Browsing by Subject "Perovskite oxides"

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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.
Investigations of cobalt-based oxides as cathode materials for intermediate-temperature solid oxide fuel cells
(2012-08) Li, Yan, doctor of materials science and engineering; Goodenough, John B.; Zhou, Jianshi; Manthiram, Arumugam; Ferreira, Paulo J.; Mullins, Charles B.
Three cobalt-based oxides operating at the Co(III)/Co(II) redox couple have been investigated as potential cathode materials for the intermediate-temperature solid oxide fuel cells (IT-SOFCs). X-ray absorption spectroscopy measurements confirmed that both the oxygen-deficient perovskite Sr[subscript 0.7]Y[subscript 0.3]CoO[subscript 2.65-delta] (SYCO) and the double-perovskite Ba₂[Co][Bi[subscript x]Sc[subscript 0.2]Co[subscript 1.8-x]][subscript O6-delta] (x = 0.1 and 0.2) (BBSC) contain high-spin Co(III) in the bulk at room temperature and thus avoid the thermally driven spin-state crossover of the Co(III) ions usually observed in other cobalt-containing perovskite oxides. Electrochemical characterizations demonstrated that both cobalt oxides operating on the Co(III)/Co(II) redox couple are equally catalytically active for the oxygen reduction reaction as those operating on the Co(IV)/Co(III) redox couple. With an LSGM electrolyte-supported single test cell and NiO+GDC as anode, the maximum power densities Pmax at 800 ºC reach 927 and 1180 mW·cm⁻² for SYCO and BBSC cathodes, respectively. The oxygen-deficient perovskites Sr[subscript 1-x]R[subscript x]CoO[subscript 3-delta] (R = Eu-Ho, Y, x [approximately equal] 0.3) are identified as a new class of cathode materials for IT-SOFCs in this dissertation. On the other hand, the layered Ba2Co9O14 (BCO) containing the low-spin Co(III) at room temperature undergoes a thermally driven spin-state crossover, which has prevented it from being evaluated as the cathode of IT-SOFCs. This problem was overcome by fabrication of a 50-50 wt.% BCO + SDC (Sm[subscript 0.2]Ce[subscript 0.8]O[subscript 1.9]) composite cathode. The addition of SDC not only improved the adhesion to the electrolyte, but also enhanced the electrocatalytic activity for the oxygen reduction reaction. The composite cathode delivers a nearly stable P[subscript max] of ~450 mW·cm-2 at 800 °C in an LSGM electrolyte-supported single test cell. In addition, the electrochemical lithium intercalation process in the monoclinic Nb12O29 was studied with a Li/Nb₁₂O₂₉ half-cell, and the results showed that it can reversibly incorporate a relatively large amount of Li-ions in the voltage window of 2.5-1.0 V at a slow discharge/charge rate while retaining structural integrity. Compared with that of the bare Nb₁₂O₂₉, samples with carbon coating show an improved rate capability. The lithium insertion mechanism into Nb₁₂O₂₉ has also been discussed in terms of sites available to the lithium ions
Low temperature scanned probe microscopy studies of magnetic oxides
(2011-05) Lee, Alfred K.; de Lozanne, Alejandro L.; Shih, Chih-Kang; Markert, John; Demkov, Alexander; Ruoff, Rodney
This dissertation is divided into two parts. In the first, the general paradigm of scanned probe microscopy is outlined with a focus on atomic force microscopy and a few of its variations. Magnetic force microscopy is covered in detail as it forms the basis of the second part of this dissertation. The core elements and extra features of the instrument are described with attention paid to the upgrades made by the author. In the second part of this dissertation, background information on perovskite oxides and the inverse spinel system, magnetite, is given. Magnetic force microscopy studies were done on three thin film systems and are detailed. In the first study, ferromagnetic manganite films were subjected to discontinuous changes in strain due to structural transitions in their barium titanate substrates. The resulting effect on the magnetic domains was observed. In the second study, the ferromagnetism of a tensile-strained LaCoO₃ film was studied across temperatures from 4.3 K to 90 K and applied fields up to [mu]₀H=1.1 T. Finally, the properties of antiphase domains in magnetite films of varying film strain due to transition metal buffer layers was probed by imaging the magnetic domains which are pinned to the antiphase boundaries.