9528826 Goodenough Previous studies of the evolution of electronic properties have revealed that nature accommodates the first-order character of the cross-overs in a variety of ways: from localized to itinerant electronic behavior, from primarily ionic to primarily covalent metal-oxygen bonding in mixed- metal oxides, from Curie-Weiss to Pauli paramagnetism in both single-valent and mixed-valent systems. Classical phase segregation, charge-density waves, or ferroelectric displacements represent static accommodations that are well- studied. This research addresses the more subtle problem of dynamic accommodations. Initially, a major emphasis will be placed on oxides with perovskite-related structures. An ABO3 perovskite contains a BO3 array of corner-shared BO6 octahedra. A mismatch between the A-O and B-O equilibrium bond lengths may be relieved by a cooperative rotation of the BO6 octahedra that bend the B-O-B bond angle from 180 . Perovskites with transition-metal B cations may contain, depending on the strength of the B-O-B interactions, either B-cation redox couples or narrow, antibonding bands with energies close to or overlapping the Fermi energy. The strength of a B-O-B interaction and therefore the bandwidth and the band occupancy of the BO3 array may be varied by isovalent and/or aliovalent substitution on the A sites. Moreover, the ca 180 B-O-B bond may accommodate the coexistence of different B-O bond strengths within the same crystallographic structure by the displacement of a bridging atom toward one B atom and away from the other on the opposite side. These displacements are generally cooperative and may be either static or dynamic. The phenomenon of high- temperature superconductivity appears to be one example of a dynamic accommodation. Another is the apparent coexistence of Mott-Hubbard and Fermi-liquid states in the perovskite system Srl-xCaxV03 where spectral weight is transferred from the latter to the former. Several techniques are de veloped for synthesizing these marginally stable oxides, and measuring the temperature dependence of resistance, magnetic susceptibility, and Seebeck coefficient under pressure. These probes can give important, if indirect, information on dynamic processes. %%% This research is motivated by three fundamental questions: 1) What is the underlying physical process responsible for the phenomenon of high-TC superconductivity? 2) How can one use the understanding of this phenomenon as a guide in the design of improved high-TC materials? 3) What are the different ways by which nature accommodates the transition from localized to itinerant electronic behavior in solids? Initial emphasis will be placed on the study of oxides with perovskite-related structures.
