ABO3 structure

The geometric relaxation described in Section 12.3.1 occurs by redistributing the bond valence between the bonds until GII and BSI both have acceptable values, but in some cases this relaxation is restricted by symmetry. In the case of per-ovskite, the cubic symmetry of the archetypal ABO3 structure (Fig. 10.4) does not allow any of the bonds to relax unless the symmetry is lowered. Thus true cubic perovskites are rare since they can only exist if the A and B ions are exactly the right size. Most perovskites have a reduced symmetry that allows the bonds to relax. For compounds in which the A-O bonds are stretched, the relaxation takes the form of a rotation of the BOg octahedra and results in a reduction of the coordination number of A. The various relaxed structures based on different expected coordination numbers were modelled in Section 11.2.2.4. 

ABO3 structures hexagonal BaTi03 high-BaMn03, BaRu03 (Table 5.6)... 

Certain cations comparable in size with form c.p. layers AO3 which can be stacked in various c.p. sequences. Smaller cations can then occupy the octahedral holes between groups of six 0 ions to form structures of the type A B 03 . Some of these structures have been described in Chapter 4, and it was noted in Chapter 5 that these structures may alternatively be described in terms of the way in which the BOg octahedra are linked together. Only vertices and/or faces are shared, and the extent of face-sharing is indicated in Table 13.5. We shall deal in detail only with the simplest of the c.p. ABO3 structures, the perovskite structure. The structure of hexagonal BaTi03 is compared with perovskite in Fig. 13.1, and in Fig. 13.2 we show sections through the 5-, 9-, and 12- layer structures to illustrate the relations between the BOg octahedra. 

Ordered anion vacancies are partly responsible for some important solid state phenomena, such as superconductivity, as well as unusual cation geometries. For example, the superconductor YBa2Cu307 is built from three oxygen-deficient blocks of the perovskite (ABO3) structure (Figure 6.10). In the parent structure, B is in an octahedral hole, coordinated to six oxygens. 

B of ABO3 structure, but mainly on the degree of substitution of the A ion with ion with lower valence [10]. 

The ideal perovskite (ABO3) structure which is a simple cubic structure with the space group Pm3m, provides the basis for the structures of a large variety of inorganic solids. The perovskits structure is conventionally described as consisting of a BO3 array... 

Figure 6.1 Different presentations of the ideal perovskite ABO3 structure and the projections of structure along [1 0 0], [1 1 0], and [1 1 1].

Figure 6.3 (a) The ideal perovskite ABO3 structure (b-e) Changes ofthe environ merit ofTi " " ions... 

The majority of new materials for SOFCs are perovskite stmctured oxides of general form ABO3.5 [23]. The ideal perovskite structure is a cubic close-packed ABO3 structure where the B-site cation sits within the octahedral interstices. Fig. 3.5. This stmcture is very flexible toward cation composition and tolerates large substitution fractions on either cation site. The Goldschmidt factor, a ratio of A, B, and O ionic radii, is often utilized to predict if a metal oxide will crystallize into the perovskite structure [24]. The A site of the commonly utilized perovskites is typically occupied by La, Ca, Sr, or Ba. The B site is typically a transition metal. Other stmctures investigated include double perovskites, apatites, and fluorites. 

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 the synthesis of higher hydrocarbons by Fischer-Tropsch, cobalt and iron are the most used metals. Under the form of trivalent cations, they have close ionic radii, which allow their crystallization in an ABO3 structure with La in the A sites. The final goal is the formation after reduction (partial or total) of an efficient catalyst for hydrocarbons synthesis. The most simple combination is to synthesize mixed La(Co cFei )03 perovskites. Bedel et al. [36] studied the preparation, by a sol-gel-like method, of these perovskites over the whole range of... 

Figure 23.4 The perovskite, ABO3 structure (From Tao, S.W. and Irvine, J.T.S. Chetn.

Figure 5.9 (a) A simple perovskite (ABO3) structure, (b) The rock-salt ordered... 

Most of the current cathode materials are based on the perovskite-type ABO3 structure with one of the conventional materials being LSM (Lai- Sr MnOs) as introduced earlier in Section 2.1.3. Other materials currently under development are based on the analogous system... 

In the rare earth perovskite oxides, the rare earth ion, R, is a relatively large ion and is hence generally the A ion in the ABO3 structure. Most of the examples in the literature are RBO3 type compounds but compounds of the type ARO3 are also known and some examples of these are listed later. In some instances the rare earth ion may substitute for some fraction of the A or B ions leading to the formation of systems of the type (R, A)B03 or A(R, B)03 respectively. 

Perovskite, which is first used to describe the oxides CaTiOs, is now extensively referring to oxides with ABO3 structure, and sometimes also to the oxides with A2BO4 structure. In some eases, earbides, nitrides, halides, and hydrides also have the ABO3 structure,[l] but practically perovskite refers to the oxides compounds. In this context, if not specified, perovskite means an oxide compound having the ABO3 or A2BO4 structure. 

Solid oxide fuel cells have been the subject of extensive research activities over the past 40 years, with significant advances made in the development of materials for anodes and cathodes and the identification of novel electrolyte materials. Developers have selected a relatively narrow compositional space to explore, focussing on the fluorite, AO2, and perovskite, ABO3, structural families. Indeed the materials currently used in SOFCs can be narrowed down to the choice of one of three electrolytes yttria stabilised zirconia (YSZ), gadolinium substituted ceria (GDC) or substituted lanthanum gallates [1], with most interest in YSZ and GDC. For the electrodes there are currently limited choices for developers, with Ni... 

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