Space group and crystal structure of the Perovskite CaTiO3 from 296 to 1720 K

Article abstract of DOI:10.1016/j.jssc.2005.06.027

The structural phase transitions of the calcium titanate perovskite CaTiO₃ were investigated by the Rietveld analysis of high-temperature neutron and X-ray powder diffraction data in the temperature range of 296-1720 K. The present work demonstrates that the compound exhibits two reversible phase transitions of orthorhombic Pbnm-tetragonal I4/mcm and of I4/mcm-cubic Pm3m at 1498±25 K and 1634±13 K, respectively. No evidence of Cmcm phase is observed between the Pbnm and I4/mcm structures. The lattice parameters discontinuously change at the Pbnm-I4/mcm transition point, while a continuous change is observed for the I4/mcm-Pm3m transformation. These results indicate that the Pbnm-I4/mcm transition is of first order and that the I4/mcm-Pm3m transformation is of either second or higher order.

Full text of DOI:10.1016/j.jssc.2005.06.027

ARTICLE IN PRESS
Journal of Solid State Chemistry 178 (2005) 2867–2872
www.elsevier.com/locate/jssc
Space group and crystal structure of the Perovskite CaTiO3
from 296 to 1720 K
1
Roushown Ali , Masatomo Yashima
Ã
Department of Materials Science and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology,
259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8502, Japan
4
Received 3 November 2004; received in revised form 2 June 2005; accepted 14 June 2005
Abstract
The structural phase transitions of the calcium titanate perovskite CaTiO
temperature neutron and X-ray powder diffraction data in the temperature range of 296–1720 K. The present work demonstrates
3
were investigated by the Rietveld analysis of high-
¯
that the compound exhibits two reversible phase transitions of orthorhombic Pbnm-tetragonal I4/mcm and of I4/mcm-cubic Pm3m
at 1498725 K and 1634713 K, respectively. No evidence of Cmcm phase is observed between the Pbnm and I4/mcm structures. The
lattice parameters discontinuously change at the Pbnm–I4/mcm transition point, while a continuous change is observed for the
I4=mcm2Pm3m transformation. These results indicate that the Pbnm–I4/mcm transition is of first order and that the
I4=mcm2Pm3m transformation is of either second or higher order.
r 2005 Elsevier Inc. All rights reserved.
Keywords: Crystal structure; Rietveld method; Phase transition; In situ measurements
1
. Introduction
between them. On the contrary, some research groups
8–10] reported one intermediate phase between the
[
Pbnm and Pm3m forms. Guyot et al. [11] and Kennedy
et al. [13] suggested two intermediate phases including a
¯
Perovskite-structured compounds are promising ma-
terials in current science and technology [1], and are of
fundamental importance in solid-state science and
geoscience [2]. The mineral perovskite CaTiO is the
parent compound of the perovskite-structured family. It
is widely used in electronic ceramic materials and in
immobilizing high-level radioactive waste [3–5]. The
¯
Cmcm form between the Pbnm and Pm3m forms.
However, the existence of the Cmcm phase has not
been established yet. Therefore, the number of existing
3
phases in the CaTiO is an unresolved issue and it differs
3
from 2 to 4, depending on the literature (Table 1).
The present work has been undertaken to examine the
structural phase transitions of CaTiO . In particular we
structural phase transition of CaTiO has been studied
3
extensively [6–18] and reported to undergo structural
3
¯
changes from orthorhombic Pbnm to cubic Pm3m
symmetry as summarized in Table 1. Vogt and Schmahl
[12] reported a direct phase transition from the Pbnm to
¯
Pm3m phase and observed no intermediate phase
focus on the possibility of the Cmcm phase between the
Pbnm and tetragonal I4/mcm structures by using not
only neutron but also X-ray powder diffraction data.
With the help of higher angular resolution of the X-ray
method, the present work reveals no possibility of the
Cmcm phase. The crystal structures of all existing phases
are refined by the Rietveld analyses of neutron powder
diffraction data with the help of the relatively large
scattering length of oxygen in neutron diffraction.
Ã
Corresponding author. Fax: +81 45 924 5630.
E-mail address: yashima@materia.titech.ac.jp (M. Yashima).
1
Present address: Department of Chemistry, University of Rajshahi,
Rajshahi-6205, Bangladesh.
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.06.027

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R. Ali, M. Yashima / Journal of Solid State Chemistry 178 (2005) 2867–2872
2868
Table 1
Existing phases of CaTiO
3
in the literature and in the present work
*XRPD and NPD denote X-ray powder diffraction and neutron powder diffraction, respectively.**No crystallographic evidence but speculation
from analogy with CaGeO
3
.
˚
1.8196 A were obtained by a vertically focusing (331)
2
. Experimental
Ge monochromator. Diffraction data were collected at
step intervals of 0.11 over a 2y range from 20 to 1521,
using 150 He counters set at 11 intervals. We have
2
.1. Sample preparation
3
CaTiO sample used in this experiment was prepared
3
utilized a furnace [20] with MoSi heaters to heat the
CaTiO rod in air under atmospheric pressure at 296,
2
by solid-state reactions. High-purity powders of CaCO3
Purity 99.99%, High-Purity-Chemical Co., Saitama,
Japan) and TiO (Purity 98.5%, Kanto-Chemical Co.,
3
(
453, 661, 868, 1069, 1173, 1270, 1322, 1373, 1423, 1473,
1523, 1548, 1572, 1598, 1622, 1674, 1671 and 1720 K.
There are two thermocouples in the furnace. One
thermocouple near the heater was used to control the
temperature and the other contacting with the rod-
sample, which monitored the sample temperature. The
2
Tokyo, Japan) were mixed and ground using an agate
mortar as dried powders and as ethanol slurries for 3 h.
The mixture was pressed into pellets under the pressure
of 200 MPa and then calcined in air at 1473 K for 12 h.
The pellets were crushed by an alumina mortar, and
then ground by the agate mortar as dried powders and
as ethanol slurries for 2 h. The powders were pressed
into a rod at 200 MPa and then sintered at 1673 K for
CaTiO rod was heated at the rate of about 10 K/min,
3
and then the sample temperature was kept constant
during each measurement, within 71 K above 1273 K
and within 72 K below 1273 K.
1
1
2 h. The sintered product with a cylindrical shape of
2 mm in diameter and of 70 mm in height was used to
2.3. High-temperature X-ray powder diffraction
measure the neutron powder diffraction profiles. A part
of the pellet was crushed and ground into powders for
X-ray powder diffraction measurements.
X-ray powder diffraction data were collected at the
sample temperatures of 1431, 1486 and 1598 K by an
X-ray powder diffractometer with a Rotaflex X-ray
generator (RINT2550V/PC, Rigaku Co. Ltd., Tokyo,
Japan) under the following experimental set up:
Bragg–Brentano geometry, CuKa radiation, voltage
40 kV, current 400 mA, divergence slit 0.51, scattering
slit 0.63 mm, receiving slit 0.15 mm. The measurement
conditions were: Step-scanning mode with step interval
2
.2. High-temperature neutron powder diffraction
We performed neutron powder diffraction experi-
ments on the Kinken powder diffractometer for high-
efficiency and high-resolution measurements, HERMES
[
19]. Incident neutrons with a fixed wavelength of

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R. Ali, M. Yashima / Journal of Solid State Chemistry 178 (2005) 2867–2872
2869
¯
of 0.021 in 2y, measuring time of 2 s, scanning range of
0–1451. The CaTiO3 powder was heated using a
furnace with Pt heaters attached to the goniometer of
the X-ray diffractometer and the sample temperature
was kept constant within 71.0 K during each measure-
ment.
diffraction profile of the Pm3m phase exhibits only the
fundamental reflections as 100, 110, 111, 200, 210 and
211. We confirmed reversible phase transitions on
heating and cooling:
2
1
498Æ25 K
1634Æ13 K
Orthorhombic
2
Tetragonal
I4=mcm
2
Cubic
Pbnm
Pm 3¯ m
2
data
.4. Rietveld analyses of neutron and X-ray diffraction
Within the temperature intervals of 25 and 13 K, no
hysteresis was observed. This sequence of transitions is
consistent with two heat anomalies observed by Guyot
et al. [11]. The two heat anomalies are attributable to the
Pbnm–I4/mcm and I4=mcm2Pm3m phase transitions,
respectively, although Guyot et al. attributed them to
Pbnm–Cmcm and Cmcm– I4/mcm transitions, respec-
tively.
Guyot et al. [11] and Kennedy et al. [13] suggested
another intermediate phase with Cmcm symmetry
between the Pbnm and I4/mcm phases (Table 1). Fig. 1
shows the X-ray powder diffraction profiles at 1431 and
1486 K just below the Pbnm–I4/mcm transition point
(1498 K). Splitting into the 044Pbnm and 404Pbnm
reflection peaks is clearly observed where h k lPbnm
stands for the h k l reflection of Pbnm phase. There is
no possibility of the Cmcm phase, because the Cmcm
yields only single 444 reflection. Furthermore, we
obtained smaller R-factor and goodness-of-fit values
(Rwp ¼ 12:3%, S ¼ 1:48) in the Rietveld refinement of
the X-ray powder diffraction data assuming the space
group Pbnm, than those by Cmcm (Rwp ¼ 13:9%,
S ¼ 1:68). We also confirmed that the existing phase
between 1431 and 1498 K is neither Imma, P4/mbm,
The structural refinements of the neutron and X-ray
powder diffraction data were performed by a Rietveld
analysis program RIETAN-2000 [21]. The peak shape
was assumed to be a pseudo-Voigt function with
asymmetry. The background of each profile was
n
approximated by a 12 polynomial in 2y ; where n had
the value between 0 and 11. The background parameters
were simultaneously refined with the lattice, peak-shape,
zero-point, scale and crystal structural parameters. The
following coherent scattering lengths were used for the
Rietveld analysis of neutron-diffraction data: bCa
4
¼
:70, bTi ¼ À3:37, and b ¼ 5:803 fm.
o
3
. Results and discussion
3
.1. Phase transitions
Neutron powder diffraction profiles of CaTiO were
3
measured in the temperature range of 296–1720 K. No
impurity phases were detected. Each peak shifted to
lower 2y position with an increase of temperature due to
the thermal expansion. The CaTiO was identified to be
the orthorhombic Pbnm phase with the GdFeO -type
¯
¯
I4/mcm, R3c nor Pm3m, but the Pbnm phase. Thus, the
Pbnm phase directly transforms into the intermediate
I4/mcm structure.
3
3
structure from 296 to 1473 K. The peak intensities of
reflections such as 120, 210, 113, 122, 212, 023 and 221
of the Pbnm phase decreased with an increase of
temperature. These Pbnm peaks disappeared between
1473 and 1523 K, indicating a phase transition from the
Pbnm to an intermediate phase. The neutron-diffraction
3.2. Structural refinement of neutron diffraction data
The crystal structure of the neutron-diffraction data
measured at 296 K was successfully refined by the
GdFeO -type perovskite structure with Pbnm giving
3
patterns show that the CaTiO sample has an inter-
3
mediate tetragonal I4/mcm phase from 1523 to 1622 K.
¯
Probable space groups of R3c, I4/mcm, Cmcm, P4/mbm
and Imma have been proposed for intermediate phases
¯
between the perovskite-type Pbnm and Pm3m structures
[22,23]. We confirmed that the space group of the
¯
intermediate phase is neither R3c, Pbnm, Cmcm,
¯
P4/mbm, Pm3m nor Imma using the neutron data of
the following crystallographic parameters: Ca 4c
x,y,0.25; Ti 4a 0,0.5,0; O1 4c x,y,0.25; and O2 8d x,y,z
(
and a ¼ 5:3789ð2Þ, b ¼ 5:4361ð2Þ, c ¼ 7:6388ð3Þ A
(Fig. 2a and Table 2). There exists a relationship
between the lattice parameters of orthorhombic phase
a , b , c and its pseudo-cubic cell parameter a : a ffi
o
o
o
p
o
pffiffiffiffiffiffiffi
pffiffiffiffiffiffiffi
2a ; b ffi 2a and c ffi 2a . At the beginning of
p
o
p
o
p
the refinement, the lattice parameters and atomic
positions were refined with isotropic atomic displace-
ment parameters where the weighted reliability factor
and goodness of fit were Rwp ¼ 7:22% and S ¼ 1:54,
respectively. Final structural refinement was performed
by using anisotropic atomic displacement parameters
for oxygen atoms, because significant improvements
were obtained in the R-factor and goodness of fit
values: Rwp ¼ 6:76% and S ¼ 1:45. Present lattice and
1
2
523–1622 K and X-ray diffraction data at 1594 K. The
11, 213, 321, 215, 323, 411, 413, 325, 431, 217, 415, 433
and 521 peak intensities of the I4/mcm phase decreased
with an increase of temperature. These I4/mcm peaks
disappeared between 1622 and 1647 K, indicating
another phase transition from the I4/mcm to high-
¯
temperature cubic Pm3m phase. The Pm3m phase has
¯
an ideal cubic perovskite-type structure. In fact the

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R. Ali, M. Yashima / Journal of Solid State Chemistry 178 (2005) 2867–2872
2870
Fig. 2. Rietveld patterns of neutron diffraction data of CaTiO3. (a)
Orthorhombic Pbnm (296 K), (b) tetragonal I4/mcm (1598 K), (c) cubic
¯
Pm3m (1720 K). The solid lines are calculated intensities and the
crosses are observed intensities. The short vertical lines show the
position of possible Bragg reflections. The differences between
observed and calculated intensities are plotted below the profiles.
Fig. 1. A selected X-ray diffraction profiles of CaTiO
1431 and 1486 K. The cross marks are observed intensities and the
3
measured at
solid lines are calculated profiles obtained from Rietveld analyses
through Pbnm and Cmcm, and showing the comparison of the curve fit
for the two space groups. Two peaks are apparent in the observed
profile up to 1486 K. The Pbnm space group produces 044 and 404
reflections and fits well with the observed profile, whereas Cmcm space
group produces only 444 reflection and worse fit is observed.
pffiffiffiffiffiffiffi pffiffiffiffiffiffiffi
2a ꢀ 2a ꢀ 2a : In the refinement of this structure,
p
p
p
anisotropic atomic displacement parameters were con-
sidered for oxygen atoms, which makes better fit
compared with that obtained with isotropic displace-
ment parameters. The present result for the I4/mcm
CaTiO is consistent with that in the literature [13].
3
structural parameters at 296 K exhibited good agree-
ment with those of single-crystal study reported by
Sasaki et al. [24]. The lattice parameters of the present
study obtained at 296 K are in agreement with those of
Kennedy et al. [13] and of Liu and Liebermann [7], but
differ a little from those of Redfern [8]. This difference
may be ascribed to a small difference in the chemical
composition.
The neutron diffraction profiles above 1634 K were
successfully analyzed by the ideal cubic structure with the
¯
space group Pm3m (Fig. 2c). The crystallographic data at
¯
1720 K of Pm3m phase are Z ¼ 1, a ¼ b ¼
(
( 3
3
c ¼ 3:8967ð1Þ A, V ¼ 59:167ð3Þ A , and D ¼ 3:82 g=cm .
x
Refinement with anisotropic atomic displacement para-
meters for oxygen gave a better fit in the Rietveld analysis
comparing with that by isotropic parameters. On the
other hand, anisotropic and isotropic thermal parameters
gave similar fit for Ca and Ti atoms in the Rietveld
analysis. Therefore, final refinement was performed with
isotropic thermal parameters for Ca and Ti atoms and
anisotropic displacement for oxygen atom (Table 2).
The results are consistent with those in the literature
[12,13,25].
The crystal structure of the intermediate I4/mcm
phase was successfully refined by the Rietveld analyses
of the high-temperature neutron diffraction data
(
Fig. 2b). The crystallographic data are tetragonal,
(
I4/mcm, Z ¼ 4, a ¼ b ¼ 5:4984ð3Þ, c ¼ 7:7828ð8Þ A, V ¼
(
3
3
2
parameters are characterized by the pseudo-cubic cell
35:29ð3Þ A , Dx ¼ 3:84 g=cm , at 1598 K. The lattice

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R. Ali, M. Yashima / Journal of Solid State Chemistry 178 (2005) 2867–2872
2871
Table 2
Lattice parameters, R-factors, positional and thermal parameters
obtained from Rietveld analyses of neutron-diffraction data for three
different phases of CaTiO
3
Temperature
Space group
296 K
Pbnm
1598 K
I4/mcm
1720 K
Pm 3¯ m
˚
a (A)
5.3789(2)
5.4984(3)
5.4984(3)
7.7828(8)
4
3.8967(1)
˚
b (A)
˚
c (A)
5.4361(2)
7.6388(3)
4
Z
1
3
(g/cm )
D
x
4.04
6.79
3.84
6.89
3.82
7.15
5.50
2.47
3.19
1.57
0.0
R
R
R
R
S
wp (%)
p
F
I
5.04
1.39
5.36
3.01
2.40
1.45
3.68
1.51
Ca
x
y
z
À0.0078(6)
0.0357(4)
0.25
0.0
0.5
0.0
0.0
0.25
B
x
y
z
0.58(7)
0.0
0.5
3.80(7)
0.0
0.0
4.1(1)
0.5
0.5
Ti
¯
Fig. 3. Lattice parameters of Pbnm, I4/mcm and Pm3m phases of
CaTiO as a function of temperature. Filled and open symbols indicate
the data obtained on heating and cooling, respectively.
0.0
0.0
0.5
3
B
x
y
z
0.34(9)
0.0736(4)
0.4828(4)
0.25
1.9(1)
0.0
0.0
1.8(1)
0.0
0.5
O1
O2
0.25
7.99
0.5
5.64
B
x
y
z
eq
0.53
0.7113(3)
0.2893(3)
0.0375(2)
0.51
0.2284(4)
0.7284(4)
0.0
The crystal structures of the three phases were success-
fully refined by the Rietveld analyses of the high-
temperature neutron diffraction data. The lattice para-
meters discontinuously change at the Pbnm–I4/mcm
transition point, while a continuous change is observed
for the I4=mcm2Pm3m transformation. These results
indicate that the Pbnm–I4/mcm transition is of first
order and that the I4=mcm2Pm3m transformation is of
either second or higher order.
B
eq
3.91
Following the phase identification discussed above, all
other neutron diffraction data measured from 296 to
720 K were successfully refined by the Rietveld
1
analyses. Fig. 3 shows the refined lattice parameters as
a function of temperature. The lattice parameters
discontinuously change at the Pbnm– I4/mcm transition
point, while a continuous change is observed for the
I4=mcm2Pm3m transition. These results indicate that
the orthorhombic-tetragonal transition is of first order
and that the tetragonal–cubic transformation is of either
second or higher order.
Acknowledgments
The neutron-diffraction study was carried out under
the PAC No. 98-174 and 99-72. We express special
thanks to Prof. Y. Yamaguchi, Prof. M. Ohashi,
Dr. K. Ohoyama and Mr. K. Nemoto for the use of
HERMES diffractometer. We would like to express our
thanks to Prof. S. Sasaki and N. Ishizawa for helpful
discussions. We acknowledge very much to T. Oketani,
Y. Hatoyama, T. Nogami, S. Utsumi, and H. Sugawara
for experimental assistance. One of the authors (Roush-
own Ali) is indebted to Ministry of Education, Culture,
Sports, Science and Technology of Japan (Monbu-
Kagaku-sho) for the financial support provided
through the Monbu-Kagaku-sho Scholarship. This
work was partly supported by TEPCO Research
Foundation, by Grant-in-Aids for Scientific Research
(B) of the Monbu-Kagaku-sho, and by the Ogasawara
Foundation.
4
. Conclusions
We have observed two subsequent phase transitions
through neutron powder diffraction measurements in
the temperature range from 296 to 1720 K. An
orthorhombic Pbnm to tetragonal I4/mcm phase transi-
tion occurs at 1498725 K. Another transformation
¯
from the tetragonal I4/mcm to cubic Pm3m phase
occurs at 1634713 K. X-ray powder diffraction data
at 1431 and 1486 K clearly showed that there is no
Cmcm phase between the Pbnm and I4/mcm regions.

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2872
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