Metal oxides containing ruthenium have a surprisingly varied range of low-temperature physical properties including the superconductor Sr2RuO4 and the metallic ferromagnet SrRuO3. The exceptionally broad manifold of itinerate and localised electronic phenomena is derived from broad Ru 4d bands and a wide number of Ru and O containing crystallographic structures. High-pressure is a powerful tool for manipulating crystal structures and tuning the associated electronic and magnetic properties. In this body of work, pressure has been utilised in two roles for the study of ternary ruthenates with perovskite and pyrochlore structures; as a synthetic method and for in situ studies (crystallographic or physical) of existing compounds. Chapter 3 details pressure dependent changes in the crystal structure of the perovskite PbRuO3 by powder x-ray diffraction up to 46 GPa and down to 20 K. PbRuO3 transformed on cooling, from orthorhombic spacegroup (Pnma) to an orbitally ordered low temperature phase, which is also an orthorhombic space group (Imma) and applied pressure reduced the critical temperature totally inhibiting the transition at 5.5 GPa. Additionally PbRuO3 was found to undergo a reversible pressure induced structural phase transition at 30 GPa and 290 K with a 10 % reduction in unit-cell volume. Indexing indicated an orthorhombic symmetry with a Pnna spacegroup. Pnna is not a spacegroup associated with perovskite or related perovskite structures despite the √2 x 2 x √2 perovskite superstructure being maintained across the transition. high-pressure resistivity and Raman measurements indicated that a metal-insulator transition accompanied the structural transition. In Chapter 4 high-pressure high-temperature (HP-HT) synthesis has been used to isolate dense phases that could not be produced at ambient pressure. The ortho-perovskite LaRuO3 with space group Pnma (# 62) was synthesised by conventional solid state methods. However to extend the series by substituting the smaller rare-earth cations, Ln3+ on the A-site of the same perovskite structure HP-HT (10 GPa and 1200° C) conditions were required. A powder diffraction study confirmed the Pnma structure of LnRuO3 where, Ln = Pr, Nd, Sm, Eu, Gd, Dy and Ho, of which the later rare-earth compounds, where Ln = Sm to Ho have not been synthesised before. Neutron powder diffraction studies of LnRuO3 where Ln = La, Pr and Nd down to 7 K suggests a ~ 10 % non-stoichiometry on the Ru site, leading to the adjusted formula LnRu0.9O3 with an unusually low Ru3.3+ valency. A possible exception to the low Ru oxidation state is EuRuO3, which has a larger unit-cell, suggesting a Eu2+Ru4+O3 charge distribution with the more common Ru4+, however, this is not concordant with magnetisation measurements. Additionally neutron diffraction suggests that the RuO6 octahedra are distorted by spin-orbit coupling. Magnetometry and resistivity measurements indicate that the compounds are semiconducting paramagnets down to 7 K. Finally in Chapter 5 is presented the analysis of a high-pressure powder x-ray diffraction experiment of the pyrochlore Tl2Ru2O7. Carried out at synchrotron facilities, we have extended the pressure-temperature phase diagram to 3.7 GPa and 25 K. Previously it had been reported that, when cooled, Tl2Ru2O7 undergoes a structural phase transition from a cubic (Fd-3m) phase to a low temperature, orthorhombic (Pnma) phase that forms Haldane chains - an unusual one-dimensional orbital ordering. As for PbRuO3 high-pressure conditions are found to inhibit the orbital ordering, to reduce the critical temperature and to suppress the transition at pressures exceeding 3.0 GPa.

A Walker-type multianvil high-pressure facility is capable of high-pressure syntheses and measurements beyond 10 GPa and has been utilized in my research to synthesize the 4d Ruthenium and Rhodium and the 5d Iridium oxides with the perovskite-related structures. Under high-pressure and high-temperature conditions, these families of oxides can be enlarged to a great extent so that enables us not only to address the long-standing problem about ferromagnetism in the perovskite ruthenates but also explore new phenomena associated with the structural and electronic properties in the iridates and rhodates. In the perovskite ruthenates ARuO₃ (A= Ca, Sr, and Ba), a systematic study of the variations of the ferromagnetic transition temperature T[subscript c] and the critical isothermal magnetization as a function of the average A-site cation size and the size variance as well as external high pressures reveals explicitly the crucial role of the local lattice strain and disorder on T[subscript c] and the nature of the localized-electron ferromagnetism. However, such a steric effect is dominated by the electronic effect in another perovskite ruthenate PbRuO₃, which is a paramagnetic metal down to 1.8 K and undergoes a first-order structural transition to a low-temperature Imma phase at Tt [almost equal to] 90 K. Bandwidth broadening due to orbital hybridization between Pb-6s and Ru-4d plays an important role in suppressing the ferromagnetism in the Sr1-zPbzRuO₃ system. The high-pressure sequence of the 9R-BaIrO₃ was explored and three more polytypes, i.e. 5H, 6H and 3C, were identified under 10 GPa. With increasing fraction of the corner- to face-sharing IrO₆/₂ octahedra, the ground states of BaIrO₃ evolve from a ferromagnetic insulator with T[subscript c] [almost equal to] 180 K in the 9R phase to a ferromagnetic metal with T[subscript c] [almost equal to] 50 K in the 5H phase, and finally to an exchange-enhanced paramagnetic metal near a quantum critical point in the 6H phase. In addition to the perovskite SrRhO₃, a new 6H polytype was synthesized for the first time under high pressure and a pressure-temperature phase diagram was given for the 6H-perovskite transformation. Restoration of the Curie-Weiss behavior in the high-temperature magnetic susceptibility [chi](T) of the perovskite SrRhO₃ resolves the puzzle about unusual dependence of [chi]⁻¹ [symbol] T² reported earlier and highlights the importance of spin-orbit coupling in the 4d and 5d transition-metal oxides. / text

With the discovery of appreciable room temperature magnetoresistance (MR) in high Curie temperature (Tc) ferrimagnetic double perovskites such as Sr2FeMoO6, research surrounding other materials of this type has expanded. Most ferrimagnetic double perovskites of the formula A2BB'O6 have non-magnetic A-site cations, such as Sr2+, Ca2+ or Ba2+. Replacing non-magnetic cations with magnetic variants offers further possibilities to tune magnetic effects. This thesis focuses on the substitution of non-magnetic A-site cations with relatively small magnetic Mn2+ cations. This substitution is made possible through the use of high-pressure/temperature (P/T) synthesis, and the characterisation of structural and magnetic properties of new phases discovered through these syntheses. The first of these new phases to be reported herein is Mn2FeReO6, which can be described as the Mn analogue of the well-known ferrimagnetic double perovskite Ca2FeReO6. These materials are well ordered with Fe3+/Re5+ on B-sites and crystallise in a P21/n structure. Mn2FeReO6 shows a high Tc of 520 K due to ferrimagnetic Fe/Re magnetic order above RT, and a large saturated magnetisation of 5.0 μB, which peaks at 75 K. Interestingly, the A-site Mn2+ (3d5) magnetic order has the effect of causing a spin reorientation of the Fe/Re sublattice observed by neutron powder diffraction (NPD) at temperatures below ~75 K. This causes the MR to exhibit the expected negative intergrain tunnelling behaviour above the transition and colossal positive behaviour below. Also reported are a series of perovskite related structures with formulae CaxMn2- xFeReO6 (x = 0.5, 1.0, 1.5). Of particular note among these is CaMnFeReO6, which exhibits 1:1 A-site ordering of Ca/Mn and adopts the P42/n space group. This material belongs to a family of newly discovered 'double double' perovskites, in which Ca/Mn order in columns pointing along the c-axis and Mn has alternating tetrahedral and square planar coordination environments. MR in this material remains negative down to 20 K, potentially due to the presence of Ca disrupting magnetic interactions between Mn2+ cations and suppressing the spin transition. Alternating coordination environments in the double double perovskite structure type were exploited in the synthesis of Ca(Mn0.5Cu0.5)FeReO6. This material also crystallises in the P42/n structure and is well ordered on B-sites, as evidenced by X-ray powder diffraction. Neutron diffraction yields, in addition to columnar order, a slight preference for Cu to occupy the square planar sites and for Mn to occupy tetrahedral sites. This doping of square planar sites with Cu has the effect of enhancing magnetic properties compared to CaMnFeReO6, increasing the saturated magnetisation, raising the ferrimagnetic ordering temperature of the B-sites from 500 to 560 K, and also having a profound effect on the observed MR effects, as a switch in the sign of the MR is observed in this material through a magnetic transition. Finally, B-site substitution has been experimented with in the synthesis of CaMnMnReO6. This also possesses the combined A and B-site orders observed in CaMnFeReO6 and an unusual magnetic structure, with perpendicular A and B-site magnetism due to frustration, deviating greatly from the magnetic structures of materials with B-site Fe/Re. In summary, this thesis compiles the synthesis and analysis of a series of new double perovskites, double double perovskites and a new 'triple double' five-fold cation ordered structure with a general formula of AA'0.5A''0.5BB'O6. These materials show that new types of structural ordering can be used to increase the number of degrees of freedom available for tuning the interplay between many different magnetic cations in different coordination environments.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20763号 / 工博第4415号 / 新制||工||1686(附属図書館) / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 陰山 洋, 教授 阿部 竜, 教授 江口 浩一 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24235号 / 工博第5063号 / 新制||工||1790(附属図書館) / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 陰山 洋, 教授 藤田 晃司, 教授 作花 哲夫 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

The rare earth pyrochlore oxides, with formula R2B2O7, are a chemically versatile family of materials that exhibit a diverse array of magnetic phenomena. In this structure the R and B site cations each form a corner-sharing tetrahedral network, a motif that is prone to intense geometric magnetic frustration. As a consequence of their magnetic frustration, rare earth pyrochlores are observed to host a number of remarkable states such as spin ice and spin liquid states. In this thesis we endeavor to explore the phase diagrams of the rare earth pyrochlores through the lens of chemical pressure. Chemical pressure is applied by varying the ionic radius of the non-magnetic B site cation, which either expands or contracts the lattice, in analogy to externally applied pressure. We apply positive chemical pressure by substituting germanium at the B site and negative chemical pressure by substituting lead at the B site. We also consider the effect of platinum substitution, which has nominally negligible chemical pressure effects. In the ytterbium pyrochlores, we find that positive chemical pressure tunes the magnetic ground state from ferromagnetic to antiferromagnetic. Remarkably, we also find that the ytterbium pyrochlores share a ubiquitous form to their low temperature spin dynamics despite their disparate ordered states. In the terbium pyrochlores, we find that positive chemical pressure promotes ferromagnetic correlations - the opposite effect of externally applied pressure. Our studies of platinum pyrochlores reveal that platinum, while non-magnetic, is able to facilitate superexchange pathways. Thus, the magnetic ground states of the platinum pyrochlores are significantly altered from their titanate analogs. The work in this thesis highlights the delicate balance of interactions inherent to rare earth pyrochlore magnetism and shows that chemical pressure is a powerful tool for navigating their phase spaces. / Thesis / Doctor of Philosophy (PhD) / Rare earth pyrochlores have the chemical formula R2B2O7, where R is a magnetic rare earth element and B is a non-magnetic element. Materials of this type are widely studied because they have a propensity to exhibit exotic magnetic properties. In this thesis, we study the effect of varying the size of the non-magnetic B site atom, which is termed chemical pressure. As B is made larger or smaller, the crystal lattice expands or contracts, mimicking the effect of externally applied pressure. High-pressure synthesis techniques were used to prepare R2B2O7 compounds with B site cations that are typically too small (germanium), too large (lead), or too unstable (platinum) under ambient pressure conditions. Our characterizations of these high-pressure materials have revealed that their magnetism is remarkably sensitive to the application of chemical pressure.

京都大学 / 新制・論文博士 / 博士(工学) / 乙第13457号 / 論工博第4197号 / 新制||工||1770(附属図書館) / (主査)教授 陰山 洋, 教授 安部 武志, 教授 江口 浩一 / 学位規則第4条第2項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24629号 / 工博第5135号 / 新制||工||1981(附属図書館) / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 陰山 洋, 教授 安部 武志, 教授 藤田 晃司 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

The perovskite structure has an incredible versatility that results in myriad compounds with varied and eccentric behaviors. Perovskite oxides have been extensively studied and used for over 60 years. In order to expand on our already thorough knowledge of these compounds, it is necessary to use modern and creative experimental techniques. High-pressure synthesis and high oxygen-gas pressure annealing techniques are used to synthesize oxygen stoichiometric RNiO₃ (R = lanthanide). The particularly rich phase diagram of this compound allows for the study of the crossover from localized to itinerant electronic behavior and from an enhanced Pauli to a Curie-Weiss law paramagnetism. Single crystals of RFeO₃ are grown in order to analyze the spin canting in these antiferromagnetic samples. The size of the rare earth-cation is used to tune the magnitude of octahedral-tilt distortions. This tuning allows distinguishing between the two possible drivers for spin canting and weak ferromagnetism in these compounds, the octahedral-tilt-dependent single-ion anisotropy and the octahedral-tilt-independent Dzyaloshinskii-Moriya interaction. Although it is a fluoride compound, KCuF₃ has been used as an analogue to transition-metal oxide perovskites such as LaMnO₃ because of the similarity of their orbital ordering. Through the use of high-temperature neutron diffraction, it is shown that the orbital ordering and Jahn-Teller distortion in this compound are not lifted at the predicted temperature. Another mechanism for orbital ordering is identified. La₂[subscript-x] Sr [subscript x] CuO₄ has long been of interest as the progenitor system of the highTc superconductors. Despite having an exceedingly well-studied phase diagram in the over-doped region of its superconducting dome, little is known about this system in the region x > 0.3 because of the difficulty of synthesizing fully oxygen-stoichiometric samples. With high-oxygen-gas-pressure annealing and high-pressure synthesis, the completion of the phase diagram up to x = 1.0 is attempted. Finally, like many iridates, post-Perovskite CaIrO₃ exhibits a very strong spinorbit coupling of its 5d electrons. Because its magnetism is very weak, traditional methods to measure the magnitude of its orbital moment and spin-orbit coupling, such as neutron powder diffraction, are not viable. In order to address this issue, direct measurement of the orbital moments was conducted by using x-ray absorption spectroscopy and x-ray magnetic circular dichroism techniques. / text

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18995号 / 工博第4037号 / 新制||工||1621(附属図書館) / 31946 / 京都大学大学院工学研究科材料化学専攻 / (主査)教授 田中 勝久, 教授 平尾 一之, 教授 三浦 清貴 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM