How Can We Control the “Element-Blocks” in Transition Metal Oxide Crystals?
Crystalline transition metal oxides show intriguing properties and important functionalities relevant to the electrical conduction, dielectricity, magnetism, and optical phenomenon. It seems that the concept of “element-blocks” and “element-block polymers” suitable to the organic polymers and organic-inorganic hybrid materials is not adequate at all for the consideration of structure and properties of transition metal oxide crystals in which ionic bonds between cations and oxide ions are predominant. However, it is possible to consider the oxygen polyhedron, at the center of which the transition metal ion is set, to be an element-block and to regard the whole crystal structure or a part of the structure as an element-block polymer. Also, one can modify the structure of element-block polymers inside the transition metal oxide crystals so as to change the electrical, magnetic, and optical properties drastically, although the process to realize the modification is not so sophisticated as organic polymers and organic-inorganic hybrid materials, for which a variety of chemical reactions are effectively utilized. We exemplify some transition metal oxide crystals for which the control of element-blocks is possible to achieve a drastic change in the magnetic, dielectric, and optical properties. We present three topics: (1) ferrimagnetism induced in spinel-type ZnFe2O4 by exchange of cations; (2) ferromagnetism caused by the change of cell volume or crystal system in perovskite-type EuTiO3, EuZrO3, and EuHfO3; and (3) piezoelectricity as well as optical second-order nonlinearity realized by oxygen octahedral rotation in Ruddlesden-Popper phases, NaRTiO4 (R: rare-earth element). The methods to control the element-blocks mentioned in the three topics are to change the way by which element-blocks are connected with each other, to change the chemical interaction or the overlap of atomic orbitals between element-blocks, and to make local displacement (rotation) of element-blocks to alter drastically the overall symmetry of the long-range structure, respectively.
KeywordsTransition metal oxide Magnetic properties Dielectric properties Electronic structure
We would like to thank many collaborators: Dr. Seisuke Nakashima (currently Shizuoka University); Prof. Hirofumi Akamatsu (currently Kyushu University); Dr. Yanhua Zong; Naoki Wakasugi; Yuya Maruyama; Toshihiro Kuge; Dr. Yoshiro Kususe; Hideo Murakami; Dr. Takahiro Kawamoto; Koji Iwata; Dr. Shunsuke Murai; Prof. Kazuyuki Hirao of the Department of Material Chemistry, Kyoto University; Prof. Isao Tanaka; Prof. Tomoyuki Yamamoto (currently Waseda University); Prof. Fumiyasu Oba (currently Tokyo Institute of Technology); Dr. Yu Kumagai; Dr. Hiroyuki Hayashi; Prof. Atsushi Togo of the Department of Materials Science and Engineering, Kyoto University; Prof. Venkatraman Gopalan; Prof. Long-Qing Chen; Dr. Arnab Sen Gupta; Dr. Shiming Lei; Dr. Fei Xue; Dr. Greg Stone of Pennsylvania State University; and Prof. James M. Rondinelli of Drexel University. Especially, we are indebted to Prof. Isao Tanaka and his research group members as well as Prof. Hirofumi Akamatsu for the theoretical calculations. Also, we thank Suguru Yoshida of the Department of Material Chemistry, Kyoto University, for the preparation of Fig. 15.1.
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