Perovskite-related structures, oxide systems

It is likely that similar oxide structures will be found in many other systems, and systematic studies, especially involving electron microscopy, will be likely to reveal many more phases in these perovskite related systems. 

Under these conditions, mixed oxide systems of well-defined ABO3 structure (perovskites) resulted as a reasonable alternative. Based on this, the aim of this chapter is to highlight and compare selected examples of perovskites and related oxides in total oxidation of VOCs. Their potential application as catalysts for the conversion of organic compounds containing carbon and hydrogen to CO, CO2,... 

For an ideal perovskite, t is unity however, the perovskite structure is also found for lower t-values (0.75 < t < 1). In such cases, the structure distorts to tetragonal, rhombohedral or other lower symmetries. Indeed, many perovskite oxides are polymorphs. In ideal perovskite structure, the atoms are in the following positions A 000, B A Vi Vi X Vi Vi 0, Vi 0 Vi, 0 Vi Vi. Moreover, the above given geometric relations are not a sufficient condition for the stabilization of the perovskite lattice, since the A and B cations must, in themselves, be stable in twelvefold and sixfold eoordination respectively. This latter condition sets lower limits for the cationie radii. In oxide systems these limits are rA > 0.90 A and re > 0.51 A [19]. 

The ample diversity of properties that these compounds exhibit, is derived from the fact that over 90% of the natural metallic elements of the periodic table are known to be stable in a perovskite oxide structure and also from the possibility of synthesis of multicomponent perovskites by partial substitution of cations in positions A and B giving rise to compounds of formula (AjfA i- )(ByB i-J,)03. This accounts for the variety of reactions in which they have been used as catalysts. Other interesting characteristics of perovskites are related to the stability of mixed oxidation states or unusual oxidation states in the structure. In this respect, the studies of Michel et al. (12) on a new metallic Cu2+-Cu3+ mixed-valence Ba-La-Cu oxide greatly favored the development of perovskites exhibiting superconductivity above liquid N2 temperature (13). In addition, these isomorphic compounds, because of their controllable physical and chemical properties, were used as model systems for basic research (14). 

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