Perovskites stoichiometry

A natural question to ask is whether this two-regime theory is consistent with the known properties of LSM. As recently reviewed by Poulsen, the defect structure of LSM has some similarities with other more reducible perovskites such as LSG and LSF. Like these other perovskites, LSM has electrical properties on the border between that of a p-type semiconductor and a metaP and becomes oxygen substoichiometric at high temperature and low as shown in Figure 35. However, unlike its more reducible cousins (which may have significant vacancy concentration at atmospheric Pq ), LSM maintains a nearly full perovskite stoichiometry above atm and in fact becomes superstoichio-... 

Barium titanate is usually produced by the soHd-state reaction of barium carbonate and titanium dioxide. Dielectric and pie2oelectric properties of BaTiO can be affected by stoichiometry, micro stmcture, and additive ions that can enter into soHd solution. In the perovskite lattice, substitutions of Pb ", Sr ", Ca ", and Cd " can be made for part of the barium ions, maintaining the ferroelectric characteristics. Similarly, the TP" ion can partially be replaced with Sn +, Zr +, Ce +, and Th +. The possibihties for forming solution alloys in all these stmctures offer a range of compositions, which present a... 

Fig. 43. Full-cell performance with hot-pressed membrane, perovskite electrodes. Cathode removal and anode generation as a function of applied current. Lines calculated from stoichiometry, 1 mol/2 F.

Numerous investigators have attempted to control the precursor structure and related solution chemistry effects with varying degrees of success, to influence subsequent processing behavior, such as crystallization tempera-ture.40-42,78,109 110 Particular attention has been given to precursor characteristics such as structural similarity to the desired product and the chemical homogeneity of the precursor species. For multicomponent films, this latter factor is believed to influence the interdiffusional distances associated with the formation of complex crystal structures, such as perovskite compounds. Synthetic approaches have been geared toward the preparation of multimetal species with cation stoichiometry identical to that of the desired crystalline phase.40 42 83 84... 

A significant problem in the combination of solid electrolytes with oxide electrodes arises from the difference in thermal expansion coefficients of the materials, leading to rupture of the electrode/electrolyte interface when the fuel cell is, inevitably, subject to temperature cycles. Insufficient experimental data are available for most of the electrolytes and the perovskites as a function of temperature and oxygen partial pressure, which determines the stoichiometry of the perovskites, to make a quantitative assessment at the present time, and mostly decisions must be made from direct experiment. However, Steele (loc. cit.) observes that the electrode Lao.6Sro.4Coo.2Feo.8O3 functions well in combination with a ceria-gadolinia electrolyte since both have closely similar thermal expansion coefficients. 

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. 

Of the structures with stoichiometry ABO3 that may be derived from that of perovskite (Sect. 2.2), the calcite structure is the simplest in the sense that it requires the fewest parameters to specify it. We earlier described this structure in terms of the parameters obtained with regular BOe octahedra that are rotated (tilted) about their 3 axes from the positions they have in (cubic) perovskite. The coordination of A goes from 12 in the cubic structure to 3 in the rhombohedral calcite structure. 

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. 

The ability of copper to take various coordinations as well as the great flexibility of the perovskite structure combine to allow significant deviations in oxygen stoichiometry that do not really change the structure but dramatically affect the superconducting properties. 

Table 9.15 Comparison of determinations of stoichiometry in thin Ba0,7Sr0 3Ti-, 03 perovskite layer measured by ICP-MS using different instruments and XRF.

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