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New Materials Designing With Unique Approaches

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SEMINARThe State Key Lab ofHigh Performance Ceramics and Superfine MicrostructureShanghai Institute of Ceramics, Chinese Academy of Sciences  

  中 国 科 学 院 上 海 硅 酸 盐 研 究 所 高 性 能 陶 瓷 和 超 微 结 构 国 家 重 点 实 验 室  

  New Materials Designing With Unique Approaches 

  Speaker 

  Mitsuru Itoh 

  (伊藤 满) 

  Materials and Structures Laboratory, Tokyo Institute of Technology 

  时间:2015年11月16日(星期一)上午 10: 30 

  地点: 四号楼十四楼第一会议室 

  欢迎广大科研人员和研究生参与讨论! 

  联系人:余建定(2507) 李勤(1708) 

  Abstract

  Freezing of the optical soft modes, i.e., the Slater and Last modes, in the phase transition of displacive-type perovskite oxides ABO3 such as BaTiO3 and PbTiO3 is a straightforward way to imagine the mechanism of ferroelectricity, as well as the collective order?disorder-type transition in a permanent dipole system such as NaNO2. In ABO3 perovskites, A-O bond disproportionality also plays an important role in the manifestation of ferroelectricity.

  Here we report the discovery of a new mechanism for the evolution of ferroelectricity in ABO3 (A,B = Al, Ga, Fe) with a non-perovskite κ-alumina-type structure, which can be a compromise between two close-packed oxygen systems, corundum and spinel. Shearing of the close-packed oxygen layers in this structure triggers polarization switching, leading to a recombination of cations from 6(4) to 4(6) coordination. The compounds with a κ-Al2O3-type structure were known to have a non-centrosymmetric space group of Pna21 and consequently was classified as a pyroelectric, the polarization direction of which cannot be switched by the external electric field, however small polarization was reported for this materials group. Corundum and spinel structures have close-packed oxygen layers in the sequence of ABAB… and ABCABC…, respectively, where A, B, and C are oxygen close-packed layers, while κ-Al2O3 has a sequence of ABACABAC……. . Cations of κ-Al2O3-type structure in the interlayer octahedral and tetrahedral sites between the oxygen close-packed layers of C and A and between B and A change their coordination under an electric field from 6(4) to 4(6), respectively. We report a ferroelectricity of κ-Al2O3-type metastable oxides of κ-Al2O3, e-Ga2O3, and e-Fe2O3 using first principles calculation and confirm the polarization experimentally in the solid solution system of GaxFe2-xO3 . The newly discovered mechanism for the evolution of ferroelectricity opens the door for the development of new ferroelectric oxides composed of abundant elements and moreover, a new concept of material design based on bond recombination caused by the shearing of the close-packed oxygen layers under an electric field, a concept distinct from that of the perovskite system

 
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