SPring-8, the large synchrotron radiation facility

Release Date

01 Oct, 2007

The reason behind the high durability of less harmful lead-free special ceramics, the expected next-generation memory material, for the writing and reading of information was clarified for the first time in joint research between Hiroshima University (Toshimasa Asahara, President), the University of Tokyo (Hiroshi Komiyama, President), RIKEN (Ryoji Noyori, President), Japan Science and Technology Agency (JST) (Koichi Kitazawa, President), and Japan Synchrotron Radiation Research Institute (JASRI) (Akira Kira, Director General).  This was achieved by Yoshihiro Kuroiwa, professor (also visiting researcher of RIKEN), and Chikako Moriyoshi, associate professor of Hiroshima University; Yuji Noguchi, associate professor, and Masaru Miyayama, professor of the University of Tokyo; and Kenichi Kato, scientist (also research scientist of JASRI), and Masaki Takata, senior scientist (also director of Research & Utilization Division of JASRI) of Takata Structural Materials Science Laboratory of RIKEN SPring-8 Center (Tetsuya Ishikawa, Director).  This research was conducted under the theme of "X-ray pinpoint structural measurement project" (research representative: Masaki Takata) within the scope of the "Novel Measurement and Analytical Technology Contributions to the Elucidation and Application of Material" project (director general: Michiyoshi Tanaka, Professor Emeritus, Tohoku University) by the Core Research for Evolutional Science and Technology (CREST) of JST.

The research and development of nonvolatile memories using ferroelectric substances with a charge-storing function even at zero voltage is ongoing all over the world.  Conventionally, piezoelectric zirconate titanate (PZT), containing harmful lead, was the mainstream of memory materials; recently, lead-free bismuthic ferroelectric substances have been developed and have attracted attention.  However, pure PZT, Bi₄Ti₃O₁₂ (BiT), has a drawback - its memory function is lost after repetitive rewriting involving polarity reversal.  Recent studies have shown that the performance of BiT with bismuth partly replaced with lanthanum, Bi_3.25La_0.75Ti₃O₁₂ (BLT), does not deteriorate even after writing and reading a trillion times, and thus it can be effectively used in practice.  The reason for such high durability was not yet clear.  To understand the reason behind the high durability of BLT, diffraction experiments were carried out on BLT using very high energy X-rays at the Powder Diffraction Beamline, BL02B2, of SPring-8.  Results showed that the atoms of newly added lanthanum and originally existing bismuth metals formed new strong chemical bonds with oxygen atoms in any direction, suppressing the separation of oxygen atoms from the metal materials.  The thus-obtained material exhibited an orderly arrangement of atoms free of oxygen defect, in which the atoms move smoothly inside the material during rewriting, resulting in the high durability.  Thus, the reason behind the excellent durability was clarified at an atomic level.  This achievement helped us obtain a clear understanding of the role of elemental replacement and provided a guideline for the development of new materials through the control of chemical bonds.  We expect that, by this magic replacement using chemical bonds, materials containing lead or other harmful elements, still being used because of the lack of alternatives, will be replaced with new more environmentally friendly materials in the future.

These achievements were published in the online version of the American scientific journal, Applied Physics Letters (10 August 2008).

Publication:
"Direct observation of oxygen stabilization in layered ferroelectric Bi_3.25La_0.75Ti₃O₁₂"
S. J. Kim, C. Moriyoshi, S. Kimura, Y. Kuroiwa, K. Kato, M. Takata, Y. Noguchi, and M. Miyayama
Applied Physics Letters 91, 062913 (2007), published online 10 August 2007

Bonding state (electron distribution) between Bi/La and O inside perovskite layer.