Crystal Structures, Ferroelectric Properties, and Chemical Reactions of SrBi 2 Ta 2 O 9 : Solid-State Chemistry of SBT M
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Y SHIMAKAWA* and Y KUBO FundamentalResearch Laboratories,NEC Corporation,Tsukuba 305-8501, Japan * [email protected] ABSTRACT
We discuss some aspects of solid-state chemistry concerning ferroelectric SrBi2Ta2O, (SBT). Crystal structure analysis by high-resolution neutron diffraction revealed considerable structural distortion in the SBT materials. This distortion, which consists of large atomic displacements of constituent ions, causes large ferroelectric spontaneous polarization. The structural distortion is more pronounced in Sr-deficient-and-Bi-excess material Sr08Bi 22Ta2O, than in stoichiometric SBT, leading to the larger remanent polarization. The enhanced structural distortion is explained in terms of an "ionic size effect" at the A-cation site. This effect was apparently observed in a series of compounds of ABi 2Ta2O9 (A=Ca, Sr, and Ba) with different size A-site cations. On the other hand, analysis of chemical reactions of SBT gave important information on the degradation of the SBT capacitors and on the reaction between the SBT materials and a Pt electrode during device fabrication. The degradation of SBT in an H2containing reducing atmosphere and its recovery by subsequent annealing in oxygen have been reproduced experimentally by using bulk ceramics and analyzed by thermogravimetric and xray-diffraction measurements. SBT decomposes into Bi metal or Bi-Pt alloy in H2 by oxygen dissociation. In the presence of Pt, the SBT degradation and the re-crystallization processes differ from those for SBT alone. INTRODUCTION
In recent years a lot of attention has been given to the use of ferroelectric thin films in nonvolatile memory applications (Ferroelectric Random Access Memories: FeRAMs). Ferroelectric Bi-layered compounds as well as PbZr,.,Ti,0 3 (PZT) perovskites are leading candidates for such devices, and the Bi-layered materials have a great advantage over PZT because of their fatigue-free nature when used in devices with Pt electrodes. Among the Bilayered materials, SrBi 2Ta2O9 (SBT) has been the most intensively studied for thin-film growth and memory device applications [1,2]. Although many of the basic structural and ferroelectric properties of SBT were studied in the early 60's [3], the information obtained at that time is not sufficient to reveal the inherent properties of the material. This paper describes some aspects of solid-state chemistry of SBT and its related ferroelectric oxides. The experimental results include crystal structures, ferroelectric properties, and chemical reactions. Recent significant developments in characterization techniques used in solid-state chemistry promise to give us new insight into these "old" materials. In the first section, crystal structures and ferroelectric properties of the stoichiometric SrBi 2Ta 2O9 and Sr-deficient-and-Bi-excess Sr0 8Bi 22Ta2O9 materials are reported [4]. In the second section, the results of the crystal structure analysis of a series of Bi-layered compounds with different A-site cations, i.e., ABi 2Ta 2O9 (A = Ca, Sr, and Ba) are
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