CHEMICAL AND PHYSICAL IMPLICATIONS OF CATIONIC AND ANIONIC MODIFICATIONS IN PEROVSKITE RELATED METAL-OXIDES

In the realm of metal oxides perovskite-related phases occupy a niche of increasing importance. Reasons for this development may be characterized as follows: the structural framework is not made up of a close oxygen packing with metal cations occupying tetrahedral and octahedral interstitial positions as in corundum or spinel type oxides, but of a mixed metal/oxygen close packing. The general formula ABO3 reflects this fact clearly: in an AO3 cubic close packing small B metal cations are accommodated in oxygen octahedra. In this structural framework many different metal combinations may be stabilized. In many cases the stabilization of unusual formal oxidation states of the present metal cations is observed. Depending on the actual metal cation combinations, the chemical reactivity and the physical properties vary within a broad spectrum. In addition and as a rather specific phenomenon, the oxygen stoichiometry is variable within a considerable range while the basic structural framework, is conserved, i.e. phases with stoichiometries ABO3-z are well known, many of them also structurally identified. Owing to the ease, with which ordered or random oxygen vacancies can be stabilized, not only high oxygen anion mobility, but also direct oxygen stoichiometry/property relations are observed. A further characteristic feature of perovskitic structural elements is their flexibility for combinations with matching structural elements resulting in composite structural frameworks. As a direct consequence, a gradual anisotropization of the basically isotropic perovskite structure into more or less low-dimensional systems is possible. Be it the variation of the metal combinations, be it the controlled generation of oxygen-deficient phases or composite structures, perovskite-related oxides offer a rich and flexible spectrum of compounds exhibiting interesting properties such as electric and ionic conductivity, piezoelectricity, ferromagnetism, superconductivity and last but not least selective reactivity as sensors or catalysts.