open access

https://doi.org/10.14943/doctoral.k13361

https://hdl.handle.net/2115/71985

R2O3-Mn2O3系ペロブスカイトの高圧合成と結晶構造と物性

英語

Magnetic property

2018-09-25

Hokkaido University

VI, 150p

Perovskite-type manganese oxide has attracted enormous attention for decades because of its useful interplay between spin, charge, and orbital degrees of freedom. It led to discovery of colossal magnetoresistance, spin-driven multiferroics, and other interesting physical properties. In this study, I investigated the perovskite-type manganese oxide in chemical systems between R2O3 (R = rare earth elements) and Mn2O3. Novel compositions were discovered by applying a high-pressure and high-temperature synthesis method, and those crystal structures and fundamental physical properties were clarified. Chapter 1 introduces the general background, and chapter 2 explains the principal technologies used in this study. Chapters 3 to 5 consist of the main outcomesand the conclusion is drawn inchapter 6. Chapter 3 describes unusual five-fold cation/charge ordering in high-pressure-synthesized RMnO3 perovskites of R = Gd-Tm and Y (at 6 GPa and 1673 K). The R3+, Mn2+, and Mn3+ cations are ordered at the perovskite A site in two separate chains consisting of R3+ and alternating Mn2+ (in tetrahedral coordination) and Mn3+ (in square-planar coordination). Mixed-valent Mn3+/Mn4+ cations are ordered in layeres at the perovskite B site; the ordering can be described as [R3+Mn2+0.5 Mn3+0.5]A[Mn3+Mn3.5+]BO6. The triple cation ordering observed at the A site is very rare, and the layered double B-site ordering is also scarcely seen. RMn3O6 crystallizes in space group Pmmn with a = 7.2479(2) Å, b = 7.4525(3) Å, and c = 7.8022(2) Å for DyMn3O6 (at 213 K), being structurally related to CaFeTi2O6. It appeared that they are non-stoichiometric and can be described as R1-δMn3O6-1.5δ with δ = -0.071 to -0.059 for R = Gd, δ = 0 for Dy, δ = 0.05-0.1 for Ho and Y, and δ = 0.12 for Er and Tm. All of them show complex magnetic behaviors with several transitions, and the magnetic properties are highly sensitive to the δ values. Chapter 4 introduces a quadruple perovskite CeCuMn6O12 synthesized under high-pressure and high-temperature conditions at 6 GPa and about 1670 K. CeCuMn6O12 crystallizes in space group Im-3 above 297 K and R-3 below the temperature with the 1:3 (Mn4+:Mn3+) charge and orbital orders. Unusually compressed Mn3+O6 octahedra found in CeCuMn6O12 closely resembles what was found for the -Q3 Jahn-Teller distortion mode in CaMn7O12. Below approximately 90 K, structural instability results in a phase separation andappearance of competing phases; and below 70 K, two R-3 phasescoexist. CeCuMn6O12 exhibits a ferrimagnetic-like transition below Tc of 140 K, and it is semiconducting with the magnetoresistance reaching -40 % (at 140 K and 70 kOe). Chapter 5 reports orthorhombic rare-earth trivalent manganites RMnO3 (R = Er – Lu), which were self-doped with Mn to form (R0.667Mn0.333)MnO3 compositions. The series was synthesized by a high-pressure and high-temperature method at 6 GPa and about 1670 K from R2O3 and Mn2O3. The average oxidation state of Mn is +3 in (R0.667Mn0.333)MnO3. However, Mn enters the A-site in the oxidation state of +2 creating the average oxidation state of +3.333 at the B site. The presence of Mn2+ was confirmed by hard X-ray photoelectron spectroscopy measurements. Crystal structures were studied by synchrotron X-ray powder diffraction. (R0.667Mn0.333)MnO3 crystallizes in space group Pnma with a = 5.50348(2) Å, b = 7.37564(1) Å, and c = 5.18686(1) Å for (Lu0.667Mn0.333)MnO3 at 293 K, and they are isostructural with the parent RMnO3 manganites. By comparing with RMnO3, (R0.667Mn0.333)MnO3 exhibits enhanced Néel temperatures of about TN1 = 106-110 K and canted antiferromagnetic properties. Compounds with R = Er and Tm show additional magnetic transitions at about TN2 = 10-15 K. (Tm0.667Mn0.333)MnO3 exhibits a magnetization reversal effect with the compensation temperature of 15 K. Chapter 6 provides the general conclusion and future prospects.

2018-09-25

甲第13361号

博士(理学)

(主査) 教授 武田 定, 教授 忠永 清治, 客員教授 山浦 一成, 准教授 長浜 太郎

総合化学院（総合化学専攻）

博士論文

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