MeCHANISM for PIEZOELECTLICITY of LANGASITE crystal under high pressure

N. Araki,^a T. Iwataki,^a K. Kakimoto,^a H. Ohsato,^a T. Kuribayashi,^b Y. Kudoh,^b and H. Morikoshi^c

^aNagoya Institute of Technology, Nagoya, Japan; ^bTohoku University, Sendai, Japan; ^cMaterials Research Centre, TDK Co., Chiba, Japan (araki@zymail.mse.nitech.ac.jp)

Langasite (La₃Ga₅SiO₁₄) is expected to be one of the new materials for surface acoustic wave (SAW) device, because of its excellent piezoelectric properties. Langsite belongs to trigonal system with space group P321 and lattice parameter of a₁=8.1674(4) and c=5.0964(8)Å. Figure 1 shows the crystal structure (a₁-c plane) of langasite. A site containing La³⁺ forms decahedron, B and C sites containing Ga³⁺ form octahedron and tetrahedron, respectively. D site containing Ga³⁺ and Si⁴⁺ with 1:1 ratio forms tetrahedron. We reported that the change in the piezoelectric properties of langasite by substitution of the cation shows a close relation to the crystal structure at atmospheric pressure [1].

In this work, we investigated the mechanism for piezoelectricity of langasite by single-crystal X-ray analysis under high pressure to confirm the mechanism proposed in the previous report.

Langasite crystal was prepared by the Czokralski method. The grown crystal was formed into sphere. Diffraction intensity data were collected with a four-circle diffractometer. Diamond anvil cell (DAC) was used under high pressure (3.3, 4.8 and 6.1GPa) measurement. Refinement of the crystal structure was performed by the full-matrix least-squares program RADY [2]. The difference of the crystal structure observed at atmospheric, 3.3 and 6.1 GPa were investigated.

The a₁-axis is preferentially shrunk compared to the c-axis and their change depended the applied pressure and followed liner functions. The cause of preferential shrinkage observed in a₁-axis is shrinkage of open space surrounded A, B and C sites under high pressure. The mechanism for piezoelectricity of langasite is explained, as follows. Under high pressure, A site is shrunk in the [100] direction accompanied with shrinkage of the open space. Polarization in A site was enhanced by applied pressure. This resulted in large piezoelectricity.

References

1 Iwataki, T., Ohsato, H., Tanaka, K., Morikoshi, H., Sato, J., and Kawasaki, K. (2001) Journal of the European Ceramic Society 21, 1409-1412.

2 Sasaki, S. (1982) XL Report, ESS, State University of New York, 1-17.