Perovskite-type oxide structure

The extensive variety of properties that these compounds show is derived from the fact that around 90% of the metallic natural elements of the periodic table are known to be stable in a perovskite-type oxide structure [74], Besides, the possibility of synthesizing multicomponent perovskites by partial substitution of cations in positions A and B gives rise to substituted compounds with a formula A, A B,. B 03 ft. The resulting materials can be catalysts, insulators, semiconductors, superconductors, or ionic conductors. 

Many ternary oxides with compositions of A B O3, A B O3, A B O3, A +B + 03, and an abundance of compounds with more complex compositions, are crystallized in perovskite structure. The perovskite structure is very flexible, allowing not only the substitution of different cations in positions A and B over a wide range of compositions Ai xA xBi xBx03, but also the introduction of vacancies or substitutions on the anion sublattice. It is for this reason that about 90% of the metallic elements of the Periodic Table are known to be stable in a perovskite-type oxide structure. 

The Incentive to modify our existing continuous-flow microunit to incorporate the square pulse capability was provided by our work on perovskite-type oxides as oxidation-reduction catalysts. In earlier work, it had been inferred that oxygen vacancies in the perovskite structure played an important role in catalytic activity (3). Pursuing this idea with perovskites of the type Lai-xSrxFeg 51 10 503, our experiments were hampered by hysteresis effects which we assumed to be due to the response of the catalyst s oxygen stoichiometry to the reaction conditions. 

The enthalpies of formation of selected perovskite-type oxides are given as a function of the tolerance factor in Figure 7.17. Perovskites where the A atom is a Group 2 element and B is a d or / element that readily takes a tetravalent state [19, 20] show a regular variation with the tolerance factor. Empirically, it is suggested that the cations that give t close to 1 have the most exothermic enthalpies of formation. When t is reduced, the crystal structure becomes distorted from cubic symmetry and this also appears to reduce the thermodynamic stability of the... 

Perovskites, 27 358 band structure, 38 131-132 crystal structure, 38 123-125 Perovskite-type oxides see also specific lanthanum-based catalysts actinide storage in radioactive waste, 36 315-316... 

Fereshteh, R., Caroline, S., James, A. F., 2002. Sphalerite activation and surface Pb ion concentration. Inter. J. Miner. Process, 67 43 - 58 Fierro, R. E., Tryk, D., Scherson, D., Yeager, E., 1988. Perovskite-type oxides oxygen electrocatalysis and bulk structure. Journal of Power Sources, 22 (3 - 4) 387 - 398... 

Figure 5. Time-averaged structure of a protonic defect in perovskite-type oxides (cubic case), showing the eight orientations of the centrai hydroxide ion stabiiized by a hydrogen-bond interaction with the eight next-nearest oxygen neighbors. ...

The special electric, magnetic, optical, superconductive and catalytic properties of perovskite-typed oxides make this group of materials attracting and widely used. Perovskites were named according to the similarity of their structure with the CaTiOs compoimd. The... 

Tejuea, LG Fierro, JLG Tascon, JMD. Structure and reactivity of perovskite-type oxides. Adv. Catal, 1989, Volume 36, 237-328. 

A wide array of ferroelectric, piezoelectric and pyroelectric materials have titanium, zirconium and zinc metal cations as part of their elemental composition Many electrical materials based on titanium oxide (titanates) and zirconium oxide (zirconates) are known to have structures based on perovskite-type oxide lattices Barium titanate, BaTiOs and a diverse compositional range of PZT materials (lead zirconate titanates, Pb Zr Tij-yOs) and PLZT materials (lead lanthanum zirconate titanates, PbxLai-xZryTii-yOs) are among these perovskite-type electrical materials. 

Nishihata et al. (2002) reported the re-dispersion of Pd in a Perovskite-type oxide. They investigated the oxidation state and the local structure of Pd by using X-ray absorption analysis. Pd occupies the -site in La2PdCo06 in the oxidized sample. For the reduced catalyst, the XAD and XANES measurements suggested the segregation of metallic Pd from the perovskite crystal. They imply that Pd also moves back and forth between the -site in the perovskite structure and sites within the lattice of Pd metal clusters dispersed on perovskite surface when the catalyst is exposed to fluctuations in the redox characteristics of the emission exhaust. 

Helmut Ullmann, Nikolai Trofimenko, Composition, structure and transport properties of perovskite-type oxides , Solid State Ionics 119,1-8 (1999). 

Several members of the MM O3 class of ternary metal oxides adopt the perovskite-type (CaTiOs) structure and are sought as worthy target materials possessing ferroelectric properties see Ferroelectricity) Among the more widely investigated members of this class are BaTiOs and SrTiOs. Clearly, use of these materials as potential memory device... 

Amo.5 Pao.5 )02 of fluorite-type structure with a statistical distribution of the metal ions (18). (Amo.s Pao.s )O2, like the analogous compounds of the rare earth elements, forms a double oxide with BaO, Ba(Amo.5, Pao.5)03 with an ordered perovskite-type of structure (19). 

J. Mizusaki, M. Yoshihiro, S. Yamauchi and K. Fueki, Nonstoichiometry and defect structure of the perovskite-type oxides Lai-xSrxFeOs-a. /. Solid State Chem., 58 (1985) 257-266. 

Y. Teraoka, M. Yoshimatsu, N. Yamazoe and T. Seiyama, Oxygen-sorptive properties and defect structure of perovskite-type oxides, Chem. Lett. (1984) 893-896. 

A primary characterization of perovskite-type oxides must include textural analysis and X-ray identification of the phase(s) present. For a more detailed characterization, structural analysis for establishing the lattice position of cations and surface analysis (by means of techniques such as XPS) for defining the surface concentration and oxidation states of cations are desirable. Consequently, information provided by these techniques will furnish the essential criteria for comparing the different preparation methods. For convenience, we will classify the methods used to date for the preparation of pure perovskite phases according to the scheme proposed by Courty and Marcilly (29) for the whole field of mixed oxides. Table I gives a survey of methods used as a function of the phenomena on which they are based. 

Addendum to Structure and Reactivity of Perovskite-Type Oxides... 

Perovskite-type oxides with A and/or B sites partially substituted present properties such as structural defects and reactivity of adsorbed and lattice oxygen that play a central role in catalytic combustion. However, preparation methods as well as temperature of calcination could affect the surface area, and most important, changes on the surface composition that will be reviewed in the following section. 

Since 1970 perovskite-type oxides (ABO3) have been suggested as substitutes for noble metals in automotive exhaust catalysts [1]. These oxides are efficient for oxidation reactions when for reduction the results obtained from the literature are dissimilar [2], mainly due to huge differences in the experimental conditions. The properties of perovskite-based catalysts are a flmction of the spin and the valence state of the metal in the B site cation, which is surrounded octahedrally by oxygen. The A site cation is located in the cavity made by these octahedra. For some perovskite-type oxides, their electronic structures have been pointed out to be similar to those of transition metals on the basis of theoretical... 

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