| Co-Investigator(Kenkyū-buntansha) |
HIRATANI Masahiko HITACHI Ltd., Central Res.Labo., University of Tokyo, Chief Researcher, 中央研究所, 主任研究員
KUDO Tetsuichi Institute of Industrial Science, University of Tokyo, Professor, 生産技術研究所, 教授 (90205097)
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| Research Abstract |
For Bismuth layer-structured oxides which is now paid attention as a ferroelectric memory device material, studies were conducted on design for intergrowth-structure ferroelectrics with two kind of perovskite layers and on realization of a layer-strucrured material showing both ferroelectric and conductive properties simultaneously. At first, effects of nonstoichiometry and lattice defects on crystal structure and ferroelectric property were examined. Nonstoichiometry (A-site deficiency in perovskite lattice) in strontium bismuth tantalate was found to give increases in Curie temperature and shift of atom positions, resulting in a large remanent polarization. By doping small amount of V into Bismuth titanate ceramics, remanent polarization increased about 3 times of non-doped material. Obtained remanent polarization was as much as the value calculated from single-crystal value for non-oriented ceramics. For the formation of intergrowth materials, the size matching of two perovskite lattice is necessary. It was found that some intergrowth-structure ferroelectrics show larger remanent polarization than those of component ferroelectrics. Stress induced by stacking different perovskite layers was assumed to be responsible for increases in lattice distortion and atom shift. Trial for making conductive perovskite layers was carried out, and substitution of Mn for Fe in Bi_5Ti_3FeO_<15> was found to be effective for an increase in p-type electronic conductivity more than four orders of magnitude. From X-ray diffraction analysis, an intergrowth-structure material with ferroelectric and conductive perovskite layers was confirmed to be formed though minor second phases are contained in samples. From these results, it was found that a marked improvement of ferroelectric property can be achieved by control of nonstoichiometry and lattice defects, and the guiding principle was obtained for design of intergrowth-structure formation and fusion of different electrical functions.
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