Unquenched orbital angular momentum in the 4d and 5d transition-metal oxides with the perovskite or the perovskite-related structure leads to a new type of Mott insulator and anomalous metallic phases. Their exotic physical properties due to strong spin-orbit coupling have attracted broad attention in recent years. The A-site cation in these perovskites does not contribute directly to the electronic states near the Fermi energy. However, as shown in high-Tc cuprates and the magnetoresistive manganites, superconductive transition temperature, the magnetic transition temperature, and the magnetization are highly sensitive to the A-cation disorder as well as the mean A-cation size. The A-cation effect on the physical properties of the 4d and 5d transition-metal oxides has not been studied systematically. Introduction of A-cation size variance and the change of mean A-cation size in a wide range is normally limited by the geometric tolerance factor at ambient pressure. A complete study of this subject requires synthesis at pressures as high as P ~ 20 GPa in some cases. The high-pressure synthesis with a large volume at P > 20 GPa has been developed in the field of geoscience to study the earth?s lower mantle. However, it remains essentially a virgin field to apply pressure over 10 GPa in a broad range of solid-state synthesis. This funding will allow the PIs not only to address key factors determining physical properties in the 4d and 5d metal oxides with unquenched orbital angular momentum, but also to explore systematically solid-state synthesis under pressure to 20 GPa.
NON-TECHINICAL SUMMARY: Perovskite oxides are technically important materials that are widely used in the microelectronic industry, in chemical plants as catalysts, and in solid-state fuel cells as both electrolyte and electrodes. The basic research outlined in this project will lead to a better understanding of perovskite oxides, which in turn benefits greatly their application. What is unique about the PIs? laboratory is that they have established several sophisticated instruments for material synthesis, characterization, and measurements and they have a long history of emphasis on the relationship between crystal structures, chemistry, their physical properties, and their engineering application. Experimental results are compared directly with theoretical calculations. The PIs emphasize high-pressure synthesis and crystal growth in the project. These characteristics have attracted scholars around the world to the laboratory. Students, post-docs, and visiting scholars with diversified backgrounds of materials science, chemistry and physics have a chance to interact with each other and to develop their own strategy of material research while working on the project. Visiting researchers have also brought with their specialties developed in their home laboratories. All these factors create a very rich environment for learning and research. In this project, the PIs also collaborate with scientists in Spain, Japan, China and Canada.
of DMR 0904282 The goal of this project is to synthesize new oxides with perovskite and perovskite-related structures under high pressure and to establish the relationship between their crystal structure and their physical properties. Transition-metal oxides with perovskite-related structures continue to drawn wide attention because of their technical as well as scientific importance. Technically, for example, they are used as components in electronic devices and in a variety of electrochemical devices. The transition between localized and itinerant d-electron behavior, for example, is not yet well-understood; it is responsible for such unusual physical properties as high-Tc superconductivity and colossal magnetoresistance. The factors that controls multiferroic behavior have yet to be clarified to where practical devices can be design. The role of spin-orbit coupling in the 4d and 5d transition metal oxides at this crossover and in the character of orbital ordering to remove an orbital degeneracy has, to date, received little attention. Moreover, the perovskite and post-perovskite phases are dominant in Earth’s lower mantle, and an understanding of their properties is important in the geosciences. High-pressure synthesis flourished in the US in the field of solid state chemistry in the 1960s and 1970s, but it, along with development of measurement under high pressure, was interrupted in the mid-1970s and fell into decline. However, high-pressure synthesis and measurement are now widely applied in Japan, Europe, and China in support of the field of materials science. Since 1990, long-term support from the NSF has enable the PIs to build up a full range of high-pressure facilities at the University of Texas at Austin. We have, for example, identified intrinsic local distortions in the orthorhombic structure, the most common form of perovskite structure. The intrinsic site distortions together with the well-recognized distortion due to octahedral-site rotations has allowed us to explain the systematic change of orbital ordering and spin ordering found in the orthorhombic RMO3 perovskites (R= rare earth, M = transition metal). Whereas most oxides with perovskite structure can be synthesized at ambient pressure, high-pressure synthesis opens an additional dimension to synthesize new compounds that can’t be made under ambient pressure, especially those oxides at the crossover from localized to itinerant electronic behavior. During the past three years, we have synthesized numerous new high-pressure phases of transition-metal oxides. Studying these new compounds has not only helped to address long-standing fundamental issues such as the ferromagnetism in the ruthenium oxides, and the anomalous electronic structure in iridium oxides, but has also deepened our understanding of how the perovskite structure responds to high pressure. High-pressure technique is an important approach in geological research. The new post-perovskite structure has been found in MgSiO3 at 120 GPa and 2400° C; the new phase has been believed to exist in the D" layer of the earth’s lowermost mantle. We have studied the perovskite to post-perovskite transition in CaIrO3 with our high pressure facilities, that led to a solution of how a highly distorted perovskite becomes unstable against the post-perovskite structure under a sufficiently high pressure. The systematic study on both perovskite and post-peroskite CaIrO3 revealed a discontinuous change of thermal conductivity on the transition from perovskite to post-perovskite structure. Since the same measurement of the post-perovskite MgSiO3 remains extremely difficulty, our result has been widely cited in the community of geophysics. Therefore, our research also has an important impact on the geosciences. In addition to highly productive research, we have trained graduate students with modern high-pressure technology and its application in material-science research.
