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C6.1.1

Property Design of SrBi2Ta2O9 by Defect Engineering Yuji Noguchi1,2, and Masaru Miyayama1 1 Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan 2 PRESTO, Japan Science and Technology Agency, 4-1-8, Kawaguchi, Saitama, Japan ABSTRACT Defect engineering is shown to be an effective to design for remanent polarization (Pr) and coercive field (Ec) in SrBi2Ta2O9 (SBT). Cation vacancies and oxygen vacancies are shown to play an essential role in the polarization properties. The ceramic samples were prepared by a solid-state reaction, and high-density ceramics (over 95% of the theoretical density) were used for the measurements of polarization and dielectric properties. High-resolution neutron powder diffraction revealed that trivalent-cation (Bi, rare-earth elements [RE=La, Ce, Pr, Nd, Sm]) substitution induces Sr vacancies in the perovskite blocks for the requirement of charge neutrality. The substitution of La with Sr vacancies (La0.33Sr0.5Bi2Ta2O9) increased 2Pr from 13 µC/cm2 (SBT) to 16 µC/cm2, and the 2Ec value (41 kV/cm) was much smaller than that of SBT (57 kV/cm). Nd-substituted SBT showed the same Pr of La-SBT, while 2Ec (140 kV/cm) of Nd-SBT (x=0.5) was much higher than that of La-SBT. The higher Ec found for Nd-SBT is attributed to partial Nd substitution at the Ta site with the creation of oxygen vacancies. The control of Ec for RE-SBT is discussed in terms of Sr vacancies and oxygen vacancies. INTRODUCTION There has been an increasing interest in ferroelectric materials because of their application to nonvolatile random access memories. Ferroelectric SrBi2Ta2O9 (SBT) is considered as a promising candidate due to its excellent high durability against a repeated switching of its polarization states.[1-6] In the crystal structure of SBT, two layers of TaO6 octahedra in the perovskite blocks (SrTa2O7) are sandwiched between bismuth oxide layers (Bi2O2), and Sr occupies the cavity between the octahedra (the A site). While the bismuth oxide layers are hard to be modify, the perovskite blocks accommodate a large variety of cations, and then the Curie temperature (TC) changes in a wide temperature range. [7-9] For SBT with TC of 295oC, larger Ba and smaller Ca ions can be substituted for divalent Sr at the A site. [7,10,11] These substitutions at the A site induce a significant change in lattice distortions in the perovskite blocks, [10,11] and TC varies from 120oC (30% Ba substitution) to above 700oC (Ca case). [7,10,11] Bi3TaTiO9 (TC =860oC [12]) and SBT form a complete series of solid solutions, and the co-substitution of Bi and Ti at the A and Ta sites in SBT also leads to a significant change in the ferroelectric properties. The partial substitution of Bi3TaTiO9 for SBT has been reported to result in a marked improvement of remanent polarization (Pr) of SBT thin films.[13] Similar to PbTiO3 that accommodates the cation vacancies created by La substitution,[14] a large amount of Sr vacancies can be introduced in the perovskite blocks of SBT by the

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