SPring-8, the large synchrotron radiation facility

RIKEN (Ryoji Noyori, President) and the Japan Synchrotron Radiation Research Institute (JASRI; Tetsuhisa Shirakawa, President) succeeded in clarifying the three different phases of Magneli phase titanium oxide (Ti₄O₇), which exist at different temperatures, using the world's highest performance X-ray photoelectron spectrometer^*1 at SPring-8 and the world's highest resolution laser photoelectron spectrometer^*2 at The Institute for Solid State Physics of The University of Tokyo. This was achieved through a joint research by Shik Shin, leader of the Excitation Order Research Team, Quantum Order Research Group, RIKEN SPring-8 Center (Tetsuya Ishikawa, Director) and a professor of the Institute for Solid State Physics, The University of Tokyo; Munetaka Taguchi, a research scientist in Prof. Shin's team; Tetsuya Ishikawa, chief scientist of the coherent X-ray optics laboratory, RIKEN SPring-8 Center; Hidenori Takagi, a professor at the Graduate School of Frontier Sciences, The University of Tokyo; Haruhiko Ohashi, an associate chief scientist at JASRI; Yasunori Senba, a research scientist at JASRI, and their colleagues.

Applications of titanium oxide in new fields, such as hydrophilic and water-repellent materials, have been developed in addition to its use as a white pigment material and in photocatalysts and photoelectrodes. Researchers have been intensively carrying out fundamental and applied research on titanium oxide. In particular, Magneli phase titanium oxides with the composition of Ti_nO2_n-1 (n = 3-9) have a unique structure and characteristics, such as a metal-nonmetal transition, metallic properties and antiferromagnetism^*3 even at extremely low temperatures. Among the Magneli phase titanium oxides, Ti₄O₇ has the highest electrical conductivity (2.75 times that of carbon) and exhibits two significant changes in electrical resistance (increases or decreases by three orders of magnitude) when its temperature is changed. It is known that Ti₄O₇ has three phases, i.e., a low-temperature phase (approximately -133 °C or lower), a high-temperature phase (approximately -119 °C or higher), and an intermediate phase. Therefore, Ti₄O₇ has been attracting attention both inside and outside Japan as a promising material for next-generation high-efficiency fuel cells that can contribute to a low-carbon society. Thus far, although research on the low-temperature phase is already underway, the electron state and electrical conduction mechanism of the high-temperature and intermediate phases have remained a mystery.

The research group determined previously unknown characteristics of Ti₄O₇ using various types of X-rays. The results indicate that the characteristics of the high-temperature phase are almost the same as those of general metals; furthermore, it was clarified that the intermediate phase between the high-temperature metal phase and the low-temperature semiconductor phase exhibits anomalous characteristics unlike those of metals and semiconductors. This achievement is expected to be an important guideline for the use of titanium oxides, such as Ti₄O₇, as materials in next-generation fuel cells.

The achievements were published online in the American scientific journal, Physical Review Letters, on 8 March 2010, prior to the printed version published on 12 March 2010.

Publication:
"Anomalous State Sandwiched between Fermi Liquid and Charge Ordered Mott-Insulating Phases of Ti₄O₇"
M. Taguchi, A. Chainani, M. Matsunami, R. Eguchi, Y. Takata, M. Yabashi, K. Tamasaku, Y. Nishino, T. Ishikawa, S. Tsuda, S. Watanabe, C.-T. Chen, Y. Senba, H. Ohashi, K. Fujiwara, Y. Nakamura, H. Takagi, and S. Shin.
Physical Review Letters, 104, 106401 (2010), published online 8 March 2010.