Boundary dislocations at a low angle grain boundary including twist component in LiNbO3

Atsutomo Nakamura1, Yuuho Furushima1, Eita Tochigi2, Yuichi Ikuhara2,3, Kazuaki Toyoura1 and Katsuyuki Matsunaga1,3

1 Department of Materials Science and Engineering, Nagoya University, Nagoya 464-8603, Japan 2 Institute of Engineering Innovation, The University of Tokyo, Tokyo 113-8656, Japan 3 Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya 456-8587, Japan

Physical and chemical properties of crystalline materials are strongly dependent on

the atomic structure of grain boundaries. Accordingly, it is essential to investigate the grain boundary structure for understanding the properties. A lot of researches have been done to investigate grain boundary structures. However, only a little is known about the atomic structure of boundary dislocations formed at the grain boundary with both tilt and twist components [1, 2]. In this study, therefore, the structure of a low angle grain boundary with both the tilt and twist components in LiNbO3 was analyzed by transmission electron microscopy (TEM).

LiNbO3 is a widely used ferroelectric material with pyroelectric, piezoelectric, electro-optic and photoelastic properties [3]. LiNbO3 is applied as surface acoustic wave filters, piezoelectric transducer devices, light wavelength converters and so on. LiNbO3 has a rhombohedral R3c structure at room temperature and R3

_

c above its Curie point of 1200oC.

LiNbO3 single crystals grown by the Czochralski method were used to fabricate bicrystals with the (12

_

10) / [101_

0] low angle tilt grain boundary with a slight twist component. The surfaces of the single-crystal plates were successively polished using diamond suspensions to achieve a mirror finish. The two single-crystal plates were subsequently joined by diffusion bonding in air at 800 °C for 10 h under an additional pressure of 0.1 MPa. The fabricated bicrystals were mechanically cut and ground, and then milled by Ar ion to obtain electron transparency. The completed samples were observed by conventional TEM (JEM-2010HC, 200kV, JEOL) and scanning TEM (JEM-2100F, 200kV, JEOL).

FIG. 1 shows the image of a typical dislocation at the boundary, which are obtained by high-angle-annular-dark-field scanning TEM (HAADF-STEM). Here, the bright points on the image correspond to the Nb columns. It can be seen that a boundary dislocation dissociates into two partial dislocations with the edge component of 1/6[12

_

10]. It was found that the separation distance between the two partials is 2 nm along [0001]. This result is in agreement with the previous report [4]. This type of dislocation compensates the tilt component at the boundary according to its total Burgers vector of 1/3[12

_

10]. FIG. 2 shows a HAADF-STEM image of a different type of dislocation obtained from

the same boundary. It was found out that a boundary dislocation dissociates into three partial dislocations with the 1/6[12

_

10] edge component. It was considered that the dissociation structure is realized to compensate the twist component in the boundary in addition to the tilt component. Interestingly, in most cases, the three partials were zigzagged as can be seen from the image.

It was thus found that the dislocation structure of low angle grain boundary changes

to the specific structure due to a slight twist component. It is expected that atomic-scale mechanisms of dislocation dissociation can be analyzed in further detail.

Acknowledgement The authors gratefully acknowledge the financial support by a Grant-in-Aid for Scientific Research on Innovative Areas "Nano Informatics" (Grant No. 25106003) from Japan Society for the Promotion of Science (JSPS). A part of this study was supported by JSPS KAKENHI Grant Numbers 24686073, 25000012, 25289225, 25630279. References [1] A. Nakamura et al., Phil. Mag., 86 (2006) 4657. [2] E. Tochigi et al., Acta Mater., 56 (2008) 2015. [3] T. Volk and M. Wohlecke, Springer Series in Materials Science 115, Lithium Niobate (2008). [4] A. Nakamura et al., J. Mater. Sci.,47 (2012) 5086.

FIG. 1. A typical HAADF-STEM image of a boundary dislocation with b=1/3[12_

10]. FIG. 2. A HAADF-STEM image showing specific dislocation dissociation formed at the low angle grain boundary accordingly to a slight twist component.

2 nm[0001]

[121

0]

[0001]

[121

0]

3 nm