Dielectric and ordering behavior in PbxCa1-xTiO3

Article abstract of DOI:10.1111/j.1151-2916.1997.tb02881.x

The dielectric properties and microstructure of Pb_xCa_1-xTiO₃ are reported. At room temperature, Pb_xCa_1-xTiO₃ is orthorhombic for compositions where x is 0.1 and 0.2, pseudocubic for composi

Full text of DOI:10.1111/j.1151-2916.1997.tb02881.x

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J. Am. Ceram. Soc., 80 [3] 653–62 (1997)
Dielectric and Ordering Behavior in PbxCa1؊xTiO3
Ramaratnam Ganesh and Edward Goo*
Department of Materials Science and Engineering, University of Southern California,
Los Angeles, California 90089–0241
The dielectric properties and microstructure of PbxCa1؊x
-
calcium atoms on the A sites on alternate {111} planes.6 Most
of the studies in this system have been conducted on the lead-
rich compositions, and no work has been conducted on the
calcium-rich compositions. Calcium titanate is paraelectric at
room temperature and lead titanate is ferroelectric; therefore,
the addition of calcium titanate to lead titanate lowers the Curie
point, and, for compositions where x is Ͻ0.5, PbxCa1ϪxTiO3 is
paraelectric at room temperature.
The purpose of the present work is to study the microstruc-
ture through X-ray diffractometry (XRD) and transmission
electron microscopy (TEM) investigations of the PbxCa1ϪxTiO3
system and examine the associated dielectric properties. Ferro-
electrics with diffuse phase transitions have been reported as
early as the 1950s.7 In this paper, we present our observations
on the effect of calcium titanate addition on the broadening of
the paraelectric–ferroelectric transitions in the PbxCa1ϪxTiO3
system and use the power-law model described by Uchino
et al.8 to measure the diffuseness parameter.
TiO3 are reported. At room temperature, PbxCa1؊xTiO3 is
orthorhombic for compositions where x is 0.1 and 0.2,
pseudocubic for compositions where x is 0.3 and 0.4, and
tetragonal for compositions where x is $0.5. TEM studies
on the calcium-rich compositions reveal the presence of
1/2{100}-, 1/2{110}-, and 1/2{111}-type superlattice reflec-
tions in three principal zone axes. An analysis of crystal
structure, based on the atomic displacements and the order-
ing of lead and calcium atoms, is reported. No ordering
behavior is observed for the compositions where x is 0.1 and
0.2; however, partial ordering of lead and calcium atoms
occurs for compositions where 0.3 # x # 0.60. The dielectric
measurements do not reveal any relaxor behavior; however,
the addition of calcium titanate to lead titanate broadens
the width of the transition region. Calculation of the dif-
fuseness parameter, ␦, from the power-law model also is
presented.
II. Experimental Procedure
I. Introduction
Conventional powder-processing techniques were used to
prepare the bulk powders. Stoichiometric amounts of the chem-
ically pure lead oxide (99.5%), titanium dioxide (99.7%), and
calcium carbonate (99.5%) were weighed and then ball milled
for 5 h in deionized water, followed by filtering and oven
drying. The ceramic powders were calcined at 850ЊC for 5 h in a
covered alumina crucible. The calcined residue was ball milled
again, and the final product was ground to a fine powder using
a mortar and pestle. The calcined powders were then cold
pressed in a hydraulic press at 2.5 ϫ 108 Pa into cylindrical
pellets 25 mm in diameter and 3 mm thick. The compacts were
then sintered at 1200ЊC for 5 h in a covered alumina crucible
that was loosely packed with powder of the same composition
to minimize the loss of lead, because of vaporization. Samples
having a density of Ն93% were obtained. The percentage of
theoretical density obtained using the lattice parameters for the
different compositions that were studied and the corresponding
crystal structure are tabulated in Table I. Polycrystalline sam-
ples with x Ͼ 0.8 were not prepared because they disintegrated
on cooling through the Curie point, because of their high c/a
ratio (1.06).
HE origin of ferroelectric behavior in oxides and an under-
standing of the nature of their ferroelectric–paraelectric
T
phase transitions have been the subject of substantial research
for the past fifty years. Ferroelectric oxide ceramics find wide-
spread applications in the electronics industry.1,2 The most
widely used ferroelectrics occur in the perovskite family with
the general formula ABO3. Several modifications of these mate-
rials, in the form of either solid solutions or dopant additions,
have been researched with the purpose of obtaining improved
piezoelectric and dielectric properties, compared to the simple
perovskite. Complex perovskites with either A-site or B-site
substitutions have been studied, although relatively lesser work
has been conducted on A-site substitutions.
The dielectric properties of lead titanate are known to be
varied by the addition of calcium titanate.3–5 Earlier studies in
the PbxCa1ϪxTiO3 systems have revealed ordering of lead and
W. Huebner—contributing editor
Grain-size measurements were performed using a scanning
electron microscopy (SEM) microscope (Cambridge Instru-
ments, Cambridge, U.K.) operating at 20 kV. The samples were
Manuscript No. 193236. Received September 7, 1994; approved March 28, 1996.
Supported by the Office of Naval Research, under Grant No. N00014-90-J-1900.
*
Member, American Ceramic Society.
Table I. Dielectric and Microstructural Data for the PbxCa1؊xTiO3 System
Composition,
Crystal
structure
Density
(% of theoretical)
Grain size,
d (␮m)
Curie temperature,
Dielectric
x
c/a
␦
␥
TC (ЊC)
constant,† km
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Orthorhombic
Orthorhombic
Pseudocubic
Pseudocubic
Tetragonal
93
93
95
99
97
98
98
98
3.4
2.7
3.0
3.2
3.5
2.4
2.3
2.9
ϳ1
ϳ1
1.5
0.6
13
29
158
254
334
1928
3000
6370
7995
9030
1.003
12
1.34
1.31
1.25
1.20
Tetragonal
1.015
1.04
1.045
9
7
5.5
Tetragonal
Tetragonal
†At TC and 1 kHz.
653

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Journal of the American Ceramic Society— Ganesh and Goo
Vol. 80, No. 3
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etched for 5 min in a solution containing 4.5% HF and 4.5%
HNO3. The grain size was measured using the circular intercept
method specified by ASTM standard E112.9 Figure 1 shows the
grain size and morphology of the Pb0.4Ca0.6TiO3 composition.
The grain sizes for the different compositions that were studied
are listed in Table I. No specific trend is noticed in the grain
size with changes in composition.
superlattice reflections were reported for compositions with a
lead content of 50–61 at.%. TEM studies of the calcium-rich
compositions (0 Յ x Յ 0.5) also reveal the presence of these
superlattice reflections. The electron diffraction patterns from
the three principal zone axes for the composition where x is 0.4
are shown in Fig. 2. Kay and Bailey10 and Koopmans et al.11
have shown that the structure of calcium titanate is a grouping
of regular TiO6 octahedra that are rotated with respect to their
ideal perovskite positions, as shown in Fig. 3. This occurs
primarily because of the small displacement of the calcium and
oxygen atoms from their ideal lattice positions. TEM studies on
CaTiO3 reveal the presence of superlattice reflections due to
such displacements.
For XRD studies, sintered specimens that had been mechani-
cally polished were used. X-ray studies were conducted using a
diffractometer (Rigaku Co., Tokyo, Japan) operating at 35 kV
and 50 mA. TEM samples were prepared by cutting circular
disks 3 mm in diameter using an ultrasonic drill. The disks were
mechanically polished and dimpled to a thickness of 20 ␮m,
mounted on a copper grid, and milled using a beam of argon
ions. For hot-stage microscopy, samples were milled directly,
without mounting onto a copper grid. Room-temperature TEM
studies were conducted using an electron microscope (Model
EM 420, Philips Electronic Instruments, Mahwah, NJ)
operating at an accelerating voltage of 120 kV. Hot-stage TEM
studies were performed using a microscope (Model JEM
2000FX, JEOL, Tokyo, Japan) operating at 200 kV. A single-
tilt hot stage (Model 628-0500, Gatan, Pleasanton, CA) was
used for this purpose. Because of the limited tilting capability
of the stage, going to the exact zone-axis conditions during
high-temperature electron diffraction studies was difficult.
Dielectric measurements were performed (Model 3322 LCZ,
Keithley Instruments, Cleveland, OH). Silver electrodes were
evaporated on the samples using a vacuum evaporator (Denton
Vacuum, Cherry Hill, NJ).
An ordered structure was proposed by King et al.6 to explain
the reflections observed in PbxCa1ϪxTiO3 at compositions simi-
lar to that at x ϭ 0.5. The orientation relationship between the
ordered structure and the perovskite unit cell is shown in
Fig. 4. Based on this ordered structure, one would expect the
presence of 1/2{110} superlattice reflections in the ͗110͘ zone
axis. However, no such reflections were observed in the ͗110͘
zone axis for the compositions where x is Ͼ0.2. For the com-
positions where x is 0.1 and 0.2, weak 1/2{110} reflections
were observed in the ͗110͘ zone axis, as shown in Fig. 5; these
reflections are similar to those observed in pure CaTiO3. Also,
X-ray studies revealed that the compositions where x is 0.1
and 0.2 had an orthorhombic structure, with lattice parameters
similar to those of pure CaTiO3. Based on these observations,
Pb0.1Ca0.9TiO3 and Pb0.2Ca0.8TiO3 were concluded to have a
crystal structure similar to pure CaTiO3. The superlattice
reflections observed in these compositions were a result of
atomic displacements similar to those observed in CaTiO3.10,11
The atom positions for the compositions where x is 0.1 and 0.2
are listed in Table II.
III. Results and Discussion
(1) Microstructural Analysis
X-ray studies on the compositions where x is 0.3 and 0.4
revealed a crystal structure corresponding to cubic symmetry.
No peak splitting was observed in the X-ray scan, as shown in
Fig. 6. However, TEM studies of these compositions revealed
the presence of 1/2{111} reflections in only one direction in
the ͗111͘ zone axis, as shown in Fig. 2(A). This suggests a
lack of threefold symmetry along the ͗111͘ direction, and,
Earlier studies in the PbxCa1ϪxTiO3 system have revealed
ordering of the lead and calcium atoms on alternate {111}†
planes.6 The presence of 1/2{100}, 1/2{110}, and 1/2{111}
†Unless otherwise stated, all reflections are indexed with respect to the perovskite
unit cell.
Fig. 1. SEM micrograph showing the grain size and morphology of the Pb0.4Ca0.6TiO3 composition.

March 1997
Dielectric and Ordering Behavior in PbxCa1ϪxTiO3
655
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(A)
(B)
(C)
(D)
Fig. 2. Electron diffraction patterns for (A) [111] zone axis, (B) [101] zone axis, (C) [100] zone axis having 1/2{110} and 1/2{100} reflections,
and (D) [100] zone axis having 1/2{110} reflections.
therefore, the compositions where x is 0.3 and 0.4 are not cubic
at room temperature. This is not contradictory because X-ray
scans average over a greater area of the sample, as compared
to electron diffraction; therefore, the different variants result
in a cubic pattern in X-ray studies. The compositions where x
is 0.3 and 0.4 are probably pseudocubic, with a c/a ratio close
to unity. For the compositions where x is Ͼ0.4, the crystal
structure exhibits tetragonal symmetry with an increasing c/a
ratio as the lead content increases. The crystal structures of
Pb0.1Ca0.9TiO3 and Pb0.2Ca0.8TiO3 have a lower symmetry, as
compared to the other compositions. Therefore, the presence
of an extra screw axis or a glide plane due to higher symmetry
would account for the absence of 1/2{110} reflections in the
͗110͘ zone axes for the compositions where x is Ͼ0.2.
High-temperature electron diffraction studies were con-
ducted to determine the nature and origin of the superlattice
reflections. Specimen warping due to lead loss at high tem-
peratures and equipment limitations precluded hot-stage TEM
studies to be conducted for temperatures of Ͼ950ЊC. Selected-
area electron diffraction patterns at different temperatures for
the compositions where x is 0.2, 0.3, and 0.5 are shown in
Figs. 7–9, respectively; the results are listed in Table III. The
1/2{100} and the 1/2{110} reflections disappear at the same
temperature, whereas the 1/2{111} reflections disappear at a
higher temperature for the compositions where x is Ն0.3.
Also, dark-field studies reveal that different regions light up
on imaging with the 1/2{111} reflections, as compared to
imaging with the 1/2{100} and 1/2{110} reflections in these
compositions.6 These observations indicate that the 1/2{111}
reflections are of a different origin, as compared to the
1/2{100} and 1/2{110} reflections. Chemical ordering of lead
and calcium atoms accounts for the presence of the 1/2{111}
superlattice reflections, and atomic shuffles are responsible for
the presence of 1/2{100} and 1/2{110} superlattice reflections
in the compositions where x is Ն0.3. Atomic shuffles similar
to those observed in pure CaTiO3 are believed to be responsi-
ble for the presence of 1/2{110} and 1/2{100} superlattice
reflections in the PbxCa1ϪxTiO3 system. This is further corrob-
orated by the evidence that BaxCa1ϪxTiO3 and SrxCa1ϪxTiO3
also show superlattice reflections identical to PbxCa1ϪxTiO3;
however, no superlattice reflections are observed for the
PbxBa1ϪxTiO312 and the PbxSr1ϪxTiO313 systems.
Also, the temperature at which the superlattice reflections dis-
appear noticeably increases as the CaTiO3 addition increases.

656
Journal of the American Ceramic Society— Ganesh and Goo
Vol. 80, No. 3
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Fig. 5. Electron diffraction patterns from the [101] zone axis of the
composition where x is 0.2, showing the presence of 1/2{110} reflec-
tions in addition to 1/2{100} and 1/2{111} reflections.
Fig. 3. TiO6 octahedra tilted with respect to their ideal perovskite
positions. Coordination number of the calcium atoms is reduced to
eight because of the rotation.11
As mentioned earlier, if atomic displacements similar to
CaTiO3 are assumed, then the presence of a glide plane would
account for the disappearance of the 1/2{110} reflections in the
͗110͘ zone axes. Theoretical electron diffraction patterns were
calculated using a program written at the University of Southern
California (Los Angeles, CA). When the displacement of oxy-
gen atoms, similar to that of CaTiO3, and the ordering of lead
and calcium atoms on alternate {111} planes were considered,
the theoretical patterns were found to be in good agreement
with the experimentally observed patterns. In the experimen-
tal diffraction patterns, the 1/2{110} superlattice reflections
appeared in the ͗111͘ zone axes along the row through the
origin because of double diffraction. This was confirmed by
the disappearance of these reflections on tilting along the row
through the origin. The superlattice reflections were weak in
electron diffraction and absent in XRD, as shown in Fig. 6.
CaTiO3 undergoes an orthorhombic-to-cubic phase transition at
1257ЊC.14 Therefore, as the addition of CaTiO3 increases, the
temperatures at which the tetragonal-to-cubic or orthorhombic-
to-cubic phase transitions occur also increases. Hence, the small
atomic displacements that are present in the structure with
lower symmetry are not present anymore as the material
becomes cubic. Therefore, the 1/2{100} and the 1/2{110}
reflections that occur because of such displacements disappear
as the material becomes cubic. For the compositions where x is
Ն0.3, the 1/2{111} reflections, however, are a result of the
ordering of lead and calcium atoms on alternate {111} planes
and, therefore, disappear at a different temperature. For the
compositions where x is 0.1 and 0.2, the crystal structure is
similar to pure CaTiO3, and no ordering of lead and calcium
atoms occurs. Hence, one would expect all the superlattice
reflections to disappear at the same temperature.
Fig. 4. Ordered structure with respect to the perovskite unit cell.