Tunability and ferroelectric relaxor properties of bismuth strontium titanate
ceramics
Wei Chen, Xi Yao, and Xiaoyong Wei
Citation: Appl. Phys. Lett. 90, 182902 (2007); doi: 10.1063/1.2734958
View online: http://dx.doi.org/10.1063/1.2734958
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APPLIED PHYSICS LETTERS 90, 182902 共2007兲
Tunability and ferroelectric relaxor properties of bismuth strontium titanate
ceramics
Wei Chen,a兲 Xi Yao, and Xiaoyong Wei
Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education, Xi’an Jiaotong
University, Xi’an 710049, People’s Republic of China
共Received 27 February 2007; accepted 5 April 2007; published online 1 May 2007兲
A systemic study was performed on bismuth strontium titanate 共Sr1−1.5xBixTiO3, 0.04艋 x 艋 0.25兲
ceramics. Dielectric properties were measured from 83 to 373 K at different frequencies. A
transformation from relaxor ferroelectrics to relaxor behavior was observed when x ⬎ 0.10. Both
diffuseness and relaxation degree increase as x rises. Tunability was found to increase with
increasing x value until 0.07, and then decrease when Bi3+ content increases. Meanwhile, their
figures of merit 共defined as tunability/loss兲 shows a maximum at x = 0.12. The change from relaxor
ferroelectrics into relaxor behavior has a positive impact on the tunability of bismuth strontium
titanate system. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2734958兴
Recently, the research about the impurity on SrTiO3
共ST兲, in particular, the replace of A site, has attracted lots of
interest. It has been shown that the cation substitution of Sr2+
by isovalent Ca2+,1 Ba2+,2 Pb2+,3 Mg2+ 共Ref. 4兲 may induce a
ferroelectric phase transition in ST. At low impurity concentration, the temperature dependence of dielectric constant reveals rather diffuse peaks with a strong frequency dispersion
characteristic of a relaxor state.1–6 Dielectric anomalies with
remarkable frequency dispersion were also reported for
nonisovalent substitution of Sr2+ by rare-earth ions7 and
Bi3+.8 In the latter case, the relaxation behavior and relaxortype behavior were attributed to the nanocluster polarization
dynamics, originating from Bi3+ ions that shift in one of the
six equivalent off-center positions in the A site of perovskite
lattice.9 As observed by Ang et al.,8 Bi impurity also can
induce a ferroelectric behavior in ST system, and this ferroelectric behavior transited into relaxor behavior with Bi increasing. This indicated that this system would be a good
model to investigate the transformation between ferroelectric
and relaxor behavior, helpful to learn and improve the dielectric or ferroelectric properties of related materials, such as
bismuth stroutium titanate 共BST兲 共Ref. 10兲 and BZT,11 and
finally to make these materials play a more important role in
devices, including actuators, transducers, as well as microwave dielectrics.12–14 However, there is a few study on the
tunability and the correlation between the tunability and the
relaxor property in this doping system. In this work,
4 – 30 mol % of Bi were doped into ST through a solid state
reaction process. Hysteresis loops were measured to demonstrate the existence of ferroelectric of some compositions.
Their permittivity as a function of temperature was also investigated in order to understand the effect of Bi content on
the tunability of SBiT ceramics.
Ceramic samples were prepared by the conventional
method with an initial amount of precursors according to the
ratio 共Sr+ Bi兲 / Ti= 1.5, assuming the substitution of Sr by Bi.
Room temperature x-ray diffraction results indicated that all
of the samples were single cubic perovskite phase. Their
density was around 96% of the theoretical density. Scanning
electronic micrograph 共SEM兲 confirmed the incorporation of
a兲
Electronic mail: lffwster@gmail.com
0003-6951/2007/90共18兲/182902/3/$23.00
Bi into ST grains and the absence of any second crystalline
phases for x 艋 0.20. The relationship between dielectric permittivity and temperature for all the samples were measured
using a HP4284 LCR meter from 100 Hz to 1 MHz in a temperature range from 83 to 373 K at a rate of 1 K / min. The
hysteresis loops were measured by TF analyzer 2000
ferroelectric-module system 共aixACCT兲. The dielectric tunability was characterized by a dc power source
共PS350/ 5000 V, 25 W兲 and multifrequency LCR meter
共TH2816兲 at room temperature.
The temperature dependence of dielectric permittivity of
Sr1−1.5xBixTiO3 共0.04艋 x 艋 0.25兲 at different frequencies is
plotted in Fig. 1共a兲. It can be seen that with the increase of Bi
content, the dielectric peaks shift from 126.9– 133.1 K
to 212.8– 226.0 K. In addition, the value shows a considerable change from over 2000 for x = 0.04 to less than 1000 for
x = 0.25. Meanwhile, the overall losses of the SBiT ceramics
are significantly suppressed as the Bi concentration increased
in Fig. 1共b兲. In particular, the loss around the room temperature decreases gradually from above 0.05 to below 0.005 as
the Bi content x increases up to 0.25.
It is noted that there is a clear shift of permittivity peak
and loss peak with increasing frequency for all compositions.
Taking the two fringed samples as examples 关Fig. 1共a兲兴, the
shift becomes larger when Bi increases. Furthermore, their
peak values decrease with increasing the frequency, which
shows a typical behavior of relaxor ferroelectrics. For the
samples below 10 mol %, there were a second dielectric
peak and a loss peak in higher temperature, which are believed to be attributed to the oxygen-vacancy related
relaxations.9
In the paper of Ang and Yu,15 the coexistence phenomenon of dielectric relaxor modes and ferroelectric relaxor
modes between 0.0033艋 x 艋 0.133 Bi content, transited into
ferroelectric relaxor at x 艌 0.133. However, the relationship
between dielectric permittivity and temperature for the
Sr1−1.5xBixTiO3 ceramics suggested that there was a different
dielectric behavior compared with the report.15 The hysteresis loops of several typical samples confirmed it, as illustrated in Fig. 2. For the samples of 4% and 7% Bi concentration, in the left side of dielectric peak temperature, the
hysteresis loops were detected at 93 K, as shown in Fig. 2共a兲.
90, 182902-1
© 2007 American Institute of Physics
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182902-2
Appl. Phys. Lett. 90, 182902 共2007兲
Chen, Yao, and Wei
FIG. 2. Hysteresis loops for different compositions at different
temperatures.
FIG. 1. 共Color online兲 Temperature dependence of dielectric properties for
all compositions at 10 kHz, and for the samples of x = 0.04 and x = 0.25 at
different frequencies.
However, these loops became distorted when elevating temperature. In the measured hysteresis loop, the remnant polarization 共Pr兲 decreased with Bi increasing, and the Pr value
was small on the whole. There was no hysteresis loop for the
compositions above 10% Bi concentration. Figure 2共b兲
showed the result for the compositions of 20% Bi around the
dielectric peak temperature. This suggested that the nominal
remnant polarization was linear variant with the electric
field, which indicated the loss type loop rather than ferroelectric hysteresis loop.
The above results demonstrated the existence of weak
ferroelectricity for the samples below 10% Bi concentration
and there was no hysteresis loops found above 10% Bi. Accordingly, the description15 that the samples above x = 0.133
showed ferroelectric relaxor behavior was not exact. In addition, in the report from Ang et al.,8 there were slim hysteresis loops for the samples below 5.33% Bi concentration in
the lower temperature. In present letter, this value corresponding to 5.33% is believed to be higher.
In addition, the parameters of ⌬Trelax and ⌬Tdiffuse共1 kHz兲
were introduced. The specific symbols of the relaxation degree and the diffuseness degree are defined, respectively, below
⌬Trelax = T␧m共100
kHz兲
− T␧m共100
Hz兲
,
共1兲
⌬Tdiffuse共1
kHz兲
= T0.9␧m共1
kHz兲
− T␧m共1
kHz兲
.
共2兲
As shown in Fig. 3, both relaxation degree and diffuseness
degree display an increasing trend in the whole scope. For
the samples with low x value, both parameter values were
small, implying the near normal ferroelectric behavior; however, with increasing Bi content, the two parameters increase
quickly, implying that this system is becoming more relaxorlike.
The tunability of all samples was measured at room temperature and under 10 kHz. Tunability was usually used to
describe ferroelectric behavior.16–18 In the recent paper by
Wei and Yao,19 polar nanoregion response contributes to the
relaxation and tunability. As we know, typical relaxor has
slight tunability. However, the tunability was rather strong in
FIG. 3. Relaxor degree and diffuse degree variant with Bi content.
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182902-3
Appl. Phys. Lett. 90, 182902 共2007兲
Chen, Yao, and Wei
共3兲, the tunability is directly proportional to ␧r共T , 0兲. For
SBiT, ␧r共T , 0兲 decreases with increase of Bi content and thus
the tunability of SBiT shows a decrease with increasing Bi
content, too.
Because of the appearance of another relaxation for the
samples below x = 0.10 near room temperature, the tunability
results in this range are believed to have a tight relation with
the strengths of the oxygen-vacancy related relaxation.9 And
in this range, the loss was also rather strong, so the merit
decreased naturally. However, the ceramics above 12% impurity concentration could be a promising candidate for microwave applications for their large merit.
In summary, hysteresis loops and tunability demonstrated that the change from relaxor ferroelectrics into relaxor behavior had a positive impact on the tunability of the
materials. From the applied aspect, the samples above 12%
Bi concentration might be a promising candidate for microwave applications because of the relatively large figures of
merit.
FIG. 4. Dielectric tunability and their figures of merit of all compositions at
room temperature at 10 kHz. The inner picture is the tunability of the compositions above 0.12.
classical ferroelectric, such as BST and PST.20,21 Therefore,
tunability can be a good way to judge the ferroelectricity.
It can be seen in Fig. 4 that the tunability was remarkable before 10% Bi concentration, particularly in the compositions of 4% and 7%. There was an increasing trend from
1% to 7% Bi concentration. This demonstrated the fact that
Bi impurity can induce ferroelectricity in SrTiO3.8,22 Furthermore, ferroelectricity also became strong with Bi increasing
until 7%. The tunability of the samples above 10% was weak
and the values were smaller than 1%. Furthermore, in this
range, the tunability showed a trend of linear decreasing. It
proved that ferroelectricity of the system was weak gradually. Meanwhile, the relaxor behavior was dominant with Bi
increasing. However, their figures of merit 共defined as
tunability/loss兲 共Ref. 16兲 decrease with Bi increasing until
x = 0.10 and reach the maximum at x = 0.12, subsequently,
exhibit monotonously decrease with the Bi content.
The above experimental results above x = 0.10 can be
elucidated in terms of the phenomenological theory of Devonshire and the simplified expression proposed by
Johnson:23,24
␧r共T,0兲
= 兵1 + 关␧o␧r共T,0兲兴3B共T兲E2其1/3 ,
␧r共T,E兲
共3兲
where the left is the tunability ratio, ␧o is the vacuum dielectric constant, ␧r共T , 0兲 is the relative permittivity at temperature T without dc bias, B共T兲 is the phenomenological coefficient of the fourth power of polarization and it has a weak
relation with the temperature, and E is the external dc bias
field.
Since the SBiT ceramics become more relaxorlike with
increasing Bi content, the relaxation degree increases with
the Bi content and the permittivity versus temperature curves
become lower and broader 共Figs. 1 and 3兲. According to Eq.
This work was supported by the Ministry of Science and
Technology of China through the 973 project under Grant
No. 2002CB613304 and NSFC under Grant No. 50402015.
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