Department of Physical Science, Hiroshima University^* Research Center for Advanced Science and Technology, University of Tokyo, Japan^**
○Sayaka Kimura^* Su Jae Kim^* Chikako Moriyoshi^* Yoshihiro Kuroiwa^* Yuji Noguchi^**

Lead-free materials have attracted much attention for the application to FeRAM. Bismuth layer-structured ferroelectric Bi₄Ti₃O₁₂ (BIT), which consists of (Bi₂O₂)²⁺ layer and (Bi₂Ti₃O₁₀)^2- perovskite layer, is one of the candidates due to its large spontaneous polarization. One of the problems to be overcome is, however, that BIT does not have a sufficient endurance for repetitive switching of its polarization states. Recently La-substituted BIT, that is Bi_3.25La_0.75Ti₃O₁₂ (BLT), has been reported to be fatigue-free. To clarify the La/Bi substitution effect and the origin of the fatigue-free characteristics of BLT, we performed synchrotron radiation (SR) powder diffraction experiment of BIT and BLT.
Powder diffraction data were obtained at BL02B2 in SPring-8 using the large Debye-Scherrer camera equipped with an imaging plate as a two-dimensional detector. High-energy X-rays (35 keV) were used in this experiment. High-energy SR powder diffraction has advantages of being free of problems of extinction and absorption effects, and is quite useful for samples such as Bi₄Ti₃O₁₂ and Bi_3.25La_0.75Ti₃O₁₂ consisting of an extremely heavy element Bi and a light element O. The crystal structure parameters and electron charge density distributions of BIT and BLT were determined by the MEM/Rietveld analysis.
We confirmed that La atoms substitute for Bi atoms only in the perovskite layer, as has been reported. An orbital hybridization between Bi-O in the perovskite layer along the a-axis was observed for BIT, whereas in BLT, in addition to the hybridization, another hybridization along the b-axis in the perovskite layer was revealed. We conclude that this hybridization stabilizes oxygen and induces the fatigue-free property in BLT.