Perovskite structure compounds with

A2M2O76 Sr Have layered perovskite structure. Samples with both Ta and Nb also studied. Strontium niobate compound is ferroelectric at room temperature. In contrast, the tantalum analog is paramagnetic. 426, 447 149... 

A novel system was introduced to the catalytic field by Matsuda et al. (1993). A solid of the general formula ALaNb207 is made up of the layer compound LaNb207 with double perovskite structure interleaved with A atoms. If the interlayer compound is water, they call the compound HLa2Nb707 to indicate the acidic character of this system. They report the results obtained when 1- and 2-butanol were reacted over this solid at temperatures between 453 and 623 K. The secondary alcohol was much more reactive, as expected for an acid catalyst. Consistent with this, no dehydrogenation product was formed when... 

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. 

Oxyhalides have a halide ion substituted for an oxygen ion in the perovskite structure. As with the oxynitrides, addition of a halogen requires charge compensation, typically via a change in the nominal cation valence. Oxyhalides with a formula ABOjF are generally approximated as ionic compounds, so that... 

In late 1990s it was shown that the proton transport can be also realized in perovskite-related compounds with structural oxygen vacancies. The niobates and tantalates of alkaline-... 

Rez] Reznitskii, L.A., Thermochemistry of Anion-Deficient Perovskites and Compounds with Brownmillerite Structure , Russ. J. Phys. Chem., 64(8), 1197-1199 (1990), translated from Zh. Fiz. Khim., 64, 2228-2231 (1990) (Thermodyn., Calculation, 14)... 

Zener, C. 1951. Interaction between the (/-shells in the transition metals. II. Ferromagnetic compounds of manganese with perovskite structure. Physical Review 82 403-405. 

The same analysis can be applied to compounds with a more complex formula. For example, the oxide LaCoCL, which adopts the cubic perovskite structure, usually shows a large positive Seebeck coefficient, of the order of +700 jjlV K-1, when prepared in air (Hebert et al., 2007). This indicates that there are holes present in the material. The La ions have a fixed valence, La3+, hence the presence of holes must be associated with the transition-metal ion present. Previous discussion suggests that LaCo03 has become slightly oxidized to LaCoCL+j, and contains a population of Co4+ ions (Co3+ + h or Coc0)- Each added oxygen ion will generate two holes, equivalent to two Co4+ ... 

The configurational entropy term, given by the degeneracy, gc, is included in AfG but not in AfGc. Let us assume the existence of two compounds with different formal oxidation states for the B atom, ABO3 and ABO2.5. The two compounds have the same (perovskite-type) structure and the non-stoichiometric phase... 

Table 11.1 lists the resulting low-temperature phases calculated for this set of compounds. Where experimental data are available (marked with a star) the predicted structures are those observed at low temperatures. Inverse denotes a perovskite structure in which a large divalent ion is 12-coordinate and a smaller univalent ion 6-coordinate. Unit cell dimensions are predicted to within 1% of the measured values. 

The perovskite structure and its variant and derivative structures, and superstructures, are adopted by many compounds with a formula 1 1 3 (and also with more complex compositions). The ideal, cubic perovskite structure is not very common, even the mineral CaTi03 is slightly distorted (an undistorted example is given by SrTi03). 

These copper-oxide compounds crystallize in the perovskite structure and superconductivity is based on the (hole or electron) doping in the copper-oxide planes. This is the reason why these materials can be regarded as being 2D. The first compound of the family was La2 i Sr i Cu04 with Tc 38 K, which soon led to YBa2Cu307 5 with Tc — 92 K for 5 < 1 (Bums, 1993). The non-copper oxide electron-doped perovskite Bai-jcK cBiOa exhibits superconductivity near 30 K for 0.3 < X < 0.5 (Cavaeta/., 1988). 

ASOa-perovskites of Table 6 extends from Dza through Can to Ci. Distinct band-splittings are not observed, however, 13) (Fig. 2). As is demonstrated for the YCra Ali-a 03-system, the Oo-transition is situated at 13600 to 13700 cm i independent of the Cr3+ concentration (Fig. 2) leading to a B55 of 705 cm (Table 3), which is more than 5% lower than for the corundum and spinel compounds discussed. This is in agreement with the possibility of relatively strong Ji-bonds in the perovskite structure, which could be inferred from spectroscopic and crystallographic results 3) as well as from NMR-data and MO-calculations 33) for Ni2+,... 

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 syntheses of several simple AB03 compounds by Art Sleight led him to the first example of oxide superconductors possessing the ordered perovskite structure. It was already known that BaPbOg, a distorted perovskite with pseudo-orthorhombic symmetry, exhibited metallic conducting properties to very low temperatures, but... 

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. 

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