open access

https://doi.org/10.14943/doctoral.k15680

https://hdl.handle.net/2115/91206

ペロブスカイト型5d遷移金属酸化物の高圧合成、結晶構造と物性

英語

5d transition metal Oxides

2023-12-25

Hokkaido University

Ⅲ, Ⅲ, 128p.

Perovskite-related materials have gained significant popularity in recent years due to their unconventional and outstanding physical and chemical properties, including ferroelectricity, multiferroicity, magnetoresistance, and exchange bias effects. Notably, materials related to perovskites that incorporate 5d elements have emerged as a leading and potentially applicable research area in inorganic and solid-state chemistry. This is attributed to the competition between local and itinerant d electrons, which can yield a range of beneficial characteristics, including frustrated magnetism, metal-insulator transitions, high-Tc superconductivity, and more. In this study, I successfully synthesized several 5d perovskiterelated compounds using the high-pressure high-temperature method and subsequently investigated their crystal structures, as well as their magnetic and electrical transport properties. Chapter 1 provides an overview of the research background, encompassing the fundamental crystal structure, mechanisms related to magnetism, material conductivity models referenced in this work, and the historical significance of high-pressure technology in materials science. Chapter 2 predominantly outlines the experimental details and characteristic techniques employed in this study. Chapter 3 presents the investigation of two double perovskite oxides, Cd2FeOsO6 and Cd2FeReO6, obtained under high-temperature and high-pressure conditions, with a space group of P21/n. Structure and composition determinations were confirmed by synchrotron powder Xray diffraction and electron probe microanalysis. Magnetic and electronic property measurements revealed that Cd2FeReO6 exhibits ferrimagnetic order well above room temperature at ~460 K, with a soft ferrimagnetic feature. Additionally, a tunneling-type magnetoresistance as large as 37% was discovered in Cd2FeOsO6, while first-principles calculations indicated its half-metallic state. In Cd2FeOsO6, significant exchange bias (EB) effects were detected under remarkably small cooling fields, revealing a unique mechanism　and providing valuable guidance for the advancement of materials exhibiting significant EB effects. Both materials behave as semiconductors, in accordance with a three-dimensional variable-range hopping transport model. These results not only provide a new platform for enhancing advanced EB-effect materials but also demonstrate that A-site Cd-occupied double perovskites constitute a promising class of oxides for exploring new materials with practical significance, particularly in the realm of soft and hard magnets or in the field of spintronics. Chapter 4 primarily focuses on the KSbO3-type Fe-doped Bi3Re3O11 and Bi3Os3O11 compounds, synthesized under high-pressure and high-temperature conditions. Doping with Fe up to 29 atomic% was achieved under these conditions. The crystal structures and chemical compositions of Bi3Os2.45Fe0.55O11 and Bi3Re2.13Fe0.87O11 were determined through synchrotron powder X-ray diffraction and electron probe microanalysis. Both crystal structures were elucidated using a KSbO3-type model with the space group Pn-3. Measurements of magnetic and electronic transport properties revealed that Bi3Os2.45Fe0.55O11 exhibited a ferrimagnetic transition at the highest magnetic ordering temperature of 490 K in the KSbO3-type structure, while Bi3Re2.13Fe0.87O11 displayed a spin-glass behavior below 22 K. The magnetoresistance at 5 K and 90 kOe was nearly zero for Bi3Os2.45Fe0.55O11, but -10% for Bi3Re2.13Fe0.87O11. These findings suggest that KSbO3-type 5d oxides, which have thus far exhibited only weak temperature-dependent paramagnetism, represent a class of compounds that can be transformed into spintronic materials through doping with 3d elements, thereby paving the way for the development of new KSbO3-type materials with both theoretical and practical significance. Chapter 5 introduces ABO3-type perovskite oxides MnV0.5Nb0.5O3 and MnV0.5Ta0.5O3, prepared under high-pressure and high-temperature conditions. Structure analysis using synchrotron XRD patterns revealed that both of them crystallize in GdFeO3-type structure with the space group Pnma. Magnetic measurements confirmed that MnV0.5Nb0.5O3 and MnV0.5Ta0.5O3 order ferrimagnetically at 17 and 18K, respectively. Both phases are semiconductors, with activation energies of 0.13 eV and 0.31 eV. Chapter 6 presents the overall conclusions and future prospects based on this body of work.

2023-12-25

甲第15680号

博士(理学)

(主査) 教授 松井 雅樹, 教授 忠永 清治, 客員教授 山浦 一成, 客員准教授 辻本 吉廣

総合化学院（総合化学専攻）

博士論文

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