Perovskite electronic properties

There has been only one report to date on superconductivity in the BaPbj S Oj perovskite series (11). From BaPbOs, where no superconductivity is observed, Tc rises to approximately 3.5K for x=0.25-0.3 and drops rapidly to below 1.5K for larger values of x. Because the BaSbOs perovskite does not exist, a complete solid solution is not expected. The limiting Sb concentration appears to be x=0.5. The electronic properties have not been studied in any detail, but the visual appearance of the materials suggests considerable changes within the solid solution... 

In a series of papers, Matsumoto et al. [86, 280, 371-374] reported the results of extensive studies of the electrocatalysis of oxygen reduction on LaNi03 and related perovskite oxide electrodes from the viewpoint of their bulk solid state structural and electronic properties. The main conclusions of their studies are... 

Tejuca, Fierro, and Thscdn review the perovskite class of metal oxides. Many challenging concepts have been associated with these. They called early attention to the relationship of catalytic activity to electronic properties they have been seen as potential substitutes of platinum and more recently the discovery of superconductivity has rekindled the interest in the electronic connection of oxide catalysts. 

The cubic perovskite phase is an ideal polymorph, that we employ in this section only to describe the trends in the calculated properties with hybrid functionals on changing the mixing parameter. Comparison with experiment will be performed later. Here we shall consider separately the structural and electronic properties of the materials. 

We analyse how the electronic properties of perovskite-structured materials change as a function of the mixing parameter a by making use of the model TB Hamiltonian, as detailed earlier. In particular, we examine the calculated band-structure of the materials along the T-X direction of reciprocal space, from which we can evaluate the effective parameters Aa and p. 

The properties of perovskite materials are heavily dominated by their oxygen content, as weU as by donor- and acceptor-type impurities. An essential contribution to the knowledge of the structural and electronic properties of point defects in these materials comes from theoretical approaches. The results of large-scale computer semiempirical and first-principles modeling of point defects, polarons and perovskite sohd solutions can be found in [722], focusing mostly on KNbOa and KTaOa. 

The flexibility of the perovskite crystal structure and the opportunity to accommodate various dopants offer the possibihty to tailor material properties. The design of catalytic or electronic properties, such as ionic or electronic conductivity, is in the focus of solid oxide fuel cell (SOFC) research activities. Perovskite-type oxides are therefore a well-investigated class of materials and commonly applied as functional layers in SOFCs, as porous microstructured cathode layer on the air electrode side [32-36] and very recently as anode on the fuel side [35]. On a research level, perovskite-type oxides are also apphed as gas-tight electrolyte to separate anode and cathode compartments [37,38] or as an interconnector material [39,40]. Beside the stoichiometry and crystal structure, processing... 

Similarly to flame-made titanates, the precise control of the stoichiometry and material purity has an influence on the electronic properties of perovskite-type oxides such as LSC, LSCF, and BSCF. While conductivity is comparable with the highest reported in the literature (Figure 4.5), other unique properties are documented for these flame-made compositions. LSC, LSCF, and BSCF from a FSS process feature a pronounced shift of the temperature, at which the maximum conductivity is observed. The FSS process resulted in materials with an exceptional electronic conductivity, which may better match to a SOFC operated at intermediate or low temperatures. For example, LSCF cathodes based on flame-made nanopowders have shown polarization resistances in the range of 0.7 Q. cm at 592 °C [59], which are among the lowest reported for thick film layers of this material stoichiometry. Similar conclusions were drawn with respect to LSC-based cathodes, for which very low overpotentials were documented [60]. 

Mixed oxide systems of well-defined ABO3 structure (perovskites), although not exclusively, are more stable in the presence of such compoimds and appeared 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 heavy hydrocarbons and aromatics and their halogenated derivatives. The perovskite structure encompasses a wide array of materials with differing physical, chemical, and electronic properties. 

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