Perovskite-type compounds

Perovskite-type compounds, especially BaTiO, have the abiUty to form extensive soHd solutions. By this means a wide variety of materials having continuously changing electrical properties can be produced ia the polycrystaUine ceramic state. By substituting ions for ions, T can be... 

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. 

Sugiura, H. and Yamadaya, T., High Pressure Studies on the Perovskite-Type Compound BaBiOs. Physica 139 140B 349 (1986). 

Galasso, F. S. "Structure, Properties and Preparation of Perovskite-Type Compounds", Pregamon Press Hungary, 1969 Chap. 4. 

Only a small fraction of the enormous number of perovskite-type compounds is listed in Table 10.4.1. For instance, substitution of the lanthanide element... 

La for Sm will generate another isostructural series. In view of the common occurrence of perovskite-type compounds, it is worthwhile to elaborate their structural characteristics and important trends. 

Thin films, to attain enough sensitivity and response time, of oxide materials normally deposited on a substrate are typically used as gas sensors, owing to their surface conductivity variation following surface chemisorption [183,184], Surface adsorption on a Sn02 film deposited on alumina produces a sensitive and selective H2S gas sensor [185]. In addition, a number of perovskite-type compounds are being used as gas sensor materials because of their thermal and chemical stabilities. BaTi03, for example, is used as sensor for C02 [183],... 

Wollan, E. O. Koehler, W. C. 1955 Neutron diffraction study of the magnetic properties of the series of perovskite-type compounds [La xj, CajMn03. Phys. Rev. 100, 545-563. (doi 10. 1103/PhysRev. 100,545)... 

Franke and Winnick [105], using a K2S2O7 based electrolyte dispersed within the interstices of an inert K2Mg2(S04)3 matrix, were able to achieve removal efficiencies of SO2 greater than 99% at current efficiencies near 100%. A porous electrode constructed of a perovskite-type compound, Lao 8Sro.2Co03, was found to be conductive and stable in the corrosive cell environment. 

PbFg, but RbF and CsF give the perovskite-type compounds MPbFg. Potassium and rubidium also form non-stoichiometric compounds MnPb4 p2-n where n == 0.2 to 0.3. These have an anti-aAgI structure (p. 153) with additional F ions. 

For instance, in the paraelectric phase of the first discovered perovskite-type compound, barium titanate BaTi03, the Ti cation is at the center of the oxygen octahedron, which is itself centered in a cubic cell with the cations... 

Francis S. Galasso, Structure, Properties, and Preparation of Perovskite-Type Compounds (Pergamon Press, New York, 1969). 

Conversion of syngas to oxygenate organic chemicals over perovskite-type compounds has been also studied by Broussard and Wade (223). These authors examined several perovskites with La, Ce, Nd, Sr, and Ba... 

Structural characterization of the support indicates that the lanthanum active component is the perovskite-type compound LaA103. The experimental results are interpreted as the formation of a three-fold superstructure of LaA103 which is in a good interfacial crystallographic fit with 5-AI2O3 and by the epitaxial relationship between the platinum and the support. 

As an attempt to simulate real operating conditions of automotive converters, a laboratory bench has been designed and ageing procedures determined to reproduce simultaneous chemical and thermal modifications encountered by catalysts in the exhaust line. Characterization of commercial samples after ageing according to different temperature cycles evidences formation of both platinum/rhodium alloys and cubic perovskite-type compound, CeA103. Simultaneously with the formation of cerium aluminate, a thermal stabilization of catalysts is observed, in terms of mean noble metal particles size and concentration of rhodium in alloyed phases. An interpretation based on the crystallographic adaptation of alumina, cerium aluminate and ceria is proposed. 

The twin structure in small LSGMO ciystals tends to form chevronlike wall configurations that allows for a stress-lfee co-existence of four different orientation states. This pattern of domain walls is expected to be characteristic also for other perovskite-type compounds with a sequence of ferroelastic phase transitions related to those of LSGMO. Examples are mixed conductivity perovskites, which are used as electrodes and interconnectors in SOFC batteries. 

Solid solutions based on RAIO3 perovskite-type compounds... 

Galasso, F.S. (1969) Structure, Properties and Preparation of Perovskite-type Compounds, Pergamon Press, Oxford. 

The situation is different for complex oxide systems. Although this topic is beyond the scope of this chapter, one such case is worth mentioning. This deals with the MAnOj (M denotes an alkaline earth), perovskite-type compounds, which provide stability for the An(IV) oxidation state. Data for several lanthanide and for Am compounds of this type have been published (Goudiakas et al. 1990) data for Cm and... 

Obayashi H, Kudo T. Perovskite-type compounds as electrode catalysts for cathodic reduction of oxygen. Mater Res Bull 1978 133 1409-13. 

Although Goldschmidt s tolerance factor t is indeed very useful for the exploration of new compounds with perovskite structure, it is only a necessary condition but not the sufficient condition for the formation or the stability of perovskite structure [145]. Many systems having t in the range of 0.75—1.0 do not form perovskite-type compound. 

SVM applied to mathematical modeling to differentiate the perovskite-type compound forming systems and the AX-BX2 system with 0.75 < t <1.00 but without 1 1 intermediate compound formation ... 

Table 8.2 lists the data of some AX-BX2 systems (X = Cl, Br, I) having tolerance factors in the range of 0.75—1.00. Some of them form perovskite-type compounds, and the others have no 1 1 intermediate compound. SVM has been used to differentiate these two types of systems. 

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