Perovskites formulations

An interesting aspect of the multiple perovskites is their ability to accommodate mono- and divalent cations at the B-site, which is not possible with a simple perovskite formulation. 

This process was used to synthesize a number of perovskite formulations, which were then extensively characterized for their catalytic properties such as oxygen storage capacity [49], water vapor sensitivity [50], conversion of CO and exhaust gas emissions [48], sensitivity to CO adsorption [39,40], reduction of NO by propene [51-53], reduction of NO either by CO [54] or by C3H5 [55-58], synthesis of higher alcohols [56,59], CO hydrogenation [60], oxidation of stearic acid [61], and methanol [62]. 

Over the past 35 years, Takahashi and Iwahara have measured oxide ion conductivity in many different perovskites [53]. They first reported fast ion conductivity in Ti- and Al-based compositions and continued to measure a range of perovskite formulations as shown in Figure 4.14. From this figure, it is clear that A1 or Mg doped CaTiOs exhibits the highest conductivity. 

The most important of these are perovskite structure solids with a formula A2+b4+o3 that can be typified by BaCeC>3 and BaZrCV The way in which defects play a part in H+ conductivity can be illustrated by reference to BaCeCV BaCeC>3 is an insulating oxide when prepared in air. This is converted to an oxygen-deficient phase by doping the Ce4+ sites with trivalent M3+ ions (Sections 8.2 and 8.6). The addition of the lower valence ions is balanced by a population of vacancies. A simple substitution reaction might be formulated ... 

The transient generation of anion vacancy of fresh perovskite during calcination as well as the generation of adsorbed oxygen (O2 ) was believed to involve the process as formulated in Eqs (1) and (2). 

Ordered Perovskite-type Compounds, A2(BB )06 Systems Cubic Fmim A feature of the perovskite structure is that, with the proper substitutions, many types of ordered structures can readily be formed. This can be accomplished by the substitution of two suitable metal ions (with different oxidation states) in the octahedral sites of the structure. In this case the unit ceil is doubled along the three cubic axes to generate an 0.8 A unit ceil (Figure 15). Partial substitution of different transition metal ions in the octahedral sites is also possible the general formulation for these compounds would be A2(B2 xB x)06. The parentheses in this formulation enclose atoms occupying the octahedral sites in the structure. 

The simplest quaternary derivative with the perovskite structure would be one in which two different transition metals might occupy the B-site position. This can be formulated as A(B,1 2B1 2)03, or preferably A2(B B0O6. These compounds can then crystallize with a doubled unit cell, if ordering occurs on the octahedral metal sites. Further compositional and structural adaptions could be obtained, as shown below, all possessing an overall 1 1 3 ratio of A B 0 atoms. In all the following examples and formulations, the proper stoichiometry will be maintained, and oxygen will be the principal anionic species. 

The general formulation of these oxides, (ACuOg.x)m(AO)n, reflects for each of them the number m of copper layers which form each perovskite slab, and the number n of AO layers which form each rock salt-type slab (the AO layers which lie at the boundary of the perovskite slabs and rock salt type slabs can only be counted as for 1/2). Thus all these oxides (2-34) can be represented by the symbol [m,n] in which m,n will be integral numbers. In most of these oxides one observes for one compound only one m and n value, corresponding to single intergrowths. 

A formulation of structures by layers, such as that represented by the sequence (1) or (2), allows one to derive in a simple way the coordination of the atoms. In the case of perovskite, for example, the atom A of a layer (AX)C is surrounded by twelve atoms X, four located at the corners of the same mesh, and eight at the midpoints of the edges of the (BX2)0 layers above and below (AX)C. The coordination polyhedron is a cuboctahedron. In the case of the rock salt structure, the atom A of a layer (AX)C has coordination six, being surrounded by four atoms X at the corners of the same mesh, and by two atoms X at the center of the meshes (AX)0 above and below (AX)C. In this case the coordination polyhedron is an octahedron. 

The first breakthrough superconductors were formulated as La2-rBarCu04 ft (jt < 0.2. unspecified but small) and have the tetragonal, layered K2NiP4 perovskite structure. They had a critical temperature of about 35 K.42... 

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