[20a-P6-1~6] 9.1 誘電材料・誘電体

▲ [20a-P6-1] Origin of material softening in BiFeO₃-BaTiO₃ lead-free piezoelectric ceramic

〇(D)Sangwook Kim¹、Gopal Prasad Khanal¹、Hyun-Wook Nam¹、Ichiro Fujii¹、Shintaro Ueno¹、Chikako Moriyoshi²、Yoshihiro Kuroiwa²、Satoshi Wada¹ (1.University of Yamanashi、2.Hiroshima University)

キーワード：Material softening

Lead-free piezoelectric ceramics have been studied for replacement of lead-based piezoelectric materials. BiFeO₃-BaTiO₃ (BFBT) is one of them. An interesting property of the BFBT ceramics is that maximum piezoelectric response is exhibited meanwhile the ferroelectric property decreasing trend. This phenomenon is related to material softening. The material softening in the BFBT ceramics is distinguished from other piezoelectric materials such as (K,Na)NbO₃. Although the material softening can be easily seen in the Bi-based lead-free piezoelectric materials, the origin of the material softening in the BFBT ceramics is unclear. In this study, the BFBT lead-free piezoelectric ceramics were fabricated using a solid state reaction method and the origin of the material softening was investigated. The material softening was exhibited in the wide composition range, and the maximum material softening was exhibited at the composition of 0.70BiFeO₃-0.30BaTiO₃ (BF30BT). The synchrotron radiation X-ray diffraction patterns of the BF30BT ceramics showed single, undivided diffraction peaks, which suggested the material had cubic-like symmetry. The single peak shape was maintained above the Curie temperature (T_C). Rietveld refinements indicated that the crystal structures were indeed rhombohedral at 300 K and cubic at 950 K. It is worthy to mention that the Bi ions were off-centered in both the rhombohedral and cubic structures. The material softening might be related to the Bi off-centering. Structural parameters such as lattice constant, bond-length, and Bi off-center displacement were calculated from the final structure refinement results, as shown in Fig. 1. The lattice constant of the BFBT ceramics was increased with increasing BT concentration. The maximum Bi off-center displacement and minimum Bi-O bond-length were observed in the BF30BT ceramics. More detail of the results will be discussed in the presentation.