Defect perovskite oxides

Besides PTBs, A-site defective perovskite oxides are known to be formed when B = Ti. Nb.Ta and soon "13. Such compounds exhibit metallic properties and perovskite structures when the B atom occurs in a low oxidation stale. Compositions such as A0 5Nb03 (A = Ba. Pb etc.) where niobium is in the highest oxidation state adopt non-perovskite network structures. An interesting example20-21 of a A-site defective perovskite is Cu 5Ta03 which crystallizes in a pseudocubic perovskite structure. The unit cell is orthorhombic with a = 7.523, />= 7.525 and c = 7.520 A and eight formula units per cell. Tantalum atoms form... 

The superconducting behavior in oxygen defect perovskite oxides is found to depend on the amount and order of oxygen in the structure. In the case of YjBa C y, the highest and sharpest transitions are related to ordering of one-dimensional Cu-O ribbons in the structure which are in turn coupled to a network of adjacent 2-dimensional Cu-O sheets. The isostructural rare earth derivatives of YjBa-jC C y are found to display similar behavior. 

In recent years, research on catalysts for the ATR of hydrocarbons has paid considerable attention to perovskite systems of general formula ABO3. In the perovskite stmcture, both A and B ions can be partially substituted, leading to a wide variety of mixed oxides, characterized by structural and electronic defects. The oxidation activity of perovskites has been ascribed to ionic conductivity, oxygen mobility within the lattice [64], reducibility and oxygen sorption properties [65, 66]. 

Perovskites constitute an important class of inorganic solids and it would be instructive to survey the variety of defect structures exhibited by oxides of this family. Nonstoichiometry in perovskite oxides can arise from cation deficiency (in A or B site), oxygen deficiency or oxygen excess. Some intergrowth structures formed by oxides of perovskite and related structures were mentioned in the previous section and in this section we shall be mainly concerned with defect ordering and superstructures exhibited by these oxides. 

In the 1960s, all the tools needed to treat hydrogen defects in oxides [41-43] were fundamentally developed by Wagner and collaborators. However, up to the end of the 1970s, real developments in the field did not take place. During the last years of this decade, some significant studies were carried out [44-48], After that, it was realized that the introduction of defects in some perovskite structures determine the protonic conductivity of these materials. In this regard, Iwahara and... 

Defects in perovskite oxides can be due to cation vacancies (A or B site), amon vacancies or anion excess. Cation-deficient oxides such as A,WOj give rise to oxide bronze structures, W03 itself representing the limiting case of the A-sile deficient oxide A-site vacancies are seldom ordered in these metallic systems. B-site vacancies are favoured in hexagonal perovskites and ordering of these vacancies gives rise to superstructures in some of the oxides. 

Partial substitution of A and B ions is allowed, yielding a plethora of compounds while preserving the perovskite structure. This brings about deficiencies of cations at the A-or B-sites or of oxygen anions (e.g. defective perovskites). Introduction of abnormal valency causes a change in electric properties, while the presence of oxide ion vacancies increases the mobility of oxide ions and, therefore, the ionic conductivity. Thus, perovskites have found wide apphcation as electronic and catalytic materials. 

Lattice dynamics in bulk perovskite oxide ferroelectrics has been investigated for several decades using neutron scattering [71-77], far infrared spectroscopy [78-83], and Raman scattering. Raman spectroscopy is one of the most powerful analytical techniques for studying the lattice vibrations and other elementary excitations in solids providing important information about the stmcture, composition, strain, defects, and phase transitions. This technique was successfully applied to many ferroelectric materials, such as bulk perovskite oxides barium titanate (BaTiOs), strontium titanate (SrTiOs), lead titanate (PbTiOs) [84-88], and others. 

S. Adler, S. Russek, J. Reimer, M. Fendorf, A. Stacy, Q. Huang, A. Santoro, J. Lynn, J. Baltisberger and U. Werner, Local structure and oxide-ion motion in defective perovskites. Solid State Ionics, 68 (1994) 193-211. 

Ullmann, H., and Trofimenko, N.J. 2001. Estimation of effective ionic radii in highly defective perovskite-type oxides from experimental data. Journal of Alloys and Compounds 316, 153-158. 

When compared to the case of the Co/Si02 catalysts, it can be seen that whereas over SmCo/Si-823 the temperatures required for total acetone combustion increase slightly, over the SmCo/Si-1073 catalyst these temperatures increase by more than 100 K. These results are somewhat similar to the ones obtained with the series of silica-supported manganese oxide catalysts. Despite the possible presence of SmCo03 in the catalyst calcined at 1073 K and the good catalytic performance of this perovskite in the acetone combustion [14], it seems likely that the behaviour of the SmCo/Si-1073 catalyst be mainly due to a poor dispersion of the supported metal oxides. This point is of particular importance for the perovskite oxides because the severe sintering taking place at temperatures above 973 K not only reduces the exposed surface area but also the density of surface defects active in oxidation reactions [23]. 

The defect perovskites that have been more studied in heterogeneous catalysis were those having in position A an alkaline, alkaline-earth, or lanthanide element and in position B a first-row transition metal. We will discuss here some examples of nonstoichiometric perovskites, paying attention preferentially to the concentration and type of defects that are formed. The influence of these defects in the catalytic performance of these oxides has been clearly established in a number of cases. Some relevant examples will be discussed in Section VII. 

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