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Lee, Hak-Sung,Mizoguchi, Teruyasu,Yamamoto, Takahisa,Kang, Suk-Joong L.,Ikuhara, Yuichi Elsevier 2007 Acta materialia Vol.55 No.19
<P><B>Abstract</B></P><P>The structural relaxation and formation energies of intrinsic vacancies in cubic-BaTiO<SUB>3</SUB> were studied by using a first-principles plane-wave-based pseudopotential calculation. The calculated defect formation energies and the atomic relaxations were compared with previously reported data for SrTiO<SUB>3</SUB> [Tanaka T, Matsunaga K, Ikuhara Y, Yamamoto T. Phys Rev B 2003;68:205213]. It was found that the formation energies of the partial Schottky species, VTi2-+VO2+ and VBa2-+VO2+, are the lowest in the oxidized condition. In contrast, as the oxygen chemical potential decreased, namely the reduced condition, the O vacancy becomes dominant in cubic-BaTiO<SUB>3</SUB>. Concerning the atomic relaxation, it was found that BaTiO<SUB>3</SUB> and SrTiO<SUB>3</SUB> show a difference in magnitude. It was also found that the overall vacancy formation energy in BaTiO<SUB>3</SUB> is higher than that in SrTiO<SUB>3</SUB>.</P>
Direct Determination of Cationic Disordering in Sodium Bismuth Titanate
Choi, Si-Young,Ikuhara, Yuichi Korean Society of Microscopy 2012 Applied microscopy Vol.42 No.3
The relaxor ferroelectric feature in lead-free perovskite oxides, where the dipoles are randomly oriented and they can be feasibly aligned parallel to the external bias, is attracting lots of attention in the field of piezoelectric materials science, since it is one of candidates to replace the toxic lead-based materials that are still being commercially used. However, the origin of relaxor characteristic and its related atomic structure are still ambiguous. In this study, $Na_{1/2}Bi_{1/2}TiO_3$, chosen as a model relaxor system, was found to exhibit a cationic-disordered atomic structure; and furthermore the nonpolar atomic structure and its related oxygen tilting were ascertained via annular bright field imaging skill. We also found that this cationic disordering gives rise to the local formation of atomic vacancies.
Choi, Si-Young,Lim, Jong Bong,Ikuhara, Yuichi,Suvorov, Danilo,Jeon, Jae-Ho Cambridge University Press 2013 Microscopy and Microanalysis Vol.19 No.5
<B>Abstract</B><P>Two kinds of Sr2FeReO6 (SFRO) samples, pristine SFRO and Re-excess SFRO, were prepared and we visualized the local atomic structure in terms of cationic ordering in the prepared SFRO samples via high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM). HAADF-STEM results demonstrated the high degree of cationic ordering maintains in both the pristine SFRO and Re-excess SFRO samples. On the other hand, defective structures such as antiphase boundary and Re-deficient phase were observed dominantly in the pristine SFRO, and thus the poor magnetic property in the pristine SFRO is attributed to those defective structures related with the frustrated Fe/Re ordering.</P>
Yoon, B.K.,Choi, S.Y.,Yamamoto, T.,Ikuhara, Y.,Kang, S.J.L. Elsevier Science 2009 ACTA MATERIALIA Vol.57 No.7
The effect of an intergranular amorphous film on grain growth behavior has been studied in a faceted model system, BaTiO<SUB>3</SUB>. We prepared two kinds of samples with and without intergranular amorphous films but with the same grain size and density. During annealing the samples at 1350<SUP>o</SUP>C in air, abnormal grain growth occurred in samples with intergranular amorphous films while grain growth was inhibited in samples with dry boundaries, indicating the presence of a pinning force in samples with dry boundaries. To compare the mobilities of dry and wet boundaries, single crystal and polycrystal bilayer samples with or without amorphous films were prepared and annealed at 1340<SUP>o</SUP>C. In contrast to the observed grain growth behavior in polycrystals, the growth of the single crystal into the polycrystal with dry boundaries was faster than that into the polycrystal with wet boundaries, demonstrating the higher mobility of a dry boundary, unlike the conventional understanding.
Choi, Si-Young,Chung, Sung-Yoon,Yamamoto, Takahisa,Ikuhara, Yuichi WILEY-VCH Verlag 2009 ADVANCED MATERIALS Vol.21 No.8
<B>Graphic Abstract</B> <P>Through a combination of aberration-corrected HAADF-STEM and the EELS at the atomic level, it is found that added La in CaCu<SUB>3</SUB>Ti<SUB>4</SUB>O<SUB>12</SUB> is selectively substituted for Ca, rather than Cu. The present study suggests that imaging based on STEM combined with EELS can be utilized as a powerful tool that reveals the atomic-level distribution of cation dopants in complex oxides. <img src='wiley_img/09359648-2009-21-8-ADMA200802728-content.gif' alt='wiley_img/09359648-2009-21-8-ADMA200802728-content'> </P>
Orientation-Dependent Arrangement of Antisite Defects in Lithium Iron(II) Phosphate Crystals
Chung, Sung-Yoon,Choi, Si-Young,Yamamoto, Takahisa,Ikuhara, Yuichi WILEY-VCH Verlag 2009 Angewandte Chemie Vol.121 No.3
<B>Graphic Abstract</B> <P>Eisenkationen, die als „Antisite“-Defekte Lithiumpositionen in dotiertem Lithiumeisenphosphat (LiFePO<SUB>4</SUB>) besetzen (rot im Bild), sind bevorzugt längs der b-Achse ausgerichtet. Rastertunnelelektronenmikroskopische Bilder dieser Defektanordnungen sprechen dafür, dass über die Verteilung der „Antisite“-Defekte in LiFePO<SUB>4</SUB> ein verbesserter Lithiumionentransport erreicht werden kann. <img src='wiley_img/00448249-2009-121-3-ANGE200803520-content.gif' alt='wiley_img/00448249-2009-121-3-ANGE200803520-content'> </P>