint pyrochlore ring
[arrowed in Fig. 2(b)] was also observed. Since this dif-
fraction pattern arose from the area which was completely
within the perovskite particle, the only reason for the ex-
istence of the pyrochlore ring is that the perovskite particle
is actually sitting on top of a much thinner pyrochlore film;
in other words, the perovskite phase grows on top of the
pyrochlore film.
Figure 3 is a set of TEM micrographs showing the
development of the perovskite phase as a function of an-
nealing temperatures. For an annealing temperature at or
below 550 “C, the microstructure has a mottled structure
and the corresponding electron diffraction shows a single-
diffuse ring which indicates an amorphous structure. More
careful examination of the diffuse ring reveals that the d
spacing of the diffuse ring corresponds to the d spacing of
the most intense (222) ring of the pyrochlore phase. Based
on this information, we can assume that the pyrochlore
phase has already nucleated and formed very minute clus-
ters at temperatures as low as 450 “C. At 650 “C annealing,
well-defined pyrochlore grains are formed and the corre-
sponding sharp diffraction-ring pattern signifies a well-de-
lined grain structure. In addition to the presence of the
fine-grain pyrochlore phase, a few clusters of three to four
perovskite grains are found and appear to grow out from
the pyrochlore phase, as can be seen in Fig. 3(e). At a
higher annealing temperature, more perovskite grains are
formed and each grain can be identified by the character-
istic diffraction spot pattern; see Fig. 3(f). At a 750 “C
annealing, all of the pyrochlore phase has transformed to
perovskite phase, as shown in Fig. 3(i). Nevertheless, the
perovskite-transformation temperature is not unique for
the free-standing PZT films, but found to be thickness de-
pendent. Very thin PZT films in our study did not trans-
form to perovskite phase even after 750 “C annealing. For
instance, many samples with tapering film thicknesses had
1432
Appl. Phys. Lett., Vol. 60, No. 12, 23 March 1992
i
the perovskite phase formed at the thicker areas whereas
the thinner regions remained in a pyrochlore structure.
From the theoretical calculations, the density of the
perovskite phase is 8.05 g/cm3 and that of the pyrochlore
phase is 7.55 g/cm3. Thus, a decrease in volume is expected
to accompany the pyrochlore-perovskite transformation.
For the free-standing films, especially for the very thin
samples, the strain energy required to form the perovskite
phase is diminished due to the strain relaxation in the di-
rection normal to the film. Hence, the transformation tem-
perature observed is always lower than the transformation
temperature determined by the x-ray method. Neverthe-
less, the TEM study of the PZT thin films has demon-
strated the pertinent evolution of the pyrochlore to perov-
skite transformation.
In conclusion, a new TEM sample-preparation method
was suggested and used in this study. The microstructural
changes of PZT films were studied by TEM. The pyro-
chlore phase is shown to have very fine-grain structure and
the perovskite phase is observed to grow on the pyrochlore
phase at a temperature around 650 “C. The discrepancy of
the transformation temperature is discussed and can be
attributed to the size effect of the thin films.
This study is partially supported by DARPA through
a project from ONR.
’ M. Sayer, Proceeding of the Sixth IEEE International Symposium on
Applications oJ‘FerroeIectrics, edited by W. Smith (Lehigh University,
Bethlehem, PA, 1986), pp. 560-568.
“A. Okada, J. Appl. Phys. 48, 2905 (1977).
‘M. Oikawa and K. Toda, Appl. Phys. Lett. 29, 491 ( 1976).
4H. Megaw, Ferroelectricity in Crystats (Methuen, London, 1957), p.
142.
‘G. Yi, Z. Wu, and M. Sayer, J. Appl. Phys. 64, 2717 (1988).
‘K. E. Wiedemann, M. S. thesis, Virginia Polytechnic Institute & State
University, 1983.
‘Powder Diffraction File Alphabetical Index: Inorganic Phases (JCPDS.
Swarthmore, PA, 1986).
sB. Cullity, Elements of X-ray Diffraction (Addison-Wesley, Reading,
MA, 1956), p. 261.
C. K. Kwok and S. B. Desu
1432
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