Photon Factory Activity Report 2017 #35 (2018)

BL-10A/2012G540

Single-crystal X-ray diffraction study of tetragonal and cubic perovskite-type PbTiO3Akira Yoshiasa1*, Tomotaka Nakatani1, Akihiko Nakatsuka2,Tsutomu Mashimo1 and Kazumasa Sugiyama 3
1
Graduate School of Sciences and Technology, Kumamoto Univ. Kumamoto 860-8555, Japan
2
Graduate School of Science and Engineering, Yamaguchi University, Ube 755-8611, Japan
3
Institute for materials research, Tohoku University, Sendai, Miyagi 980-8577, Japan.
1 Introduction
Perovskite-type PbTiO3 has been intensively studiedfor a long time because of their useful functional
properties such as ferroelectric properties at ambientconditions. PbTiO3 shows a ferroelectric-paraelectricphase transition at around 763 K. The low temperature
tetragonal phase is polar and ferroelectric with theacentric space group P4mm and crystal system of hightemperature form is cubic (Pm3m). In order to examine
the relationship between the structural changes in thetetragonal and cubic phases of PbTiO3 and its
ferroelectric phase transition mechanism, we have carriedout the single crystal X-ray structure analyses in the widetemperature range 298 K to 928 K. In this study [1], wehave determined the temperature dependence of thedistortion in the ferroelectric phase and of mean squaredisplacements for each atom. The possibility of the Ti
displacement from central position of the TiO6 octahedronin the cubic phase is assessed together with themechanism of ferroelectricity.
2 Experiment
A single crystal used for X-ray measurements wascarefully ground to a sphere 140(1)μm in diameter andput into a silica-glass capillary. Temperature fluctuation
during the data collections was kept within ± 0.2 K.
Single-crystal X-ray diffraction measurements werecarried out with a four-circle diffractometer at the BL10A beam line of the Photon Factory, Tsukuba, Japan,
using monochromatized synchrotron X-ray (λ = 0.70006Å) radiation. The unit-cell parameters at each temperature
were determined by the least-squares method from a setof 25 reflections within the range of 45  2  50.
Intensity data were collected within 2  2  120 foreach temperature by continuous–2 scan mode, andconsequently reflections between 1105 and 590 weremeasured for each phase. After the corrections forLorentz-polarization factors and absorption effects, theintensity data were averaged in Laue symmetry 4mm andm3m to give independent reflections between 620 and
129 for the tetragonal and cubic phases, respectively. All
the measured reflections and all the independent
reflections after averaging were observed with accuraciesof Fo  3(Fo), and the latter sets of reflections wereemployed for the structure refinements. The structurerefinements were carried out by minimizing the function
w(Fo–Fc)2 using a full matrix least-squares program
RADY. The alternative of the indexing hkl or hk-l are

possible because of the acentric structure for thetetragonal phase.
3 Results and Discussion
Figures 1 and 2 show the temperature dependence ofTi-O1 and Pb-O2 bond distances. The differences
between the longer and shorter distances show that Ti andPb occupy off-center positions in TiO6 and PbO12
polyhedra, respectively. The differences between thelonger and shorter bond distances obviously decrease
with increasing temperature. This shows that thedistortion in each polyhedron decreases and cation moves
from off-center position to center position in eachpolyhedron with increasing temperature toward the phasetransition point. [1].

Fig. 1: Temperature dependence of the longer and shorter
Ti-O1 distances in TiO6 octahedron.

Fig. 2: Temperature dependence of the lomger and shorter
Pb-O2 distances in PbO12 polyhedron.
References[1] A.Yoshiasa, T.Nakatani, A.Nakatsuka, M.Okube,K.Sugiyama and T. Mashimo, Acta Cryst., B72, 381-368(2016).
* yoshiasa@sci.kumamoto-u.ac.jp