Room-temperature deformation behavior of SrTiO3 single crystals with different chemical compositions

Atsutomo Nakamura1, Kensuke Yasufuku1, Yuho Furushima1, Kazuaki Toyoura1

and Katsuyuki Matsunaga1,2 1 Department of Materials Science and Engineering, Nagoya University, Nagoya 464-8603, Japan 2 Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya 456-8587, Japan

Most ceramics are brittle at room temperatures. This is believed to be because

large translation vectors in crystal structures of ceramics leads to the large Burgers vector of dislocations. In contrast, strontium titanate (SrTiO3) single crystals exhibit significant ductility even at room temperature [1] although SrTiO3 is a ceramic with the large translation vector. A physical origin of room-temperature ductility of SrTiO3 remains still unclear. Recently, Takehara et al reported intense segregation of point defects at the dislocations in SrTiO3 [2]. Here the dislocations bring about easy slip system at room temperature plastic deformation. Thus, concentration of point defects at dislocations has a potential to dominate plastic deformation behavior of SrTiO3.

In this study, therefore, uniaxial deformation tests were conducted for SrTiO3 single crystals with different chemical compositions, in order to reveal the influence of point defects on room-temperature plastic deformation of SrTiO3. It is known that the formation of Sr vacancies arises easily in SrTiO3 because of the low formation energy [3]. It is expected that the ratio of Sr/Ti = 1 in growing up single crystals induce more Sr vacancies. In other words, single crystals grown up with more Sr/Ti ratio could result in less Sr vacancies in SrTiO3.

The single crystals grown by Velneuil method were used for the deformations tests in this study. For investigating influence of different chemical composition, single crystals grown using the starting powders with two composition ratios of Sr/Ti = 1.00 and 1.04 were employed. Figure 1 shows the shapes and crystallographic orientation of the specimens for deformation. Specimens were deformed in compression for [100] at a strain rate of 1.0 x 10-5 / s in air at room temperature. In this case, four equivalent slip systems of [1

_

01](101), [101]( 1_

01), [01_

1](011) and [011](01_

1) will work as primary slip system because all the Schmid factors are 0.5. The deformed specimens were observed by optical microscopy to analyze the slip lines formed on the specimen surfaces.

Figure 2 shows the obtained stress-strain curves. It can be seen that stress gradually increase with increasing strain and specimens give rise to failure finally. It was found that SrTiO3 single crystal grown from the powder with a Sr/Ti ratio = 1.04 clearly exhibit lower deformation stress and larger failure strain, as compared to SrTiO3 of Sr/Ti = 1.00. It was confirmed from the surface observation of the deformed specimens that the specimens with the two different composition formed similar slip lines. It appears that macroscopic activity of slip systems themselves do not depend on the chemical composition.

In general, more point defects bring about higher stress in deforming single crystals because point defects can affect structure and mobility of dislocations. The lower yield and flow stress as in Fig.1 suggest that less point defects were involved in single crystal with a Sr/Ti ratio = 1.04. As mentioned above, SrTiO3 with a Sr/Ti ratio = 1.00 is likely to form more Sr vacancies. It is suggested that less Sr vacancies result in the lower deformation stress for Sr/Ti ratio = 1.04. In addition, single crystal with a Sr/Ti ratio =

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1.04 exhibited larger failure strain as in Fig.1. Here note that failure stress for a Sr/Ti ratio = 1.04 also has distinctly high value. This indicates that less interaction between dislocations and point defects tends to prevent failure. Acknowledgement The authors gratefully acknowledge the financial support by a Grant-in-Aid for Scientific Research on Innovative Areas "Nano Informatics" (grant numbers 25106002) from Japan Society for the Promotion of Science (JSPS). A part of this study was supported by JSPS KAKENHI Grant Numbers 15H04145, 15H02210, 26630311 and 25630279. References [1] Brunner et al., J. Am. Ceram. Soc. 84 (2001) 1161. [2] Takehara et al., J. Mater. Sci. 49 (2014) 3962. [3] Tanaka et al, Phys. Rev. B. 68 (2003) 205213.

FIG. 1. Schematic illustration of shapes and crystallographic orientation of the specimens for deformation tests.

FIG. 2. Stress-strain curves of SrTiO3 single crystals compressed for [100]. (a) Sr/Ti =1.00, (b) Sr/Ti = 1.04 as starting powder for crystal growth.

Stre

ss,σ

(MP

a)

0

100

200

300

400

0 5 10 15 200

100

200

300

400

0 5 10 15 20Strain, ε (%) Strain, ε (%)

(a) (b)

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AMTC Letters　Vol. 5 (2016) ©2016 Japan Fine Ceramics Center