Browsing by Subject "High-pressure synthesis"
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Item The high pressure synthesis, crystal growth and physical properties of transition metal perovskites(2013-12) Marshall, Luke Gordon; Goodenough, John B.; Zhou, JianshiThe 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.Item High-pressure synthesis of the 4d and 5d transition-metal oxides with the perovskite and the perovskite-related structure and their physical properties(2010-05) Cheng, Jinguang; Zhou, Jianshi; Goodenough, John B.; Manthiram, Arumugam; Lozanne, Alex De; Markert, John T.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.