Introduction:
Transition metal oxides represent a large class of compounds with a uniquely wide range of electronic properties. Some of these properties, like the magnetism of loadstone, have been known since antiquity. Others, like high-temperature superconductivity have been discovered only recently and indeed would have been thought of being impossible 20 years ago. Transition metal oxides may be good insulators, semiconductors, metals or superconductors. Many of them display a metal-to-insulator transition (MIT) as a function of an external control parameter (usually temperature, pressure or chemical composition). The differences of electrical conductivity are also reflected by drastic changes of other physical properties related to the electronic structure.
The electrical, magnetic and optical properties of transition metal oxides find a rich field of important technical applications. A classical example is the wide use of ferrites in electronic devices. Further examples of suitable technological applications include wide gap semiconductors, superconductors and thermoelectric materials, to mention just a few. Apart from these exciting electronic properties, some transition metal oxides exhibit a remarkable mechanical and high-temperature stability together with a strong resistance against corrosion, thus forming ideal coating materials. Several transition metal oxides may also serve as catalysts. It was the discovery of high-temperature superconductivity in the cuprates and, subsequently, of the colossal magneto-resistance effect (CMR) in the manganates that triggered a tremendous research effort in transition metal oxides during the last decade. It also demonstrates that these well-known compounds reveal new, puzzling and fascinating properties implying both a challenge of our physical understanding and the promise to adequately respond to an increasing demand of advanced materials in industry and technology.
The rich diversity of physical properties of the transition metal oxides originates from the cooperative behavior of different microscopic degrees of freedom of the electronic system (charge, orbital and spin degrees of freedom) and its coupling to the lattice. Based on the long-range Coulomb repulsion and the Pauli exclusion principle electronic correlations play a crucial role in determining the electronic structure. A proper description of the electronic structure represents a formidable quantum mechanical many-body problem of strongly interacting particles. The challenge is to isolate the underlying physics against a background of irrelevant complexity. Transition metal oxides represent an ideal laboratory for the study of electronic correlations since dramatic changes of behavior can be induced by relatively small and controllable changes in families of closely related materials.
A characteristic feature of the transition metal oxides is the intimate relationship of electronic and lattice degrees of freedom. This is reflected by many cases in which an electronic (magnetic) transition is accompanied by a change of the crystallographic structure. Though these structural changes are most commonly rather slight distortions of a parent structure, they may lead to drastic changes of the physical properties. A proper characterization of the crystallographic structure is therefore an essential prerequisite for meaningful interpretations of electronic properties. Therefore, in the next chapter a short survey of the crystal structures of transition metal oxides and their mutual relationships is given. The structural details as determined by extensive x-ray and neutron diffraction experiments and its physical consequences for the investigated compounds are presented in the corresponding chapters. After focusing on the crystallographic aspects of transition metal oxides in chapter 2, subsequently we summarize the electronic interactions in these compounds in chapter 3. The study of the electronic interactions in transition metal oxides has a long history and very different models and calculational approaches have been developed, thus forming an extremely vast field of research. We therefore only briefly outline well established physical concepts which will be applied for interpreting the results obtained for specific compounds. The following main body of this work, chapters 4 - 7 present the studies of the electronic properties of selected examples of (pseudo-) ternary transition metal oxides. In order to give an orientation, table 1 ranges the investigated compounds according to different aspects which determine their overall physical properties.
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