Contribution of structure to temperature dependence of resonant frequency in the (1 - x)La(Zn1/2Ti1/2)O3·xATiO3 (A = Ca, Sr) system

Article abstract of DOI:10.1111/j.1151-2916.2001.tb00737.x

The crystal structure and microwave dielectric properties of the (1 - x)La(Zn_1/2Ti_1/2)O₃·xSrTiO₃ and (1 - x)La(Zn_1/2Ti_1/2)-O₃·xCaTiO₃ system were investigated. X-ray powder diffraction showed that cation ordering disappeared at x > 0.3 for both systems. However, infrared spectra demonstrated that short-range cation ordering could exist at x = 0.4. Permittivity and the temperature coefficient of the resonant frequency (τ_f) of both systems exhibited nonmonotonic variations with composition. Both systems exhibited a τ_f of zero at the same composition of x = 0.5 although the τ_f of SrTiO₃ was about two times larger than that of CaTiO₃. The behavior of the permittivity and τ_f were described by the tilting of oxygen octahedra and cation ordering. The relation between τ_f and cation ordering of La(Zn_1/2Ti_1/2)O₃ was discussed in conjunction with the experimental results on metal halides. It is suggested that cation ordering induced a negative τ_f and suppressed the increase of permittivity for compositions between x = 0 to x = 0.5 for (1 - x)La(Zn_1/2Ti_1/2)O₃·xSrTiO₃ and (1 - x)La(Zn_1/2Ti_1/2)-O₃·xCaTiO₃ systems.

Full text of DOI:10.1111/j.1151-2916.2001.tb00737.x

J. Am. Ceram. Soc., 84 [4] 753–58 (2001)
journal
Contribution of Structure to Temperature Dependence of Resonant
Frequency in the (1 ؊ x)La(Zn1/2Ti1/2)O3⅐xATiO3 (A ؍ Ca, Sr) System
Seo-Yong Cho,† Hyuk-Joon Youn,† Hyo-Jong Lee,*,‡ and Kug Sun Hong†
School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, Korea and
Micro-Radiowave Technology, Ltd., Seoul, 150-010, Korea
term depends on the structure of the compound.7 In the case of
perovskites, compounds having a tilt of the oxygen octahedra
appear to exhibit a positive ␶ε.10 For example, the correlation
The crystal structure and microwave dielectric properties of
the (1 ؊ x)La(Zn1/2Ti1/2)O3⅐xSrTiO3 and (1 ؊ x)La(Zn1/2Ti1/2)-
O3⅐xCaTiO3 system were investigated. X-ray powder diffrac-
tion showed that cation ordering disappeared at x > 0.3 for
both systems. However, infrared spectra demonstrated that
short-range cation ordering could exist at x ؍ 0.4. Permittivity
and the temperature coefficient of the resonant frequency (␶f)
of both systems exhibited nonmonotonic variations with com-
between the tilt of oxygen octahedra and ␶ was investigated for
ε
(Ba,Sr)(Mg1/3Ta2/3)O3 and (Ba,Sr)(Zn1/3Nb2/3)O3 systems.2,11
The sign and magnitude of ␶ changed rapidly near the composi-
ε
tion at which antiphase tilting appeared. Sr(Zn1/3Nb2/3)O3, which
has octahedral tilting, shows positive ␶ε, while Ba(Zn1/3Nb2/3)O3
without tilting exhibits a negative ␶ε.2 Such coincidence between
position. Both systems exhibited a ␶ of zero at the same
f
composition of x ؍ 0.5 although the ␶f of SrTiO3 was about two
times larger than that of CaTiO3. The behavior of the permit-
tivity and ␶f were described by the tilting of oxygen octahedra
positive ␶ and octahedral tilting was also found in several
ε
A2ϩB4ϩO3 type complex perovskites compounds.12 It is sug-
gested that the existence of octahedral tilting reduces the restoring
force and makes the C term less negative.10 More recently, a
short-range modulation of the crystal structure rather than a change
and cation ordering. The relation between ␶ and cation
f
ordering of La(Zn1/2Ti1/2)O3 was discussed in conjunction with
the experimental results on metal halides. It is suggested that
in oxygen octahedral tilting was suggested as the origin of the ␶
ε
cation ordering induced a negative ␶ and suppressed the
and the permittivity variation in the (Ba,Sr)(Zn1/3Nb2/3)O3 sys-
f
tem.13
increase of permittivity for compositions between x ؍ 0 to x ؍ 0.5
for (1 ؊ x)La(Zn1/2Ti1/2)O3⅐xSrTiO3 and (1 ؊ x)La(Zn1/2Ti1/2)-
O3⅐xCaTiO3 systems.
Structurally, tilting of oxygen octahedra has similar effects as
cation ordering in 1:1 type complex perovskites. These factors
result in doubled unit cells and face-center symmetry.1 Hence, if
the C term is affected by octahedral tilting because of a structural
change, then it is also possible that cation ordering could induce a
I. Introduction
positive ␶ε. However, the relation between cation ordering and ␶
(or ␶f) is still unclear.
ε
HE dielectric properties of perovskite compounds are strongly
related to their structural characteristics, such as cation order-
T
La(Zn1/2Ti1/2)O3 (LZT) is a promising microwave dielectric
material with a dielectric constant of 34 and low dielectric loss (tan
ing and the orientation of the oxygen octahedra.1,2 Recently,
complex perovskites and their solid solutions were investigated
and found to be promising dielectrics for microwave applica-
tions.3,4 Dielectric materials for microwave applications should, in
general, satisfy three conditions: high permittivity, low dielectric
loss, and small temperature coefficient of resonant frequency (␶f).
These properties can be understood in terms of the structural
characteristics. The permittivity of dielectrics can be calculated
using the Clausius–Mossotti relation and additivity rule;5 however,
structural factors, such as cation compression or rattling, can
induce a difference between the calculated and measured permit-
tivity.5,6 Cation ordering has a great effect on the dielectric loss
and phase transition characteristics of complex perovskites.1
␦ Ͻ 10Ϫ4 at 6 GHz), but its large ͉␶ ͉ Ͼ 50 ppm/°C requires
f
modification for applications as a dielectric resonator.14 SrTiO3
(ST) and CaTiO3 (CT) have a positive ␶f, so mixing LZT with ST
and CT is expected to adjust the ␶ of LZT near zero. The
f
above-mentioned relation between octahedral tilting and ␶ does
f
not apply to CaTiO3. That is, CaTiO3 has inphase and antiphase
octahedral tilting, but its ␶f is positive.10 In a structural aspect, ST
is cubic but CT is orthorhombic with octahedra tilting. LZT has
cation ordering and octahedral tilting. Therefore, when the
LZT–ST system is compared to LZT–CT system, their structural
changes with composition should be quite different. It is the
purpose of this article to discuss the relation between structure and
microwave dielectric properties of LZT–ST and LZT–CT. In
␶
is mainly determined by the temperature coefficient of
f
permittivity (␶ε).3 ␶ can be divided into three terms: A, B, and
ε
particular, the behavior of ␶ is discussed in terms of cation
f
C.7–9 A, B, and C terms are related to a decrease in the number of
polarizable particles per unit volume as the temperature increases,
the increase of the polarizability of a constant number of particles
with the increase of temperature, and the dependence of polariz-
ability on temperature with constant volume, respectively. The C
ordering.
II. Experimental Procedure
Powders of (1 Ϫ x)La(Zn1/2Ti1/2)O3–xSrTiO3 and (1 Ϫ x)-
La(Zn1/2Ti1/2)O3–xCaTiO3 with compositions from x ϭ 0 to 1
were prepared using the conventional mixed oxide method. La2O3,
CaCO3, SrCO3, ZnO, and TiO2 with high purity (Ͼ99.9%) were
weighed and mixed for 24 h with stabilized ZrO2 media and ethyl
alcohol. The mixed powders were calcined at 1200°C for 2 h and
then ball-milled for 24 h. The milled powders were pressed into
disks 8 mm in diameter and 2–4 mm thick under a pressure of
1000 kg/cm2. Pellets were sintered at 1550°C for 2 h in air.
X-ray diffraction (XRD, Model M18X, Mac Science, Tokyo)
data was collected using CuK␣ radiation and a graphite monochro-
mator in the 2␪ range of 10°–60°. Step scanning was used with a
H. Chan—contributing editor
Manuscript No. 189487. Received March 18, 1999; approved January 28, 2000.
Supported by the Korea Science and Engineering Foundation (KOSEF) through the
Research Center for Thin Film Fabrication and Crystal Growing of Advanced
Materials (RETCAM) at Seoul National University.
*Member, American Ceramic Society.
†School of Materials Science and Engineering, Seoul National University.
‡Micro-Radiowave Technology.
753

754
Journal of the American Ceramic Society—Cho et al.
Vol. 84, No. 4
step size of 0.005° and a count time 1 s/step to investigate the
Table I. Approximate Structure Change in
†
1
existence of
⁄2
(111) superlattice reflections. For infrared (IR)
(1 ؊ x)La(Zn1/2Ti1/2)O3–xSrTiO3
reflectivity measurement, samples were ground and polished using
1 ␮m diamond paste. IR reflectivity in the 30–4000 cmϪ1 range
was measured using FTIR spectroscopy (Model DA8-12,
BOMEM, Canada). The reflectivity spectra were evaluated by
means of standard Kramers–Kronig analysis.
Microwave dielectric properties of sintered samples were mea-
sured using a network analyzer (Model 8720C, Hewlett Packard,
Palo Alto, CA). Permittivity was measured using the post-
resonator method.15 An Invar cavity was used for the measurement
of the temperature coefficient of resonant frequency because of its
low thermal expansion coefficient (Ͻ 2 ppm/°C).
Cation
Cation
Inphase
tilting
Antiphase
ordering
displacement
tilting
1
1
1
1
⁄
2
(111)
⁄
2
(210)
⁄
2
(310)
⁄
2
(311)
0 Յ x Յ 0.3
0.3 Ͻ x Յ 0.5
0.5 Ͻ x Յ 0.9
X
X
X
X
X
X
X
X
†Based on XRD data only.
III. Results
(1) X-ray Diffraction Analysis
In Fig. 1, XRD patterns of the (1 Ϫ x)La(Zn1/2Ti1/2)O3–
xSrTiO3 [LZT–ST] samples are shown. The reflections were
indexed on the basis of the doubled pseudocubic perovskite
subcell. Several superlattice reflections were observed in the XRD
pattern of La(Zn1/2Ti1/2)O3. The origin of each superlattice reflec-
tion was explained in the previous paper.16
⁄2 (111) superlattice
1
reflections were due to the cation ordering in La(Zn1/2Ti1/2)O3.
1
1
⁄ (311) and ⁄ (310) reflections resulted from antiphase and
2
2
inphase tilting of the oxygen octahedra, respectively.17 From the
XRD patterns in Fig. 1, approximate structure changes were
1
suggested and listed in Table I. ⁄
2
(111) superlattice reflections
disappeared at x Ͼ 0.3, which indicated that cation ordering was
disrupted at compositions of x Ͼ 0.3, however, short-range cation
ordering could exist at x Ͼ 0.3 since the XRD method is generally
sensitive to long range ordering. Figure 2 shows the variation with
1
1
2
composition of
⁄
2
(111) and ⁄ (210) superlattice reflections of
LZT–ST system in more detail.
Figure 3 shows the XRD pattern of the (1 Ϫ x)La(Zn1/2Ti1/2)-
O3–xCaTiO3 [LZT–CT] system. Superlattice reflections were also
found but their variation with composition was quite different from
1
1
2
those of the LZT–ST system, that is,
⁄
2
(210),
⁄ (311), and
1
⁄
2
(310) reflections existed in the entire range of composition.
1
1
2
Fig. 2.
⁄ (111) and ⁄ (210) superlattice reflections of (1 Ϫ x)La(Zn1/2-
2
Ti1/2)O3–xSrTiO3 system ((a) x ϭ 0.2, b) x ϭ 0.3, c) x ϭ 0.4, d) x ϭ 0.5,
e) x ϭ 0.6).
Table II shows the approximate structure change in the LZT–CT
system. This difference was related to the fact that CaTiO3 has a
distorted structure with tilting of oxygen octahedra and cation
displacements, whereas SrTiO3 had an ideal cubic structure.17 The
1
⁄2 (111) reflection disappeared at x Ͼ 0.3, as it did in the LZT–ST
system.
The dielectric properties of perovskites have been explained by
the tilt scheme of oxygen octahedra.2,10,12 If the tilt scheme of
oxygen octahedra determines the ␶ of the LZT–ST and LZT–CT
f
systems, then the dielectric behaviors of the LZT–CT and LZT–ST
systems should exhibit different trends with compositions.
(2) Microwave Dielectric Properties
Figures 4 and 5 show the permittivity and the ␶f of the LZT–ST
and LZT–CT systems, respectively. The trend of permittivity
variation with composition can be described using the mixture
rule, i.e., 1/εtotal ϭ v1/␧ ϩ v2/␧ where vi and εi are the volume
1
2
fractions and permittivity of each phase i, respectively.16 The
dotted line in the figures of permittivity indicates the calculated
values. As can be seen in the figure, calculated values coincide
with the measured values. The variation of ␶ with composition
f
also shows mixturelike behavior.
It is interesting that the dielectric behaviors with composition of
the LZT–ST and LZT–CT systems exhibit similar trends. The ␶f of
CaTiO3 is ϳ600 ppm/°C, less than half of the value for SrTiO3
Fig. 1. XRD patterns of the (1 Ϫ x)La(Zn1/2Ti1/2)O3–xSrTiO3 system.

April 2001 Contribution of Structure to Temperature Dependence of Resonant Frequency in the LZT-ST and LZT-CT System
755
Fig. 3. XRD patterns of the (1 Ϫ x)La(Zn1/2Ti1/2)O3–xCaTiO3 system.
Table II. Approximate Structure Change in
†
Fig. 4. Permittivity and ␶ of the (1 Ϫ x)La(Zn1/2Ti1/2)O3–xSrTiO3
system.
(1 ؊ x)La(Zn1/2Ti1/2)O3–xCaTiO3
f
Cation
Cation
Inphase
tilting
Antiphase
ordering
displacement
tilting
1
1
1
1
⁄
2
(111)
⁄
2
(210)
⁄
2
(310)
⁄
2
(311)
0 Յ x Յ 0.3
0.3 Ͻ x Յ 1
†Based on XRD data only.
X
X
X
X
X
X
X
cation ordering, cation displacement, and tilting of oxygen octa-
hedra. Cation ordering and octahedral tilting are known to induce
extra modes in the IR spectra.11 Through the comparison of
BaZrO3 (cubic) and SrZrO3 (orthorhombic with octahedra tilting),
it is suggested that the appearance of the additional infrared active
mode at ϳ300 cmϪ1 is a common characteristic of the perovskite
compounds with tilted oxygen octahedra.11 The mode assignment
of the ordered Ba(B1Ј/2B1Љ/2)O3 (Fm3m) was recently suggested.19
According to the authors, modes in the 280–320 cmϪ1 range were
assigned to BЈOBЉ stretching and the strength of this mode was
used as a measure for the degree of B-site ordering. Considering
these works, the new modes found in LZT between 200 and 500
cmϪ1 could be related to cation ordering and tilting of oxygen
octahedra. These modes were found at 0 Յ x Յ 0.5, which
indirectly indicates that short-range cation ordering might exist at
x ϭ 0.4, although an unambiguous interpretation of each mode
cannot be provided yet.
(Ͼ 1100 ppm/°C). However, the composition having a zero ␶
value is the same, x ϭ 0.5, for the two systems. The ␶ values of
f
f
LZT–ST system in the composition range from x ϭ 0 to x ϭ 0.5
do not change significantly, i.e. from Ϫ52 ppm/°C to 0 ppm/°C,
whereas those from x ϭ 0.5ϳ1 exhibit a rapid increase from
0 ppm/°C to 1100 ppm/°C. This indicates that certain factors
suppressed the increase of ␶f in the composition range from x ϭ 0
to x ϭ 0.5.
(3) Far-IR Spectroscopy
Far-infrared spectroscopy work was done on the LZT–ST and
LZT–CT samples to examine the short-range structure variation.
Figure 6 shows the reflectivity spectra for the LZT–ST system. In
the case of simple perovskite structures (Pm3m), three IR active
modes were observed, as can be seen in the IR spectra of SrTiO3
(x ϭ 1) in Fig. 6. The lowest-frequency mode corresponded to
AOBO3 translations, the second mode to BO6 bending, and the
third mode corresponded to BOO6 stretching.18 The IR spectra of
LZT were more complex than that of ST. Several new modes could
be found in the range from 200 to 500 cmϪ1. The general shape of
the spectra did not change within the composition range from x ϭ
0 to x ϭ 0.5. Figure 7 shows the results of Kramer–Kronig
analysis. Existence of modes between 200 and 500 cmϪ1 was
found at x ϭ 0.5.
Figure 8 depicts the IR spectra of LZT–CT system. The
lowest-frequency mode at 150 cmϪ1 corresponds to CaOTiO3
translations, the second mode at 180 cmϪ1 to TiOO6 stretching,
the resonance mode at 440 cmϪ1 to TiOO3 torsion, and the
resonance mode at 550 cmϪ1 to BO6 bending.18 The modes
between 200 and 500 cmϪ1 exhibited nearly the same tendency as
those in LZT–ST system within the composition between x ϭ 0
and x ϭ 0.5. This phenomenon was confirmed by the results of
Kramer–Kronig analysis shown in Fig. 9. If the modes between
200 and 500 cmϪ1 were originated by only oxygen octahedra
tilting, then the modes should be found at x Ͼ 0.5 because 1⁄
2
(310)
1
and ⁄
2
(311) superlattice reflections were found at x Ͼ 0.5 in the
XRD pattern of Fig. 3.
The exact mode assignment of LZT was beyond the scope of
this work but possible reasons for some modes could be suggested
by considering the IR spectra and XRD patterns together. Struc-
turally, LZT has three additional characteristics compared to ST:
Consequently, from the XRD and IR results, it can be said that
cation ordering existed within the composition range of x ϭ 0 to
x ϭ 0.5 in the LZT–ST and LZT–CT system.

756
Journal of the American Ceramic Society—Cho et al.
Vol. 84, No. 4
Fig. 6. Infrared reflectivity spectra of the (1 Ϫ x)La(Zn1/2Ti1/2)O3–
xSrTiO3 system.
Fig. 5. Permittivity and ␶ of the (1 Ϫ x)La(Zn1/2Ti1/2)O3–xCaTiO3
f
system.
coincidence of the tilted structure and sign change of ␶ε was found
in the LZT–ST system. However, in contrast to the case of the
LZT–ST system, in phase and antiphase octahedra tilting existed in
the entire range of compositions for the LZT–CT system, so the
IV. Discussion
sign change of ␶ could not be related to the tilting of oxygen
f
octahedra. In fact, as mentioned earlier, the criterion of oxygen
octahedral tilting could not be applied to CaTiO3 itself because it
Temperature coefficient of resonant frequency (␶f) is given by
the equation ␶ ϭ Ϫ(␣ ϩ ␶ε/2) where ␣ is the linear thermal
f
has a positive ␶ with octahedra tilting. These results imply that
f
expansion coefficient and ␶ε the temperature coefficient of permit-
tivity.3 In the case of ceramics, ␶f is directly influenced by ␶ε since
the magnitude of ␣ is generally constant and insignificant com-
there could be another factor controlling the ␶ of the LZT–CT
f
system. In addition, it is very important that the sign of ␶f changes
at the same composition (x ϭ 0.5) for LZT–ST and LZT–CT
system, despite of the large difference in the ␶f values between ST
pared to that of ␶ε. ␶ is composed of three terms: A, B, and C:7
ε
͑ε Ϫ 1͒͑ε ϩ 2͒
and CT. This meant that the ␶ of the LZT–ST and LZT–CT
systems at the composition of x Յ 0.5 was determined by a certain
structural property of LZT.
f
␶ ϭ
␧
͑ A ϩ B ϩ C͒
ε
The A and B terms indicate the direct effect of volume expansion
due to a temperature increase, and the C term represents a
dependence of the polarizability on temperature with a constant
volume. A relative magnitude of the C term to (A ϩ B) generally
determines the sign and magnitude of ␶ε.2,7 Literature on metal
halides and oxides shows that the C term depends on the structure
of the compounds rather than composition.7,8 For example, halides
having CsCl structure (Pm3m) have a negative ␶ε and those having
NaCl structure (Fm3m) exhibit a positive ␶ε.
Perovskite structures can be described by three components:
tilting of anion octahedra, displacements of cations, and distortions
of the octahedron.17 Cation ordering should be added in the case of
complex perovskites. Among these components, only the tilting of
oxygen octahedra have been investigated in conjunction with
␶ε.2,11,20 For example, in the case of Ba1ϪxSrx(Zn1/3Nb2/3)O3
Then, what structural features changed near x ϭ 0.5 for the two
systems? Through a comparison of Tables I and II, cation
1
displacement can be ruled out at x ϭ 0.5 because the
⁄2 (210)
reflection was found at x Ͼ 0.5 in the LZT–CT system, whereas it
disappeared at Ͼ0.5 in the LZT–ST system. Cation ordering
disappeared at the same composition of x Ͼ 0.3 for the two
systems, but the ␶ of the two systems changed at x ϭ 0.5.
f
However, the structure classifications in Tables I and II are based
on XRD results and IR spectra in Figs. 6–9 demonstrated that
short-range ordering could exist at 0.3 Ͻ x Յ 0.5 and disappeared
at x Ͼ 0.5. This indicates that cation ordering should be considered
as a critical factor which could determine the sign of ␶f.
The question of interest is how ␶ or ␶ can be associated with
f
ε
cation ordering. Structurally, cation ordering in 1:1-type perovs-
kites such as Pb(Sc1/2Ta1/2)O3 (PST) has two effects. First, the
cation ordering in PST induces a doubled unit cell with face-
centered symmetry.1 That is, ordered PST has Fm3m symmetry
and disordered PST exhibits Pm3m. Second, cation ordering in
PST reduces the space for the rattling of the Pb2ϩ cation.21 These
two effects can be related to the variation of ␶ε.
(A) Effect of Symmetry Change: Cation ordering in LZT
induces a symmetry change from Pm3m to Fm3m. In the case of
metal halides, the symmetry change is matched to the variation of
system, the sign of ␶ changes from negative to positive when the
ε
structure changes from an untilted structure to a tilted structure.
Three structural components were commonly found in the
LZT–ST and LZT–CT system, but their variations with composi-
tions were quite different. In the case of the LZT–ST system, the
1
sign of ␶ changed at x ϭ 0.5 and the
⁄
3
(310) superlattice
f
reflections, showing in phase octahedra tilting, also disappeared at
this composition, as shown in Fig. 4 and Table I. That is, the

April 2001 Contribution of Structure to Temperature Dependence of Resonant Frequency in the LZT-ST and LZT-CT System
757
Fig. 8. Infrared reflectivity spectra of the (1 Ϫ x)La(Zn1/2Ti1/2)O3–
xCaTiO3 system.
The hypothesis of reduced rattling space could also explain the
behavior of permittivity in the LZT–ST and LZT–CT systems at
0 Ͻ x Ͻ 0.5. Permittivity of oxides at microwave frequencies can
be calculated using Clausius—Mossotti relations and ion additivity
rules5 but several compounds showed discrepancies between the
measured and calculated values.6,23 Properties of the compounds
such as ionic or electronic conductivity, ferroelectricity, or cation
compression/rattling were suggested as possible explanations.5,6
For example, the measured permittivity of La(Mg1/2Ti1/2)O3 and
LnAlO3 (Ln ϭ lanthanide) were much smaller than the calculated
values.6,23 This phenomenon was explained by cation compres-
sion. In the case of LZT–ST and LZT–CT systems, the degree of
increase of permittivity is much smaller at 0 Ͻ x Ͻ 0.5 than those
at 0.5 Ͻ x Ͻ 1. This can be explained by the reduced rattling space
with the cation ordering at 0 Ͻ x Ͻ 0.5.
Fig. 7. Infrared conductivity spectra ␴(␻) obtained from Kramer–Kronig
analysis for the LZT–ST system.
the sign of the C term and ␶ε, as is the case for NaCl and CsCl.
Their composition and permittivities are almost the same but the
sign of ␶ε is opposite.8 The space group of CsCl is Pm3m and that
of NaCl is Fm3m. The effect of symmetry on ␶ε is more evident for
RbBr.8 It has the NaCl structure and a positive ␶ under normal
ε
conditions. This transforms to the CsCl structure at high pressure
and the ␶ also changes to a negative value. This relation between
ε
structure and ␶ε is applicable to the LZT–CT and LZT–ST systems.
That is, the disappearance of cation ordering of LZT at x ϭ 0.5
resulted in a symmetry change from Fm3m to Pm3m, and ␶
ε
Considering the above explanations, how can the cation order-
ing in the LZT–ST and LZT–CT systems be maintained at x ϭ
0.5? The solid solution between a complex perovskite and a simple
changes from positive to negative (␶ changes from negative to
f
positive value) at the same composition.
(B) Effect of Compression: Most perovskite compounds ex-
hibit a constant value of the (A ϩ B) term.2 Therefore, the sign and
magnitude of ␶ε is determined by the magnitude of the C term. The
origin of the C term is explained by an ion moving in a potential
well.8 As temperature increases, the restoring force constant ( f )
increases because the ions move from a potential minimum to a
displaced position. The magnitude of the C term decreases as the
restoring force increases, since the polarizability is proportional to
1/f.
It was reported that the ordering of cations with different sizes
reduces the rattling space for cations, compared to a disordered
state.21 Differences in rattling space between ordered and disor-
dered states increases with the size difference of the cations. The
ion size of Zn2ϩ and Ti4ϩ is 0.74 and 0.605 Å,22 respectively, and
the size difference is ϳ22%. This indicates that cations in ordered
LZT are under stress, compared to those in the disordered state.
Therefore, the magnitude of the C term in the ordered state could
be smaller than that in the disordered state, which means the sign
perovskite has been widely studied. Typical examples are Ba(Zn1/3-
Ta2/3)O3–SrTiO3, Ba(Mg1/3Ta2/3)O3–BaSnO3, and Pb(Sc1/2Ta1/2)-
O3–PbTiO3.24–26 In all these cases, cation ordering is disrupted by
the small addition, a few mole percent, of a simple perovskite. In
this experiment, cation ordering was found even with the addition
of several tens of percent. The maintenance of cation ordering in
LZT–CT and LZT–ST systems can be explained as follows.
First, 1:1-type cation ordering is generally an easier process
than 1:2-type ordering. For example, ordering of Ba(Mg1/3Ta2/3)-
O3 is hard to achieve.27 This system needs more than a 10 h
annealing at 1600°C. Cation ordering of LZT has been found in the
samples sintered even at 1300°C for 2 h, which means that the
cation ordering of LZT is less sensitive to the addition of foreign
atoms than that of Ba(Mg1/3Ta2/3)O3. Second, the charge differ-
ence between the two B-site cations (Zn2ϩ and Ti4ϩ) is not
changed when CaTiO3 or SrTiO3 has been added to LZT. The
driving forces for cation ordering are the size and valence
difference between cations.28 In the case of Pb(Sc1/2Ta1/2)O3–
PbTiO3, introducing Ti4ϩ onto the shared Sc3ϩ ϩ Ta5ϩ B-site
reduces the valence difference between B-site ions. However, in
of ␶ of an ordered LZT is dominated by the (A ϩ B) term and is
ε
therefore positive.

758
Journal of the American Ceramic Society—Cho et al.
Vol. 84, No. 4
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Perovskites on the Temperature Coefficient of the Relative Permittivity,” J. Appl.
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3W. Wersing, “High Frequency Ceramic Dielectric and Their Application for
Microwave Components”; pp. 67–119 in Electronic Ceramics, Edited by B. C. H.
Steele. Elsevier, New York, 1991.
4M. Furuya and A. Ochi, “Microwave Dielectric Properties for Ba(Mg1/3Ta2/3)O3⅐
A(Mg1/2Ta1/2)O3 (A ϭ Ba, Sr, and Ca) Ceramics,” Jpn. J. Appl. Phys., 33, 5482–87
(1994).
5R. D. Shannon, “Dielectric Polarizabilities of Ions in Oxides and Fluorides,”
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Perovskite Substrate Materials for High-Temperature Superconductors,” J. Mater.
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7A. J. Bosman and E. E. Havinga, “Temperature Dependence of Dielectric
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8E. E. Havinga and A. J. Bosman, “Temperature Dependence of Dielectric
Constants of Crystals with NaCl and CsCl Structure,” Phys. Rev., 140, A292–A302
(1965).
9G. A. Samara, “Temperature and Pressure Dependence of the Dielectric Constants
of the Thallous Halides,” Phys. Rev., 165, 959–69 (1968).
10R. C. Kell, A. C. Greenham, and G. C. E. Olds, “High-Permittivity Temperature-
Stable Ceramic Dielectrics with Low Microwave Loss,” J. Am. Ceram. Soc., 56,
352–54 (1973).
11T. Nagai, M. Sugiyama, M. Sando, and K. Niihara, “Anomaly in the Infrared
Active Phonon Modes and Its Relationship to the Dielectric Constant of
(Ba1Ϫx)Srx(Mg1/3Ta2/3)O3 Compound,” Jpn. J. Appl. Phys., 35, 5163–67
(1996).
12I. M. Reaney, E. L. Colla, and N. Setter, “Dielectric and Structural Characteristics
of Ba- and Sr-based Complex Perovskites as a Function of Tolerance Factor,” Jpn.
J. Appl. Phys., 33, 3984–90 (1994).
13J. S. Kim, J. H. Lee, Y. S. Lim, J. W. Jang, and I. T. Kim, “Revisit to the
Anomaly in Dielectric Properties of (Ba1ϪxSrx)(Zn1/3Nb2/3)O3 Solid Solution Sys-
tem,” Jpn. J. Appl. Phys., 36, 5558–61 (1997).
14S. Y. Cho, M. K. Seo, K. S. Hong, and S. J. Park, “Influence of ZnO Evaporation
on the Microwave Dielectric Properties of La(Zn1/2Ti1/2)O3,” Mater. Res. Bull., 32,
725–35 (1997).
15B. W. Hakki and P. D. Coleman, “A Dielectric Resonator Method of Measuring
Inductive Capacities in the Millimeter Range,” IRE Trans. Microwave Theory Tech.,
[July] 402–10 (1960).
16S. Y. Cho, I. T. Kim, and K. S. Hong, “Crystal Structure and Microwave
Dielectric Properties of (1 Ϫ x)La(Zn1/2Ti1/2)O3–xSrTiO3 System” Jpn. J. Appl.
Phys., 37, 593–96 (1998).
Fig. 9. Infrared conductivity spectra ␴(␻) obtained from Kramer–Kronig
analysis for the LZT–CT system.
17A. M. Glazer, “Simple Ways of Determining Perovskite Structures,” Acta
Crystallogr., Sect. A, A31, 756–61 (1975).
18C. H. Perry, D. J. McCarthy, and G. Rupprecht, “Dielectric Dispersion of Some
Perovskite Zirconates,” Phys. Rev., 138, A1537–A1538 (1965).
19I. M. Reaney, J. Petzelt, V. V. Voitsekhovskii, F. Chu, and N. Setter, “B-site
Order and Infrared Reflectivity in A(BЈBЉ)O3 Complex Perovskite Ceramics,”
J. Appl. Phys., 76, 2086–92 (1994).
the case of LZT–CT and LZT–ST systems, the valence difference
in B-site ions does not change with increasing CT and ST contents,
so the driving force for ordering could be maintained.
20V. Sivasubramanian, V. R. K. Murthy, and B. Viswanathan, “Microwave
Dielectric Properties of Certain Simple Alkaline Earth Perovskite Compounds as a
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21K. Uchino, L. E. Cross, R. E. Newnham, and S. Nomura, J. Phase Transitions,
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V. Conclusion
The behavior of permittivity and ␶f in (1 Ϫ x)La(Zn1/2Ti1/2)O3–
xSrTiO3 and (1 Ϫ x)La(Zn1/2Ti1/2)O3–xCaTiO3 was discussed in
conjunction with structural changes. Dielectric properties of both
systems exhibited mixturelike behavior, which was characterized
as suppressed increase of permittivity and ␶ in the composition
range 0 Ͻ x Ͻ 0.5. The composition at which the sign of ␶
changed was x ϭ 0.5 for both systems. XRD results showed that
cation ordering disappeared at x Ͼ 0.3 for both systems, but IR
spectra demonstrated that short-range cation ordering existed at
x Յ 0.5. Structural changes including cation ordering were
suggested as the determining factor for the sign of ␶ in
La(Zn1/2Ti1/2)O3-based perovskite systems. Effects of cation
22R. D. Shannon, “Revised Effective Ionic Radii and Systematic Studies of
Interatomic Distances in Halides and Chalcogenides,” Acta Crystallogr., Sect. A, A32,
751–67 (1976).
23R. Guo, A. S. Bhalla, R. Roy, and L. E. Cross, “Ion Polarizability Additivity Rule
and Its Application on HTSC Substrate Materials,” Ferroelectrics, 155, 43–48
(1994).
f
f
24H. Matsumoto, H. Tamura, and K. Wakino, “Ba(Mg,Ta)O3⅐BaSnO3 High-Q
Dielectric Resonator,” Jpn. J. Appl. Phys., 30, 2347–49 (1991).
25H. Tamura, T. Konoike, Y. Sakabe, and K. Wakino, “Improved High-Q
Dielectric Resonator with Complex Perovskite Structure,” J. Am. Ceram. Soc., 67,
C59–C61 (1984).
f
26J. R. Giniewicz, A. S. Bhalla, and L. E. Cross, “An Investigation of the Structural
and Dielectric Properties of the Solid Solution System (1 Ϫ x)Pb(Sc1/2Ta1/2)O3–
xPbTiO3,” Ferroelectr. Lett. Sect., 12, 35–42 (1990).
ordering on ␶ may be understood in terms of the symmetry
change and the reduced space for cation rattling.
ε
27R. Guo, A. S. Bhalla, and L. E. Cross, “Ba(Mg1/3Ta2/3)O3 Single Crystal Fiber
Grown by the Laser Heated Pedestal Growth Technique,” J. Appl. Phys., 75, 4704–08
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References
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(1980).
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