Phase transition behavior of mother phase of proton-conducting oxides, Sr1-xBaxZrO3

Article abstract of DOI:10.1016/j.tca.2011.11.037

The phase diagram of Sr_1-xBa_xZrO₃, which is the mother phase of highly proton-conducting oxides, has been established by differential scanning calorimetry (DSC), dilatometry and X-ray diffraction analysis at various temper

Full text of DOI:10.1016/j.tca.2011.11.037

Thermochimica Acta 530 (2012) 58–63
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Phase transition behavior of mother phase of proton-conducting oxides,
ଝ
Sr1 Ba ZrO3
−x
x
∗
Takayuki Sugimoto, Shinya Hasegawa, Takuya Hashimoto
College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajousi, Setagaya-ku, Tokyo 156-8550, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 August 2011
Received in revised form
−x x 3
The phase diagram of Sr1 Ba ZrO , which is the mother phase of highly proton-conducting oxides, has
been established by differential scanning calorimetry (DSC), dilatometry and X-ray diffraction analysis at
various temperatures. The phase boundaries and order of the transitions from the primitive orthorhom-
bic phase to the body-centered orthorhombic, tetragonal and cubic phases have been determined. The
temperatures of these three kinds of phase transition decreased with increasing Ba content. The jump in
the specific heat of the second-order phase transitions from primitive orthorhombic to body-centered
orthorhombic and from tetragonal to cubic also decreased with increasing Ba content. An increase in Ba
content decreased the variation in the enthalpy of the first-order phase transition from body-centered
orthorhombic to tetragonal. The temperature region where high symmetry phases, such as the cubic and
tetragonal phases, were stable expanded with increasing Ba content, which was in agreement with the
tolerance factor approaching unity.
3
0 November 2011
Accepted 30 November 2011
Available online 9 December 2011
Keywords:
Proton-conducting oxides
Differential scanning calorimetry
X-ray diffraction
Phase transitions
Thermal expansion
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
finally to cubic (Pm 3¯ m) with increasing temperature [4–8]. These
phase transitions might be a mechanical or electrical problem pre-
venting practical application.
Proton conductors have attracted much interest as electrolytes
of fuel cells, H2 sensors and other devices. Various proton conduc-
tors such as polymers, glasses and oxides have been discovered.
Oxides such as BaZrO -, SrZrO -, BaCeO - and SrCeO -based ones
Since the properties and structure of materials can be controlled
by partial cation substitution, a solid solution of Sr1 Ba ZrO3 can
x
−x
3
3
3
3
be considered the mother phase of highly proton-conducting mate-
rials with high chemical and mechanical stability. The preparation
with distorted perovskite structures have advantages over other
proton conductors in terms of mechanical property and stabil-
of Sr1 Ba ZrO has been reported by Kennedy et al. They measured
x
−x
3
ity at high temperatures. Among them, BaZrO -based oxides have
synchrotron X-ray diffraction and reported that the crystal struc-
ture of Sr1 Ba ZrO3 changes from primitive orthorhombic (Pbnm)
3
the highest proton conductivity [1] and higher chemical sta-
bility against CO2 than BaCeO - and SrCeO -based oxides [2].
−x
x
3
3
to body-centered orthorhombic (Imma), tetragonal (I4/mcm) and
finally to cubic (Pm 3¯ m) with increasing Ba content [7]. They
However, the average thermal expansion coefficient of BaZrO
3
−
6
−1
is 7.13 × 10
K
between room temperature and 1000 K [3],
also carried out synchrotron X-ray diffraction measurements at
high temperatures and proposed a phase diagram that shows a
decrease in temperature for the three kinds of phase transition with
increasing Ba content. However, their proposed phase diagram is
inconclusive since the number of measured specimens and temper-
ature were limited, and high-temperature X-ray diffraction analysis
is not suitable for the precise determination of phase transition
temperature owing to the large number of specimens required.
In addition, the thermodynamic characteristics of the phase tran-
sitions such as temperature, order, latent heat (ꢀH), jump in
specific heat (ꢀCp) and variation in volume (ꢀV), and their vari-
ation in composition, which should be considered for application,
have not yet been clarified. Thermal analyses, such as differential
scanning calorimetry (DSC) and dilatometry, enable us to perform
not only direct measurement but also precise measurement of
the thermodynamic characteristics owing to the small number of
which is lower than those of electrode materials. Although SrZrO -
3
based oxides have lower proton conductivity than BaZrO -based
3
ones, their average thermal expansion coefficient between room
−
6
−1
temperature and 1000 K is 9.67 × 10
K [3], which is close to
those of electrode materials. However, SrZrO3 undergoes three
kinds of structural phase transition between room temperature
◦
and 1200 C, which are not observed in cubic BaZrO . The crys-
3
tal structure of SrZrO3 varies from primitive orthorhombic (Pbnm)
to body-centered orthorhombic (Ibmm), tetragonal (I4/mcm) and
ଝ
This paper was reproduced from ECS Transactions, vol. 28 (11) pp. 251–258,
010 with permission from the Electrochemical Society.
Corresponding author.
E-mail address: takuya@chs.nihon-u.ac.jp (T. Hashimoto).
2
∗
0
040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2011.11.037

T. Sugimoto et al. / Thermochimica Acta 530 (2012) 58–63
59
specimens required. In this study, we have prepared various kinds
(
d) x = 0.3
66°C
of Sr1 BaxZrO3 and evaluated the phase transition behavior by
−x
721°C
a combination of thermal analysis and high-temperature X-ray
4
diffraction analysis. A phase diagram of Sr1 BaxZrO3 with high
−x
reliability has been established for the first time.
1
1
0 W/mol
0 W/mol
2
. Experimental
Ceramic Sr1 BaxZrO3 specimens were prepared by the Pechini
(
c) x = 0.2
5
96°C
−x
method [9]. Nominal amounts of powder BaCO3 (99.9%, Furu-
uchi Chem. Corp.), SrCO3 (99.9%, Furuuchi Chem. Corp.) and
ZrOCl ·8H O (99.9%, Wako Pure Chemical Industries Ltd.) were
419°C
847°C
2
2
dissolved in citric acid, HNO3 and distilled H O, respectively, and
2
(
b) x = 0.1
mixed in liquid phase. Prior to their dissolution, BaCO3 and SrCO3
were dried at 120 C for more than 12 h in air. The purity of
◦
9
73°C
ZrOCl ·8H O was confirmed from the weight of ZrO prepared by
2
2
2
7
23°C
◦
heating ZrOCl ·8H O at 1000 C in a Pt crucible. After the addition
2
2
5
97°C
10 W/mol
of citric acid and ethylene glycol, the liquid was heated at more than
◦
◦
3
00 C and burned to form a resin. The resin was calcined at 850 C
in air. The obtained powder was pressed into a pellet of 20 mmØ and
mm thickness or 5 mmØ and 15 mm height, followed by heating
(
a) x = 0.0
5
◦
◦
at 1300 C for 10 h and at 1500 C for 10 h in air.
1101°C
8
41°C
To evaluate the crystal structure of the prepared specimens and
its variation with temperature, X-ray diffraction measurements
were carried out using Cu K␣ radiation (50 kV, 250 mA, Rigaku
7
68°C
10 W/mol
◦
400
600
800
1000
1200
Co., Ltd.: RINT-2500) from room temperature to 1200 C and from
◦ ◦
Temperature (°C)
−
150 C to 100 C. To analyze the thermodynamic behaviors of
the phase transitions, the thermal expansion behaviors of the sin-
tered specimens in the temperature ranges of room temperature
Fig. 1. DSC curves of Sr1−xBaxZrO3: (a) SrZrO3, (b) Sr0.9Ba0.1ZrO3, (c) Sr0.8Ba0.2ZrO3
and (d) Sr0.7Ba0.3ZrO3.
◦
◦
◦
to 1300 C and −130 C to 500 C in air were measured by dilatom-
etry using a TMA8310 (Rigaku Co., Ltd.). Al O and SiO2 were used
2
3
◦
approximately 36.0 decreased with increasing x and completely
as the reference, materials for the sample stage and push rod for
disappeared for the specimen with x = 0.9. This can be ascribed to
the boundary of the body-centered tetragonal (I4/mcm) and cubic
¯
Pm3m) phases between x = 0.8 and x = 0.9. All the diffraction peaks
shown in Fig. S1 could be indexed assuming the validity of the
thus-proposed crystal system, showing agreement with the phase
diagram proposed by Kennedy et al. [7].
◦
the measurement in the ranges of room temperature −1300 C and
◦ ◦
−
130 C to 500 C, respectively. The DSC of powder specimens was
(
also preformed to analyze the phase transitions using a DSC8270
◦
(
Rigaku Co., Ltd.) between room temperature and 1200 C in air.
Pt was used as reference and sample pan materials. The weights
of Al O powder used for the reference and sample were about
2
3
2
2
0 mg and 21 mg, respectively. The heating and cooling rates were
0 C/min.
3
.2. Characterization of thermodynamic behavior of structural
◦
phase transition in Sr1 BaxZrO3 by DSC
−x
3
. Results and discussion
To evaluate the thermodynamic characteristic of the phase tran-
sitions in Sr1 BaxZrO , DSC was performed. Fig. 1(a) shows the
−x
3
◦
◦
3
.1. Crystal structure of Sr1 BaxZrO3 at room temperature
DSC curve of SrZrO [8]. Anomalies were observed at 768 C, 841 C
−x
3
◦
and 1101 C. The combination of the DSC curve with the cool-
ing procedure confirmed that the anomalies at 768 C and 1101 C
were reversible baseline shifts corresponding to the second-order
◦
◦
The X-ray diffraction patterns of Sr1 BaxZrO3 at room tem-
−x
perature are depicted in Fig. S1, showing agreement with
previous report [8]. The diffraction pattern of the spec-
◦
a
phase transition, whereas that at 841 C was an endothermic
imen with x = 0.0 was indexed as Pbnm orthorhombic with
a = 5.7986 A˚ , b = 5.8216 A˚ and c = 8.2095 A˚ . To clarify the varia-
peak with hysteresis indicating the first-order phase transition
[7]. This indicates that the transition from Pbnm to Ibmm and
¯
tion of the crystal system with increasing Ba content, some
peaks were magnified. Fig. S2(a) and (b) shows X-ray diffrac-
that of I4/mcm to Pm3m were of the second-order, whereas that
from Ibmm to I4/mcm was of the first-order. The values of ꢀCp
◦
◦
◦
tion peaks at room temperature in the 2ꢁ ranges of 31–37
at 768 C and 1101 C induced by the second-order phase transi-
tion and the ꢀH induced by the first-order phase transition were
13.1 J/mol K, 6.0 J/mol K and 58.1 J/mol, respectively. Fig. 1(b) shows
◦
and 70–74 for the specimens with x = 0.0–1.0, respectively. The
intensities of the 0 2 1 peak at approximately 32.5 and the
2 0 and 2 1 0 peaks at approximately 34.5 decreased with
◦
◦
1
the DSC curve of Sr0.9Ba0.1ZrO . Baseline shifts corresponding to
3
increasing x. The peaks disappeared in the diffraction pattern of the
specimen with x = 0.5. Since the peaks of h+k+l = odd should disap-
pear in body-centered orthorhombic structures, it was confirmed
that there was a phase boundary from primitive orthorhombic
the second-order phase transition from Pbnm to Ibmm and to that
¯
◦
from I4/mcm to Pm3m were also respectively observed at 597 C
◦
and 973 C. An endothermic peak indicating the first-order phase
◦
transition from Ibmm to I4/mcm was observed at 723 C. The values
◦
◦
(
Pbnm) to body-centered orthorhombic (Ibmm) between x = 0.4 and
of ꢀCp at 597 C and 973 C induced by the second-order phase
transition and the ꢀH induced by the first-order phase transi-
tion were 7.2 J/mol K, 5.7 J/mol K and 43.1 J/mol, respectively. The
ꢀH and ꢀCp of Sr0.9Ba0.1ZrO3 were smaller than those of SrZrO3.
x = 0.5. It was observed that the shape of the peak shown in Fig. S2(b)
varied between x = 0.5 and x = 0.6, indicating the phase transi-
tion from body-centered orthorhombic (Ibmm) to body-centered
tetragonal (I4/mcm). The intensity of the 2 1 1 peak for x = 0.6 at
Fig. 1(c) shows the DSC curve of Sr0.8Ba0.2ZrO . Baseline shifts
3

6
0
T. Sugimoto et al. / Thermochimica Acta 530 (2012) 58–63
x = 1.0
x = 0.9
x = 0.8
2
1
x = 0.2
x = 0.1
x = 0.0
2
1
0.5
1
0
1
0
0
2
0
2
0.5
1
1
0
1
0
0
2
0
2
0.5
1
1
0
0
0
0
2
0
500
1000
0
500
1000
0.5 x = 0.7
Temperature (°C)
1
0
0
2
Fig. 2. Temperature dependences of linear thermal expansion and thermal expan-
sion coefficient of Sr1−xBaxZrO3 (x = 0.0–0.2) (dashed arrows: first-order phase
transition; solid arrows: second-order phase transition).
x = 0.6
x = 0.5
0
0
0
.5
1
0
0
2
.5
◦
◦
were observed at 419 C and 847 C. An endothermic peak indi-
1
0
cating the first-order phase transition from Ibmm to I4/mcm was
observed at 596 C. The values of ꢀCp at 419 C and 847 C and ꢀH
were 3.1 J/mol K, 5.1 J/mol K and 34.3 J/mol, respectively. Fig. 1(d)
0
◦
◦
◦
200
400
0
200
400
Temperature (°C)
shows the DSC curve of Sr0.7Ba0.3ZrO . A baseline shift correspond-
ing to the second-order phase transition was observed at 721 C;
3
Fig. 4. Temperature dependences of linear thermal expansion and thermal expan-
sion coefficient of Sr1−xBaxZrO3 (x = 0.5–1.0). Solid arrows represent the anomaly
due to the second-order phase transition. At the temperature indicated by dashed
arrows, an anomaly due to the first-order phase transition was expected from the
results of X-ray diffraction analysis at various temperatures.
◦
however, no other second-order phase transition was detected. An
endothermic peak indicating the first-order phase transition was
◦
◦
observed at 466 C. The ꢀCp induced by the second-order phase
transition at 721 C and the ꢀH induced by the first-order phase
transition were 4.4 J/mol K and 20.8 J/mol, respectively. The DSC
curves of the specimens with x = 0.0–0.3 indicated that not only
the temperature but also ꢀCp and ꢀH decreased with increasing
Ba content. Therefore, it is surmised that the detection of the phase
transitions in the specimens with x larger than 0.4 by DSC is difficult.
In fact, no thermal anomaly due to the phase transitions could be
observed in the DSC curves of the specimens with x larger than 0.4.
Thus, thermal expansion measurements were studied as another
method of detecting the phase transitions.
◦
temperature and 900 C. In the linear thermal expansion of the
specimens with x = 0.3 and x = 0.4, a change in slope was observed
as represented by solid arrows. This anomaly was more clearly
detected in the temperature dependence of the thermal expan-
sion coefficient as baseline shift represented by solid arrows,
indicating the second-order phase transition. For the thermal
expansion coefficients, the first-order phase transition, represented
by dashed arrows, and another second-order phase transition were
also observed at lower temperatures in the specimen with x = 0.3.
The phase transition temperatures obtained by dilatometry in
Ba0.7Sr0.3ZrO3 agreed well with those observed by DSC as shown
in Fig. 1(d). It was concluded that the first-order and second-order
phase transitions existed in the specimens with x = 0.4 and that the
phase transition temperature decreased with increasing Ba con-
tent.
3
.3. Dilatometry of Sr1 BaxZrO –detection of phase transition
−x 3
with smallꢀCp orꢀH
Fig. 2 shows the temperature dependences of the linear thermal
expansion and thermal expansion coefficient of the specimens with
x = 0.0–0.2 measured by dilatometry between room temperature
Fig. 4 shows the temperature dependences of the linear thermal
expansion and thermal expansion coefficient of the specimens with
◦
and 1300 C. The thermal anomaly corresponding to the two kinds
◦
◦
of second-order phase transition and the first-order phase transi-
tion were observed in the temperature dependence of the thermal
expansion coefficient as represented by solid and dashed arrows,
respectively. The phase transition temperatures agreed with those
observed by DSC suggesting that dilatometry can be used as an
alternative method of detecting the phase transitions.
Fig. 3 shows the temperature dependences of the linear ther-
mal expansion and thermal expansion coefficient of the specimens
with x = 0.3 and x = 0.4 measured by dilatometry between room
x = 0.5–1.0 measured by dilatometry between −130 C and 500 C.
For the specimens with x = 0.5–0.9, a thermal anomaly correspond-
ing to the second-order phase transition from I4/mcm to Pm 3¯ m was
observed, as represented by solid arrows. For the specimen with
x = 0.9, the phase transition was observed below room tempera-
ture. In the specimens with x = 0.5 and 0.6, no other phase transition
was observed; however, at the temperatures depicted by dashed
arrows, the existence of the first-order phase transition can be sur-
mised from the results of the X-ray diffraction analysis at various
temperatures, as explained in the following section.
3.4. X-ray diffraction measurements at various temperatures
2
1
x = 0.4
To distinguish whether the phase transitions other than that
1
2
0
1
from I4/mcm to Pm 3¯ m exist in the specimens with x = 0.5 and 0.6,
x = 0.3
X-ray diffraction measurements were performed at various tem-
peratures.
1
0
Fig. 5 shows the X-ray diffraction patterns of Sr0.5Ba0.5ZrO3
0
200 400 600 800
0
200 400 600 800
◦
◦
between 30 C and 1200 C. The crystal structure of Sr0.5Ba0.5ZrO3
Temperature (°C)
◦
was Ibmm at 30 C, as has already been described earlier. Fig. 6(a)
and (b) shows X-ray diffraction patterns of Sr0.5Ba0.5ZrO3 from
Fig. 3. Temperature dependences of linear thermal expansion and thermal expan-
sion coefficient of Sr1−xBaxZrO3 (x = 0.3, 0.4) (dashed arrows: first-order phase
transition; solid arrows: second-order phase transition).
◦
◦
room temperature to 1200 C in the 2ꢁ ranges of 31–37 and
◦
70–72.5 , respectively. No 0 2 1, 1 2 0 or 2 1 0 peak of Pbnm

T. Sugimoto et al. / Thermochimica Acta 530 (2012) 58–63
61
1
200°C
1
1
1
9
8
7
6
200°C
100°C
000°C
00°C
00°C
00°C
1
1
9
8
7
6
100°C
000°C
00°C
00°C
00°C
00°C
00°C
500°C
5
4
3
00°C
00°C
00°C
400°C
300°C
2
1
00°C
00°C
2
1
00°C
00°C
30°C
20
40
60
80
2θ (deg [CuKα])
3
0°C
◦
Fig. 7. X-ray diffraction patterns of Sr0.4Ba0.6ZrO3 at temperatures between 30 C
and 1200 C. The diffraction patterns at 30–300 C and 400–1200 C can be indexed
to I4/mcm and Pm 3¯ m, respectively.
◦
◦
◦
2
0
40
60
80
2θ (deg [CuKα])
Fig. 5. X-ray diffraction patterns of Sr0.5Ba0.5ZrO3 at various temperatures. The
diffraction patterns at 30–100 C, 200–400 C and 500–1200 C can be indexed to
Ibmm, I4/mcm and Pm 3¯ m, respectively. The solid arrow represents the peak of the
◦
◦
◦
by dilatometry as shown in Fig. 4. The intensity of the 2 1 1 peak in
Fig. 6(a) decreased with increasing temperature and disappeared at
Al2O3 sample holder.
◦
¯
00 C, indicating a phase transition from I4/mcm to Pm3m between
5
4
◦
◦
00 C and 500 C, which was also observed by dilatometry, as
◦
shown in Fig. 4. The existence of the cubic system above 500 C
is also supported by the single peak indexed as 3 1 0 in Fig. 6(b).
The whole diffraction patterns shown in Fig. 5 could be indexed to
the above-mentioned crystal structures.
(a)
(b)
1
200°C
1
1
9
8
100°C
000°C
00°C
Fig. 7 shows the X-ray diffraction patterns of Sr0.4Ba0.6ZrO3
◦
◦
between 30 C and 1200 C. The crystal structure of Sr0.4Ba0.6ZrO3
◦
was I4/mcm tetragonal at 30 C, as previously reported. Fig. 8(a) and
(
b) shows the X-ray diffraction patterns of Sr0.4Ba ZrO from room
0.6
3
◦
◦
◦
temperature to 1200 C in the 2ꢁ ranges of 31–37 and 70.1–72.5 ,
respectively. The intensity of the 2 1 1 peak in Fig. 8(a) decreased
00°C
◦
with increasing temperature and disappeared at 400 C, indicat-
¯
◦
ing a phase transition from I4/mcm to Pm3m between 300 C and
00 C. At these temperatures, the three separate peaks shown in
7
6
00°C
00°C
◦
4
Fig. 8(b) formed a single peak, also supporting the existence of
◦
the cubic system above 400 C. The phase transition temperature
5
4
00°C
00°C
showed agreement with that evaluated by dilatometry shown in
Fig. 4. Also, all of the diffraction peaks in Fig. 7 could be indexed
using the crystal system thus deduced.
From the results mentioned above, it is surmised that the phase
3
00°C
transition temperature of Sr1 BaxZrO3 decreased with increasing
−x
Ba content. Therefore, diffraction measurements below room tem-
perature were carried out for the specimens with x larger than
0.6. Fig. 9 shows the X-ray diffraction patterns of Sr0.4Ba0.6ZrO3
2
1
3
00°C
00°C
0°C
◦
◦
between −150 C and 100 C. Except for unknown small peaks
◦
at approximately 25 , almost all the peaks were indexed as
3
2
34
36
θ (deg [CuKα])
71
72
◦
Sr0.4Ba0.6ZrO . The crystal system at 30 C can be indexed as
3
2
I4/mcm. To analyze the crystal system at the other temperatures,
some peaks are enlarged in Fig. 10(a) and (b). The diffraction peaks
Fig. 6. X-ray diffraction peaks of Sr0.5Ba0.5ZrO3 at various temperatures. 2ꢁ ranges:
(
◦
◦ ◦
a) 31–37 and (b) 70–72.5 .
below −50 C could be indexed to have an orthorhombic symmetry.
The absence of peaks with h+k+l = odd, such as the 0 2 1, 1 2 0 and
2
1 0 peaks, indicates a body-centered orthorhombic phase. A dif-
◦
◦
was observed at room temperature, indicating the body-centered
orthorhombic phase. A difference in peak shape was observed
ference in peak shape was observed between −50 C and 30 C, as
Fig. 10(b), indicating a phase transition from Ibmm to I4/mcm. This
phase transition was not clearly detected by dilatometry, proba-
bly owing to the small volume variation during the transition. The
◦
◦
between 100 C and 200 C in Fig. 6(b), showing a phase transi-
tion from Ibmm to I4/mcm, which was not satisfactorily detected

6
2
T. Sugimoto et al. / Thermochimica Acta 530 (2012) 58–63
(
a)
(b)
1
200°C
1
1
9
8
100°C
000°C
00°C
00°C
7
6
00°C
00°C
5
4
00°C
00°C
3
00°C
2
1
3
00°C
00°C
0°C
3
2
34
36
71
72
2
θ
(deg [CuKα])
◦
◦
.
Fig. 8. X-ray diffraction peaks of Sr0.4Ba0.6ZrO3 at various temperatures. 2ꢁ ranges: (a) 31–37 and (b) 70–72.5
diffraction patterns and their correspondence to the result of ther-
mal analysis are discussed in the following section.
1
00°C
3
.5. Establishment of phase diagram of Sr1 BaxZrO3
−x
3
0
−
−
0°C
°C
Fig. 11(a) shows the phase diagram of Sr1 BaxZrO3 proposed
−x
on the basis of the results of DSC, dilatometry and X-ray diffraction
analysis at various temperatures. Dotted lines represent the phase
boundary evaluated by DSC and dilatometry. ꢀ, ꢁ, ♦, ꢀ, ꢁ and ꢂ
represent the phases of Pbnm orthorhombic, Ibmm orthorhombic,
50°C
100°
¯
I4/mcm tetragonal, and Pm3m cubic phase, the mixture of I4/mcm
¯
and Pm3m and the mixture of Ibmm and I4/mcm respectively, eval-
uated by X-ray diffraction analysis at various temperatures. They
were in agreement with the phase boundary evaluated by thermal
analysis and a continuous phase boundary was drawn as a solid
line from the results of X-ray diffraction analysis only. The almost
straight lines suggest that a regular solution model i.e., a homoge-
neous solution model, is applicable to the solid solution of Sr and
Ba. The temperature region in which structures with high symme-
try, such as cubic and tetragonal structure, is stable expanded with
increasing Ba content. In the upper axis of Fig. 11, the tolerance
factor calculated using Shannon’s ionic radius [10] is shown. It can
be concluded that phases with high structural symmetry become
stable as the tolerance factor approaches unity. Fig. 11(b) shows
the phase boundaries reported by Kennedy et al. [7] with dashed
lines. The crystal systems observed in this study overlapped. The
points represented by closed symbols show a discrepancy from
their proposed phase boundary. The large number of discrepant
points indicate that the phase boundary proposed in this study is
more reliable than that reported by Kennedy et al.
−
150°
2
0
40
60
80
2
θ
(deg [CuKα])
◦
◦
Fig. 9. X-ray diffraction patterns of Sr0.4Ba0.6ZrO3 between −150 C and 100 C. The
◦
◦
◦
◦
◦
diffraction patterns at −150 C to −50 C, 0 C and 30 C to 100 C can be indexed to
Ibmm, a mixture of Ibmm and I4/mcm, and I4/mcm, respectively. The peaks repre-
sented by solid arrows could not be identified.
◦
X-ray diffraction pattern at 0 C can be indexed as a mixture of Ibmm
or I4/mcm, suggesting that Sr0.4Ba0.6ZrO3 is mixed phase.
To confirm the phase transition temperature evaluated by
thermal analysis, X-ray diffraction measurements of other spec-
imens were performed at various temperatures. Representative
diffraction patterns are summarized in Figs. S3–S8. The crys-
tal system and structural phase transition deduced from the

T. Sugimoto et al. / Thermochimica Acta 530 (2012) 58–63
63
(
a)
(b)
1
3
0
00°C
0°C
°C
−
−
−
50°C
100°C
150°C
3
2
34
36
38
71
72
73
2
θ (deg [CuKα])
Fig. 10. X-ray diffraction peaks of Sr0.4Ba0.6ZrO3 between −150 ◦C and 100 C. 2ꢁ ranges: (a) 31–38 and (b) 70.5–73.5
◦
◦
◦
.
Tolerance factor
Tolerance factor
(
a)
(b)
0.96
0.98
1.00
0.96
0.98
1.00
1
1
200
000
1200
1000
800
600
400
200
0
8
6
4
2
00
00
00
00
0
Pm3m
I4/mcm
Ibmm
Pbnm
0
.0
0.2
0.4
0.6
0.8
1.0
(BaZrO )
0.0
(SrZrO )
0.2
0.4
0.6
0.8
1.0
(BaZrO )
(
SrZrO )
3
3
3
3
Ba content
Ba content
Fig. 11. (a) Phase diagram of Sr1−xBaxZrO3 proposed in this study. Each symbol represents the crystal structure analyzed by high-temperature X-ray diffraction analysis. ꢀ,
Pbnm; ꢁ, Ibmm; ♦, I4/mcm; ꢀ, Pm 3¯ m, ꢁ, mixture of I4/mcm and Pm 3¯ m; ꢂ, mixture of Ibnm and I4/mcm. Dotted and solid lines represent the phase boundary measured by
thermal analysis and that proposed from the result of X-ray diffraction measurements, respectively. (b) Comparison between phase diagram obtained in this study (symbols)
and that reported by Kennedy et al. (dashed lines) [7]. The points where a discrepancy is observed are represented by closed symbols. The upper axis represents the tolerance
factor calculated using Shannon’s ionic radius [10].
4
. Conclusion
A reliable phase diagram of Sr1 BaxZrO3 has been established
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tca.2011.11.037.
−x
by thermal analysis and X-ray diffraction analysis at various tem-
peratures. The phase boundaries and order of the transition from
primitive orthorhombic, to body-centered orthorhombic, tetrago-
nal and cubic phase have been determined. It has been confirmed by
thermal analysis that the temperatures and ꢀCp and ꢀH values of
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(
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