Electron charge density study of Bi4Ti3O12 and Bi3.25La0.75Ti3O12 by MEM/Rietveld analysis

Department of Physical Science, Hiroshima University* Research Center for Advanced Science and Technology, University of Tokyo, Japan**
â—‹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 Bi3.25La0.75Ti3O12 (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 Bi3.25La0.75Ti3O12 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.