Photon Factory Activity Report 2017 #35 (2018)

BL-8A/2016S2-004

Structural variations of ferroelectric material (Bi1−xLax)2SiO5 (0 < x < 0.1)
J. Yamaura1,*, H. Taniguchi1,2, S. Tatewaki2, S. Yasui3, Y. Fujii4, I. Terasaki2
1
Materials Research Center for Element Strategy, Tokyo Institute of Technology, Kanagawa, Japan2
Department of Physics, Nagoya University, Aichi, Japan
3
Laboratory for Materials and Structures, Tokyo Institute of Technology, Kanagawa, Japan
4
Department of Physical Sciences, Ritsumeikan University, Shiga, Japan* jyamaura@mces.titech.ac.jpa (Å)

15.20

Bi2SiO5Monoclinic, Cc

a

15.19
15.18
050100150200250300

T (K)
5.470

5.330
Bi2SiO5
Monoclinic, Ccb5.465b (Å)

5.325

5.460

5.455
0c

50100150

5.320

200250300

T (K)

3.830

15.28

(Bi0.9La0.1)2SiO5
3.828 Tetragonal, I4/mmm

15.27a (Å)

a3.826

15.26

3.824

15.25

c

3.822
3.820
0c (Å)

References[1] H. Taniguchi et al., Angew. Chem. Int. Ed. 52, 8088
(2013).
[2] F. Izumi and K. Momma, Solid State Phenom. 130, 15(2007).
[3] H. Taniguchi et al., Phys. Rev. Mat. 2, 045603 (2018).

15.21c (Å)

Ferroelectric materials have been explored in a longwhile. They have usually an oxygen octahedral
coordination to the central transition metal, however, aferroelectric tetrahedral oxide recently reported inBi2SiO5 [1]. The ferroelectricity comes from zig-zag
tilting of SiO4 one-dimensional chains. Moreover, aninternal deformation of SiO4 tetrahedron gives rise tolarge spontaneous polarization. A ferroelectric phasetransition in Bi2SiO5 occurs with changing from a hightemperature paraelectric Cmcm phase to a lowtemperature ferroelectric Cc phase at 673 K.
In order to investigate an origin of the dielectricanomaly observed in the low-temperature, synchrotron
powder X-ray diffraction measurements were performedfor (Bi1−xLax)2SiO5 (x = 0, 0.10) using the curved imaging
plate diffractometer at the wavelength of λ = 0.99925 Å
on BL-8A, KEK-PF. The images were integrated to yield2θ-intensity data. The structure analysis was examinedwith RIETAN-FP program [2].
Figure 1 shows the lattice parameters for x = 0 on Cc
space group in monoclinic crystal system [3]. Twoanomalies were observed at 30 K and 100 K for every
crystallographic axis, corresponding to the observed
dielectric anomaly. Because neither extra peak nor
additional peak split was obtained in the diffraction
patterns at the lowest temperature, we consider that nofurther symmetry lowering exists within the experimentalresolution. In x = 0.10, the low-temperature latticeanomalies disappear as illustrated in Fig. 1. This trend isin good agreement with the x dependence offerroelectricity [3]. Taking account of the strong
correlation between ferroelectricity and crystal structure,
the low-temperature anomaly is considered to originate
from the ordered one-dimensional SiO4 chains. This issimilar to an isosymmetric phase transition inorthopyroxene, strongly related to the one-dimensional
SiO4 tetrahedral chains. Thus, we suggest thatisosymmetric transitions emerge with the present latticeanomalies observed at low-temperature.

15.24

50100

150

200

250

15.23300

T (K)Fig. 1 Lattice parameters for (Bi1−xLax)2SiO5 (x = 0,0.10) [3]. The x = 0 compound is monoclinic, Cc structureand the x = 0.1 compound is tetragonal, I4/mmm structure.
The β angle data in x = 0 is not shown here. In the x = 0
two anomalies are observed at 30 K and 100 K, while theanomalies disappear in the x = 0.1.