Akinori Hoshikawa

Publications (4)3.53 Total impact

• Article: Ferroelectric distortion and electronic structure in Bi4Ti3O12

  Yuji Noguchi, Takashi Goto, Masaru Miyayama, Akinori Hoshikawa, Takashi Kamiyama
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  ABSTRACT: The ferroelectric phase transition in Bi4Ti3O12 has been investigated through Rietveld analysis of high-resolution neutron powder diffraction and electronic structure calculations. The structural and electronic analyses show that the traditional model based on the stereoactive lone-pair 6s electrons of Bi3+ is not sufficient to explain the structural distortions in the ferroelectric state. It is strongly suggested that the hybridization of the Bi 6p and the O 2p in the perovskite layers is the trigger of the ferroelectric transition in Bi4Ti3O12, and that this orbital interaction is responsible for stabilizing the ferroelectric displacements in the perovskite layers.
  Journal of Electroceramics 04/2012; 21(1):49-54. · 1.19 Impact Factor
• Article: Neutron Diffraction Study on Lanthanum Gallate Perovskite Compound Series

  Masahiro Kajitani, Motohide Matsuda, Akinori Hoshikawa, Ken-ichi Oikawa, Shuki Torii, Takashi Kamiyama, Fujio Izumi, Michihiro Miyake
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  ABSTRACT: The crystal structures of lanthanum gallate perovskite compounds, LaGaO3, LaGa0.8Mg0.2O2.9, and La0.8Sr0.2Ga0.8Mg0.2O2.8 which is well-known as a superior oxide ion conductor, have been analyzed by neutron powder diffraction to clarify the effect of the substitutions on the perovskite structures. The crystal structure of Mg2+-substituted LaGaO3, LaGa0.8Mg0.2O2.9, belongs to the same orthorhombic space group, Pbnm, as that of LaGaO3, and the oxygen vacancies were found to be created at the planar O2 atom sites of GaO6 octahedron. On the other hand, the crystal structure of doubly Sr2+- and Mg2+-substituted LaGaO3, La0.8Sr0.2Ga0.8Mg0.2O2.8, belongs to the cubic space group Pm3̄m and the oxygen vacancies were found to be in disorder. The distortion in the perovskite structures and the oxide ion conduction mechanism are discussed on the basis of the analytical results, together with the results of X-ray diffraction and total conductivity up to 800 °C.
  08/2003;
• Article: High-temperature phase transition in lanthanum titanate perovskite La0.64(Ti0.92,Nb0.08)O3

  Masatomo Yashima, Mizuki Mori, Takashi Kamiyama, Ken-ichi Oikawa, Akinori Hoshikawa, Shuki Torii, Koh Saitoh, Kenji Tsuda
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  ABSTRACT: High-temperature phase transition of lanthanum niobium titanate La0.64(Ti0.92,Nb0.08)O3 has been studied using neutron powder diffraction and the Rietveld method. The material is orthorhombic (Cmmm) between 295 and 534 K, while it is tetragonal (P4/mmm) between 637 and 740 K. The anti-phase tilt along the b axis of the oxygen octahedron around Ti and Nb atoms was found to induce the tetragonal-to-orthorhombic phase transition. The angle of the tilt and the b/a ratio of cell parameters decrease with an increase of temperature and become 0° and unity, respectively, at a transition temperature between 534 and 637 K.
  Chemical Physics Letters 375:240-246. · 2.34 Impact Factor
• Article: Crystal structure analysis of β-tricalcium phosphate Ca3(PO4)2 by neutron powder diffraction

  Masatomo Yashima, Atsushi Sakai, Takashi Kamiyama, Akinori Hoshikawa
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  ABSTRACT: The crystal structure of sintered β-tricalcium phosphate, Ca3(PO4)2, was refined using a high-resolution neutron powder diffraction data and the Rietveld method. This material was confirmed to have a rhombohedral structure (space group R3c, Z=21). Unit-cell parameters with higher precision (a=b=10.4352(2) Å, c=37.4029(5) Å, α=β=90°, and γ=120° in the hexagonal setting) and positional parameters for oxygen with equal precision were obtained by the neutron powder diffraction technique, compared with the single-crystal X-ray diffraction data by Dickens et al. (J. Solid State Chem. 10 (1974) 232). The site Ca(4) with atomic coordinates [0.0, 0.0, −0.0851(6)] was confirmed to be very different from the other four Ca sites: The position Ca(4) is three-fold coordinated with oxygen atoms, and has lower occupancy factor of 0.43(4), and a higher isotropic thermal parameter. On the contrary, each of the Ca(1), Ca(2), Ca(3), and Ca(5) is fully occupied by one Ca atom and these positions are coordinated with seven, eight, eight, and six oxygen atoms, respectively. The bond valence sums of Ca(4) and Ca(5) are lower (0.7) and higher (2.7), respectively, than the others (1.8–2.1).
  Journal of Solid State Chemistry.