C. ZHANG et al.: THE DIFFUSE PHASE TRANSITION OF Ti-RICH (Ba 0.75Sr 0.25)T i 1+  O 3+2  DIELECTRIC CERAMICS
181–187
THE DIFFUSE PHASE TRANSITION OF Ti-RICH (Ba
0.75
Sr
0.25
)
Ti
1+  O
3+2  DIELECTRIC CERAMICS
PREHOD DIFUZIVNE FAZE V DIELEKTRI^NE KERAMIKE,
BOGATE S Ti ((Ba
0.75
Sr
0.25
)T i
1+  O
3+2  )
Chen Zhang
*
, Xin Zhong, Fang-Xu Chen, Zhu-Ming Tang
Jiangsu University of Science and Technology, Department of Materials Science and Engineering, No. 2 Mengxi Road,
Zhenjiang 212004, China
Prejem rokopisa – received: 2019-07-12; sprejem za objavo – accepted for publication: 2019-11-20
doi:10.17222/mit. 2018.144
The microstructures, dielectric properties and phase-transition behavior of Ti-rich barium-strontium-titanate-based ceramics
synthesized by the solid-state method were investigated with a non-stoichiometric level by SEM, XRD and an LCR measuring
system. It was found that all the (Ba 0.75Sr 0.25)Ti 1+  O 3+2  ceramics (  = 0.005, 0.01, 0.015, 0.02) are single-phase solid solutions
with a typical tetragonal perovskite structure. With an increasing   value, the average grain size of the (Ba 0.75Sr 0.25)Ti 1+  O 3+2  ceramics increases. The strengthened spontaneous polarization resulting from the unit-cell deformation is responsible for the
increase of the transition temperature T m as   increases. Also, the dielectric parameters   rRT and   m increase with an increasing
non-stoichiometric level. The low-temperature (T   T m) frequency dispersion of the relative dielectric constant can be found
and the diffuse phase-transition behavior is suppressed with an increasing   value in the (Ba 0.75Sr 0.25)Ti 1+  O 3+2  ceramics.
Keywords: barium strontium titanate, dielectrics, ceramics, phase transition
Avtorji so analizirali mikrostrukture, dielektri~ne lastnosti in fazne prehode barij-stroncij titanatne kermamike bogate s Ti, ki so
jo sintetizirali z metodo sintranja v trdni fazi. Analize na nestehiometri~nem nivoju so izvajali s pomo~jo SEM, XRD in LCR
preiskovalnih sistemov. Ugotovili so da vsebuje (Ba 0.75Sr 0.25)Ti 1+  O 3+2  keramika (pri   = 0,005, 0,01, 0,015 ali 0,02) enofazno
trdno raztopino s tipi~no tetragonalno peroviskitno strukturo. Z nara{~ajo~o vrednostjo d nara{~a povpre~na velikost kristalnih
zrn (Ba 0.75Sr 0.25)Ti 1+  O 3+2  keramike. Oja~ana spontana polarizacija je posledica deformacije enovite celice, kar povzro~i z
nara{~ajo~im d tudi nara{~anje temperature prehoda T m. Prav tako s pove~evanjem nestehiometrije nara{~ata dielektri~na
parametra   rRT in   m. Ugotovili so tudi, da ima z nara{~ajo~o vrednostjo d (Ba 0.75Sr 0.25)Ti 1+  O 3+2  keramika nizko disperzijo
temperaturne frekvence (T = T m) relativne dielektri~ne konstante in zatrt prehod difuzivne faze.
Klju~ne besede: barij stroncijev titanat, dielektriki, keramika, fazni prehodi
1 INTRODUCTION
Ferroelectric barium titanate (BaTiO
3
, BT) possesses
a typical perovskite structure ABO
3
, which is used in
many electronic devices, including high-permittivity
multilayer ceramic capacitors (MLCC’s).
1
The interest in
BT stems from the high relative permittivity
10,000–12,000, observed at the ferro- to para-electric
phase-transition temperature (about 130
o
C), also known
as the Curie temperature T
c
.
2
Above T
c
, in the paraelectric
state, the temperature dependence of the dielectric
constant follows the Curie-Weiss law and around T
c
,a
sharp Curie peak can always be found.
To meet the tolerances of the percentage change of
capacitance   C over a certain temperature range  T for
the different specifications of MLCCs, various dopants
are often applied in BT dielectric ceramics.
3–5
The
majority of dopants added into BT cause a drop of T
c
,
and some of them succeed in inducing the diffuse phase
transition. For example, the substitution of Zr
4+
with
isovalent Ti
4+
results in a solid solution from BT to
Ba(Ti
1-x
Zr
x
)O
3
(BZT) and causes typical relaxor
ferroelectric behavior characterized by the frequency
dispersion of the relative permittivity maximum as well
as a diffuse phase transition as x approaches 0.3.
6,7
How-
ever, some of the dopants fail to modify the temperature
stability of the capacitance, especially around the T
c
.F o r
instance, the solid solution (Ba
1-x
Sr
x
)TiO
3
(BST)
resulting from the substitution of Sr
2+
with isovalent Ba
2+
in BT still has a sharp Curie peak, even when x equals
0.4.
8
For practical capacitor applications, a further modi-
fication in BST ceramics is inevitable. Three common
methods are employed: the addition of isovalent or
aliovalent dopants such as Mg,
9
Na,
10
etc.; process
controlling especially the calcination/sintering prog -
ress;
11,12
and the usage of functionally graded materials
(FGMs).
8
Among the dopants, rare-earth elements (e.g.,
La
3+
,D y
3+
,S m
3+
) are found to be effective in controlling
the temperature dependency of the dielectric properties
in stoichiometric BST ceramics.
13–15
The diffused ferro-
electric-paraelectric phase transition accompanied by the
comprehensive properties can be obtained in Ho
2
O
3
highly doped (Ba
0.75
Sr
0.25
)TiO
3
ceramics according to our
previous study.
16
The high unchanged dielectric constant
with a slight decrease of loss tangent has been found in
Materiali in tehnologije / Materials and technology 54 (2020) 2, 181–187 181
UDK 620.1:532.7:666.3-1:546.824 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(2)181(2020)
*Corresponding author's e-mail:
czhang1981@hotmail.com (Chen Zhang)
Ba-excessive Ba
0.71
Sr
0.29
TiO
3
ceramics
17
and adding
deficient TiO
2
could bring about a diffuse phase tran-
sition in BST ceramics,
18
which provides us with the
possibility to obtain fine dielectrics for ceramic capaci-
tors by controlling the mole ratio of A-site ion to B-site
ion (n
A
:n
B
) in BST ceramics. Furthermore, the dielectric
temperature stability of the BST ceramics can be some-
how improved by applying excessive TiO
2
.
19
Previous findings sparked our interest in findind out
how the Ti-rich level influencing the dielectric properties
and phase-transition behavior in non-stoichiometric BST
ceramics modified with Ho
2
O
3
. Therefore, in this article
we report a systematic study of the microstructure,
dielectric properties and phase transition of non-stoi-
chiometric (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  (  = 0–0.02) based
ceramics [n
A
:n
B
=1:(1+  )  1].
2 EXPERIMENTAL PART
The chemical compositions of the Ti-rich BST-based
ceramics were given by the formula (Ba
0.75
Sr
0.25
)
Ti
1+  O
3+2  + 3 at.% Ho
2
O
3
(  = 0, 0.005, 0.01, 0.015,
0.02) and symbolized as R0, R1, R2, R3, R4. High-
purity BaCO
3
(>99.0 %), SrCO
3
(>99.0 %) and TiO
2
(>98.0 %) powders used as starting raw materials were
weighed, ball-milled, dried and calcined at 1080 °C for
2 h. The calcined powders were mixed with Ho
2
O
3
(>99.0 %), reground, dried and added with5%( w/%)
polyvinyl alcohol (PV A) as a binder for granulation. The
mixture was sieved through a 60-mesh screen and then
pressed into pellets. Sintering was conducted in air at
1350–1480 °C for 1–6 h. For dielectric the measure-
ments, both the flat surfaces of the specimens were
coated with BQ-5311 silver paste after ultrasonic bath
cleaning and then fired at 800 °C for 10 min.
The crystal structures of the specimens were
confirmed by X-ray diffraction analysis (XRD, Rigaku
D/max 2500v/pc) with Cu-K
  radiation. The surface
morphologies of the as-sintered specimens were ob-
served using the SEM (JSM-6480 ESEM). The tem-
perature dependence of the dielectric constant and loss
tangent was measured at 1 kHz, 10 kHz, 100 kHz and
500 kHz from –185 °C to 250 °C with a TZDM-200-300
automatic LCR measuring system. The percentage of
permittivity variation at 1 kHz (  
r
/  r
) from –30 °C to
85 °C used for accessing the temperature stability of the
relative dielectric constant is determined using the
following Equation:
Δ      
  r
r
r r
r
=
−
×
t RT
RT
100% (1)
where   rRT
is the relative dielectric constant at room
temperature;   rt
is the relative dielectric constant at any
other temperature. All the above microstructure anal-
yses and dielectric measurements were conducted using
the samples sintered at 1450 °C for 2 h.
3 RESULTS AND DISCUSSION
The X-ray diffraction patterns of the as sintered
(Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  bulk ceramics are shown in
Figure 1. It appears that the stoichiometric ceramic
(Ba
0.75
Sr
0.25
)TiO
3
and the non-stoichiometric (Ba
0.75
Sr
0.25
)
Ti
1+  O
3+2  ceramics (  = 0.005, 0.01, 0.015, 0.02) are
single-phase solid solutions with a typical perovskite
structure. No obvious secondary phase is detected, even
for the (Ba
0.75
Sr
0.25
)Ti
1.02
O
3.04
ceramics based on the
X-ray diffraction patterns. According to the magnified
view of the diffraction peaks presented in Figures 1b
and 1c, the (002)/(200) and (202)/(220) diffraction peaks
C. ZHANG et al.: THE DIFFUSE PHASE TRANSITION OF Ti-RICH (Ba 0.75Sr 0.25)T i 1+  O 3+2  DIELECTRIC CERAMICS
182 Materiali in tehnologije / Materials and technology 54 (2020) 2, 181–187
Figure 1: XRD patterns for (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics
can be seen in all the (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics,
indicating that the studied ceramics possess tetragonal
perovskite structure (space group P4mm).
Also, a slight shift of the diffraction peaks to higher
two-theta values with the increasing   value is observed,
which indicates that the unit-cell volumes of the
(Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics decrease as the non-
stoichiometric level of the Ti ions present in the ABO
3
perovskite structure increases. Similar phenomena have
been previously reported in Ca-substituted BST
ceramics
20
and in our previous work for Ti deficient
(Ba
0.75
Sr
0.25
)Ti
1-  O
3-2  ceramics.
21
Initially, the A-site vacancies V
A
"
and oxygen vacan-
cies V
o
••
remain in the un-doped (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  calcined powders according to the following point-defect
reaction equation:
(Ba
0.75
Sr
0.25
)Ti
1+  O
3+2    0.75Ba
Ba
+ 0.25Sr
sr
+
(1+  )Ti
Ti
+ (3+2  )O
O
+   V
A
"
+   V
o
••
(2)
After Ho
2
O
3
doping, the A-site vacancies V
A
"
are
partially filled up by Ho
3+
ions forming the point defects
Ho
A
•
and the leftover A-site vacancies V
A
"
taking the
negative charge remain there to compensate the positive
charge of Ho
A
•
. After that, the Ho
3+
ions begin to
substitute the host ions (including both the A-site ion and
B-site ion) because the concentration of Ho
3+
doping
ions (6 at.%) is much more than that of the A-site
vacancies (less than or equal to 2 a/%). And the other
two point defect reactions happen:
Ho
2
O
3
  2Ho
A
•
+ V
A
"
+3 O
O
(3)
Ho
2
O
3
  2Ho
Ti
ª
+ V
o
••
+3 O
O
(4)
Therefore, there are totally four kinds of point defects
Ho
Ti
ª
, Ho
A
•
, V
A
"
and V
o
••
in the Ho
2
O
3
doped
(Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics. With the increase of   ,
the amount of Ho
3+
ion occupying the A-site increases,
while that in the B-site decreases due to the fixed
concentration of Ho
3+
ions. The radii of the host Ba
2+
and
Sr
2+
ions are 0.161 nm (in 12 coordination) and 0.144 nm
(in 12 coordination), respectively, while the radius of the
host Ti
4+
ion is 0.061 nm (in 6 coordination). The Ho
3+
doping ion with a moderate radius of 0.0901 nm can
cause the shrinkage of the unit cell by entering the A-site
or the unit-cell expansion by getting into the B-site due
to the radius difference between the Ho
3+
ion and the
host ion. Apparently, the obtained shrinkage of the
unit-cell volume with the increase of   in
(Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics is exactly attributed to the
above-mentioned difference of the Ho
3+
distribution in
the host lattice site when increasing   .
Figure 2 shows the surface morphologies of the
as-sintered (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics. All the
(Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  samples possess dense microstruc-
tures and a fine grain size distribution can be found in
C. ZHANG et al.: THE DIFFUSE PHASE TRANSITION OF Ti-RICH (Ba 0.75Sr 0.25)T i 1+  O 3+2  DIELECTRIC CERAMICS
Materiali in tehnologije / Materials and technology 54 (2020) 2, 181–187 183
Figure 2: SEM micrographs of (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics: a)   =0 ,b )  = 0.005, c)   = 0.01, d)   = 0.015, e)   = 0.02
C. ZHANG et al.: THE DIFFUSE PHASE TRANSITION OF Ti-RICH (Ba 0.75Sr 0.25)T i 1+  O 3+2  DIELECTRIC CERAMICS
184 Materiali in tehnologije / Materials and technology 54 (2020) 2, 181–187
Figure 3: Temperature dependence of the relative dielectric constant and loss tangent for (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics at different
frequencies: a)   =0 ,b )  = 0.005, c)   = 0.01, d)   = 0.015, e)   = 0.02
ceramics with a small   value. However, with the
increase of   , the abnormal grain growth becomes
distinct, as shown in Figures 2c, 2d and 2e.The average
grain size of the (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics analyzed
using the Nano Measurer software is collected in
Table 1. It is obvious that the average grain size of the
(Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics increases with the in-
creasing   value.
Table 1: Average grain size of (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics
Sample No. R0 R1 R2 R3 R4
Grain size(μm) 2.1 2.6 2.8 2.9 3.1
Figure 3 shows the temperature dependence of the
relative dielectric constant and dielectric loss for the
(Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics at 1 kHz, 10 kHz,
100 kHz and 500 kHz. The dielectric constant peaks
corresponding to the ferroelectric-paraelectric phase
transition can be observed for all the (Ba
0.75
Sr
0.25
)
Ti
1+  O
3+2  ceramics. The relative dielectric constant at
room temperature (25 °C)   rRT
, the loss tangent at room
temperature (25 °C) tan   RT
, the maximum of dielectric
permittivity   m
at 1 kHz and the temperature corres-
ponding to this permittivity maximum T
m
are collected in
Table 2. The   rRT
,t a n  RT
and   m
increase with the
increasing   value, in general. The (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics with low  value, such as R1 and R2, possess a
high relative dielectric constant (>6000) and a low
dielectric loss (<0.004) at room temperature. With the
increase of  value, the T
m
increase from –1 °C for R0 to
6 °C for R2 and then to 12 °C for R4. The rise of T
m
is
also found in samarium-doped Ba
0.68
Sr
0.32
TiO
3
ceramics.
15
The charged vacancies, such as A-site vacan-
cies V
A
"
and oxygen vacancies V
o
••
in (Ba
0.75
Sr
0.25
)
Ti
1+  O
3+2  ceramics, as mentioned previously, give rise to
the distortion of the ABO
3
perovskite unit cells and thus
a stronger deviation of the B-site ion from the center of
the oxygen octahedron. It is exactly this stronger
deviation that explains the enhancement of the
spontaneous polarization, in other words, the increased
tetragonality in tetragonal BST perovskites. Therefore,
the increase of T
m
with the increasing  is observed. The
increase of  rRT
and  m
with increasing  is also attributed
to the strengthened spontaneous polarization. It has been
reported that the smaller grains in ferroelectric ceramics
inhibit the formation of large ferroelectric domains and
thus reduce the effective contribution to the total polari-
zation.
22
Furthermore, the (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ferro-
electric ceramics with a smaller grain size contain more
non-ferroelectric grain boundaries but relatively fewer
ferroelectric grains than those with a larger grain size.
Therefore, the increased average grain size gives another
reason for the increase of   rRT
and   m
with increasing   .
The   
r
/  r
at 1 kHz from –30 °C to 85 °C for the
(Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics is shown in Table 2. The
  
r
/  r
of R4 sample (+20.0 % – –68.7 %) satisfies the
Y5V standard (  
r
/  r
  +22 % to –82 % from –30 °C to
85 °C) according to the ceramic capacitor classification
of the Electronic Industries Association (EIA).
18,23
The low temperature (T   T
m
) frequency dispersion
of the relative dielectric constant can be found in
(Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics. The dielectric loss of
(Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics shows the strong
frequency dependence across the whole temperature
range.
Table 2: Dielectric parameters for (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics
at 1 kHz
Sample
No.
    rRT
tan  RT
  m    r
/  r
(%)
R0 0 4469 0.0017 6565 +46.9– –60.8
R1 0.005 6159 0.0022 8123 +31.9– –67.1
R2 0.01 6478 0.0039 8330 +28.6– –66.6
R3 0.015 6172 0.0122 7842 +27.1– –70.5
R4 0.02 8119 0.0220 9742 +20.0– –68.7
The diffuse phase transition is always characterized
by a deviation from the Curie-Weiss law in the vicinity
of the Curie temperature (T
c
). It is known that the
dielectric permittivity of a normal ferroelectric above the
Curie temperature follows the Curie-Weiss law described
by the following Equation (5):
1
0
  r
TT
C
=
−
(5)
where T
0
is the Curie-Weiss temperature and C is the
Curie-Weiss constant. Figure 4 shows the inverse of the
relative dielectric constant for (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics as a function of temperature at 1 kHz. The
plots (for the T > T
m
part) are fitted linearly according to
Equation (5) and the Curie-Weiss temperature T
0
ob-
tained from the fittings is listed in Table 3.
Obviously, the dielectric behavior of the (Ba
0.75
Sr
0.25
)
Ti
1+  O
3+2  ceramics with a low  value shows a deviation
from the Curie-Weiss law at temperatures above the T
m
.
C. ZHANG et al.: THE DIFFUSE PHASE TRANSITION OF Ti-RICH (Ba 0.75Sr 0.25)T i 1+  O 3+2  DIELECTRIC CERAMICS
Materiali in tehnologije / Materials and technology 54 (2020) 2, 181–187 185
Figure 4: Temperature dependence of the inverse relative dielectric
constant for (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics at 1 kHz (The symbols:
experimental data; the solid line: fitting to the Curie-Weiss law.)
Then the parameter   T
m
, often used to characterize the
degree of the deviation from the Curie-Weiss law and
defined as follows, is calculated (Table 3):
  T
m
= T
cw
– T
m
(6)
where T
cw
denotes the temperature from which the
dielectric permittivity starts to deviate from the Curie-
Weiss law and T
m
represents the temperature of the
dielectric constant maximum. With the increase of the  value,   T
m
decreases from 79 °C (for R0 sample) to
1 °C (for R4 samples), which indicates that the diffuse
transition behavior is suppressed with an increasing   value and the diffuse phase-transition behavior of
(Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics even vanishes at   =
0.02.
Table 3: Temperature parameters for (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  cera-
mics at 1 kHz
Sample No.   T 0 (°C) T cw
(°C) Tm (°C)   T m (°C)
R 002 17 8– 17 9
R1 0.005 16 59 7 52
R2 0.01 4 37 6 31
R3 0.015 –16 18 8 10
R4 0.02 –37 11 12 1
A modified Curie-Weiss law has been proposed to
describe the diffuseness of the ferroelectric phase tran-
sition:
11
  
  r
TT
C
−=
−
m
m
()
(7)
where   and C
’
are assumed to be constant. The para-
meter  reveals the characteristic of the phase transition:
for  = 1, a normal Curie-Weiss law is obtained; for  =2,
a complete diffuse phase transition is described. The
parameters   can be obtained by a linear fitting of the
data through ln(1/  r
–1/  m
) ~ ln(T–T
m
). The plots of
ln(1/  r
–1/  m
) as a function of ln(T–T
m
) for (Ba
0.75
Sr
0.25
)
Ti
1+  O
3+2  samples are shown in Figure 5. The   =
1.47026, 1.46651, 1.30255 and 1.03711, respectively,
for R0, R2, R3 and R4 ceramics. The   value decreases
as the   increases, clearly implying that the
(Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics with a higher
non-stoichiometric level exhibit a weaker diffuse
ferroelectric-paraelectric phase transition, which is in
accordance with the previous conclusion drawn from the
discussion of  T
m
. It is found that the smaller grain size
is beneficial to obtaining a more broadened Curie peak
in the BST ceramics due to the grain-boundary effect.
24
The average grain size of the (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  samples increases with the increasing   value, as
mentioned previously. Therefore, the diffuse phase
transition for high  samples is weaker than that for low
  ones.
Figure 3 also provides the dielectric response of the
(Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics at various frequencies.
The relative dielectric constant (in the T<T
m
range)
decreases with the increase of the test frequency. The   m
and T
m
of the (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics at 1 kHz,
10 kHz, 100 kHz and 500 kHz are shown in Table 4. The
T
m
for the R1, R2, R3, and R4 samples shifts only 1 °C
towards higher temperature with increasing frequency.
For the R0 sample, there is no shift in T
m
at all. The   m
for all the (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics shows an
obvious frequency dispersion.
Table 4:   m
and T
m
for (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics at different
frequencies
Sam-
ple
No.
    m
T
m
(°C)
1 kHz
10
kHz
100
kHz
500
kHz
1
kHz
10
kHz
100
kHz
500
kHz
R0 0 6565 6494 6414 6383 –1 –1 –1 –1
R1 0.005 8123 8021 7901 7912 7 8 8 8
R2 0.01 8330 8215 8077 7964 6 7 7 7
R3 0.015 7842 7695 7528 7396 8 8 9 9
R4 0.02 9742 9469 9152 8870 12 12 12 13
4 CONCLUSIONS
Ti-rich (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics for low-freq-
uency capacitor applications were prepared by a conven-
tional solid-state method. The microstructures, dielectric
properties and ferroelectric-paraelectric phase transition
were investigated with a non-stoichiometric level by
SEM, XRD and an LCR measuring system. The follow-
ing was observed:
1) All the (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics (  = 0.005,
0.01, 0.015, 0.02) are single-phase solid solutions with a
typical tetragonal perovskite structure.
2) With the increasing   value, the abnormal grain
growth occurs and the average grain size of the
(Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics increases.
3) The enhanced spontaneous polarization takes the
main responsibility for the increase of the transition
C. ZHANG et al.: THE DIFFUSE PHASE TRANSITION OF Ti-RICH (Ba 0.75Sr 0.25)T i 1+  O 3+2  DIELECTRIC CERAMICS
186 Materiali in tehnologije / Materials and technology 54 (2020) 2, 181–187
Figure 5: Plots of ln(1/  r
–1 /   m
) as a function of ln(T–T
m
)f o r
(Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics (The symbols: experimental data;
the solid line: fitting plot)
temperature T
m
as well as the increase of the dielectric
constant   rRT
and   m
when   increases.
4) The low-temperature (T   T
m
) frequency disper-
sion of the relative dielectric constant can be found and
the diffuse phase-transition behavior is suppressed with
an increasing   value in the (Ba
0.75
Sr
0.25
)Ti
1+  O
3+2  ceramics.
Acknowledgements
This work is sponsored by Suzhou Pant Piezoelectric
Tech. Co. Ltd and the National Demonstration Center for
Experimental Materials Science and Engineering Edu-
cation (Jiangsu University of Science and Technology).
This work is also funded by the Priority Academic
Program Development of Jiangsu Higher Education
Institutions (PAPD).
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C. ZHANG et al.: THE DIFFUSE PHASE TRANSITION OF Ti-RICH (Ba 0.75Sr 0.25)T i 1+  O 3+2  DIELECTRIC CERAMICS
Materiali in tehnologije / Materials and technology 54 (2020) 2, 181–187 187