Perovskites exhibit

Doped manganite perovskites exhibiting CMR have the general formula REi fM f]V[n03 where RE represents a rare earth element and M a divalent metal such as Cu, Cr, Ba, or Pb. The divalent RE ions and divalent M ions occupy the A sites in the perovskite structure (Figure 9.14) and have 12-fold coordination to oxygen. The Mn ions occupy the octahedral B sites. (1- of the manganese ions... 

RE2CUO4 perovskites exhibit important and varying magnetic and electrical characteristics, and they are broadly studied as potential high-Tc superconductive materials. At room temperature, they show p-type semiconducting behaviors, and are used as electrode materials in fuel batteries. The catalytic properties of the perovskite oxides also make them effective in various oxidation and reduction reaction, hence they are considered as promising substitutes to the classical Pt/Rh-based catalysts applied to automotive pollution control. 

Abstract A review is made on the developments in the last two decades in the held of the Jahn-Teller effect on itinerant electrons in Jahn-Teller crystals. Special attention is paid to the current status of the researches on the fullerene superconductors and the manganite perovskites exhibiting the colossal magnetoresistance. Present knowledge about the polarons and bipolarons in the typical Jahn-Teller model systems is also summarized, together with some original results of our own. 

The late transition metal-containing perovskites exhibit high electronic conductivities. In the materials which receive prime interest for oxygen delivery applications, the electronic contribution at high temperature of operation is usually predominant. The values for e.g., Lai rxCoi.yFey03 at 800°C in air... 

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). 

Consequently, we are in the startup phase of our program. Our first task was to identify candidate perovskite oxide materials with high protonic conductivities. We have identified ytterbium doped strontium cerate and yttrium doped strontium zirconate materials as possible electrolyte materials. Barium cerate perovskites exhibit higher protonic conductivity, but the reactivity with carbon dioxide would require pretreatment of the steam. 

The perovskite family is a typical representative of complex oxides. Many of the perovskites exhibit interesting physics that includes ferro- and piezoelectricity, high electronic and ionic conductivities, diverse magnetism, colossal... 

Several perovskites exhibit metallic conductivity, typical examples being LaTi03, LaNi03, SrV03, AM0O3 (A = Ca, Sr, Ba), Re03, and A O. Metallic conductivity in similar perovskite oxides is due to a strong cation-anion-cation interaction [21,24,34]. 

Ferroelectric substances perovskites exhibit no permanent dielectric dipoles in the p electric phase (which belongs to the centrosymmetrical symmetry group /w3/w). The dipole moments appear in ferroelectric phase as a result of the spontaneous displacements of ions. Such phase transition is therefore called displacement-type phase transition. 

Overall, some of these perovskites exhibit a coloration that is comparable to inorganic pigments currently utilized by the industry (Figure 12.1). In other cases, however, the color strength is too weak to support their use as colorants. At all events, the only perovskite pigment produced at industrial scale is the red Cr-YAlOs [9,10,12,29,44-47]. Thus, the following sections will focus on yttrium orthoaluminate and related compounds. 

For comparison piuposes, the catalytic activity of several perovskites in NO + CO reaction is presented in Table 25.4, adopted from Ref. [64]. The activity of Rh/Al203 has also been included as a reference value. Obviously, some perovskites exhibit satisfactory activity compared to Rh/Al203. Good stability and SO2 tolerance are also often reported for perovskite materials. 

It is far beyond the scope of this chapter to review the electronic structures and properties of all metal oxides, or even all of the important metal oxide stmc-ture types. Instead, this section covers some featnres of one stmctural family, perovskite, in some detail. In doing so, it is hoped that the important concepts will be illnstrated in snch a way that they can be widely appUed. Of course, the choice of the perovskite stmctnre as an illnstrative example is not a random choice. The perovskite family of componnds is very extensive, encompassing most of the periodic table. Fnrthermore, perovskites exhibit nearly every type of interesting electronic or magnetic behavior seen in oxides (ferromagnetism, ferroelectricity, piezoelectricity, nonlinear optical behavior, metaUic condnctivity, snpercondnct-ivity, colossal magnetoresistance, ionic conductivity, photoluminescence, etc.). One important property that is not readily found among perovskites, transparent conductivity, is the focus of Section 6.7. 

It appears that cubic perovskites exhibit higher diffusivities of protons than less symmetrical lattices. This difference is most easily rationalized by the oxide ion sites being equivalent, so that there are no sites that act as traps by requiring a higher activation energy for the liberating jump. 

Just as the magnetic properties of rare earth perovskites exhibit very diverse behavior, the electrical conductivities also show wide variations. Some compounds have been utilized for their dielectric properties, others show... 

LaMnOs-based perovskites exhibit intrinsic p-type conductivity due to changes in the Mn valence. The electrical conductivity of these materials is greater than 10 S cm at 700°C. The electrical conductivity is enhanced by replacing La with lower valence cations (such as Ca ", Sr ) or doping with other cations (Mg ", Co ". etc.) for application as a cathode material [21-23,26]. 

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