open access

https://doi.org/10.14943/doctoral.k13807

https://hdl.handle.net/2115/79299

ペロブスカイトに関連する水銀およびオスミウム酸化物の高圧合成と結晶構造と物性

英語

magnetic property

2019-09-25

Hokkaido University

vii, 153p

Transition metal oxides with partially filled 3d and 4d shells have dominated materials research in past decades, while 5d oxides are being of great current interest as 5d oxides exhibit spectacular phenomena. The unique properties of 5d oxides stem from the nature of 5d electrons themselves. On one hand, upon descending the periodic table from the 3d to 5d series, the d orbitals become spatially extended, resulting in smaller on-site Coulomb repulsion for 5d electrons. On the other hand, the spin-orbit coupling increases considerably in 5d orbital, leading to enhanced splitting degenerate orbitals and bands. The interplay between spin-orbit coupling, Coulomb repulsion and crystal-electric field can lead to novel phases of matter. In this thesis, eight 5d oxides were synthesized under high-pressure and high-temperature conditions to elucidate the novel physical properties involving 5d oxides. Chapter 1 is the introduction of background behind this thesis. Chapter 2 provides information about the experimental methods and technologies used in this thesis. In Chapter 3, HgPbO3 is introduced, which was synthesized at 6 GPa and its crystal structure was studied by single-crystal X-ray diffraction and powder synchrotron X-ray diffraction measurements. HgPbO3 crystallizes into a hexagonal structure (R-3m) with a = 5.74413(6) Å and c = 7.25464(8) Å. The metallic behavior was observed from temperature dependence of resistivity measurement for HgPbO3, but its weak temperature dependence of the electrical resistivity, significant diamagnetism, and remarkably small Sommerfeld coefficient imply that HgPbO3 is a semimetal. The discovery of semimetal in oxide compounds is quite rare, which could be useful for the advanced material research and the development of multifunctional devices. Chapter 4 summarizes two new osmium triple perovskite oxides synthesized at 6 GPa and 1100 °C; a polymorph of the triple perovskite Ba3CuOs2O9, which exists in the orthorhombic phase (Cmcm), is reported. The crystal structure of Ba3CuOs2O9 transfers to a II hexagonal structure (P63/mmc) under a high-pressure and high-temperature condition. Although Ba3CuOs2O9 maintains the 6H perovskite-type lattice basis, the distribution of Cu and Os atoms are dramatically altered over the lattice. The new distribution brings about distinct magnetic properties; for example, the hexagonal Ba3CuOs2O9 exhibits a ferrimagnetic transition at 290 K in stark contrast to the antiferromagnetic transition at 47 K exhibited by the orthorhombic Ba3CuOs2O9. Furthermore, it was achieved by replacement of Cu by Ni that the Tc was successfully increased to 370 K for Ba3NiOs2O9. In this chapter, a new class of 3d-5d hybrid ferrimagnetic materials with high-Tc emerged from the application of high-pressure technique. Ba3CuOs2O9 as well as Ba3NiOs2O9 could serve as a worthwhile platform for developing high-Tc ferrimagnetic material. In Chapter 5, the study of substitutional series double perovskite oxides Sr2Cr1-xNixOsO6 (x = 0, 0.25, 0.5, 0.75 and 1) prepared at 6 GPa is presented. Sr2Cr1-xNixOsO6 (x = 0, 0.25, 0.5, 0.75 and 1) exhibit structural transition from cubic to tetragonal at x = 0.5. The former three samples (x = 0, 0.25 and 0.5) exhibit ferrimagnetic ordering below Tc, while the other two Ni-rich samples (x = 0.75 and 1) are antiferromagnetic. Remarkable variation of the magnetic properties among the five samples seems not to be simply explained by the structural transition, but rather by 5d electron nature. The series of Ni-substituted Sr2CrOsO6 likely gives an insight into the effect of spin-orbital coupling on the novel physical properties. In Chapter 6, the general conclusion is provided, and prospects for future research of 5d oxide are presented.

2019-09-25

甲第13807号

博士(理学)

(主査) 特任教授 武田 定, 客員教授 野口 秀典, 客員教授 山浦 一成, 准教授 長浜 太郎, 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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