Layered perovskite-related

One possible mechanism which must be considered is the excitonic mechanism. We suggested that mixed valence, layered perovskite related structured materials would be best suited as... 

Figure 20. Temperature dependence of conductivity for LSFC samples and LSNF sample with a layered perovskite- related structure sintered at 1500 (LSFCo 2) or 1100 C (all other samples).

In Chapter 12 the layered perovskite, La2Ni04 (65917), was used as an example of a structure which displays lattice-induced strain. This compound is typical of the large class of perovskite-related structures. All show some degree of lattice-induced strain and, because the mechanism of relaxation depends on the details... 

Fig. 16.2 Idealized structures of some perovskite-related layered oxides and their fluoride relatives.2)...

Table 16.3 Perovskite-Related Layered Oxides Studied for Photocatalysis... 

Microdomains within the perovskite-like slabs of layered perovskite phases in the Ba-Bi-O system illnstrate these ideas.The structures are layered perovskites with K2Nip4 related structures and gross changes in the Ba to Bi ratio are accommodated by the formation of a homologous series Ba +iBi 03 +i, made up of slabs of BaBi03 perovskite structure (see Section 8.4). In addition, subtle changes in the Ba to Bi ratio are accommodated by microdomains of ordered structure within the perovskite slabs themselves. These microdomains are ordered fragments of perovskite structure, and cation variation occurs at the interface between the microdomains in such a way that local excess or deficit of one cation over the other is accomplished. 

Table 10. Perovskite-related layered oxides studied for photocatalysis.

Perovskite-related Oxides.—The perovskite-related oxides have been studied extensively in recent years because of the large variety of device applications for which these materials are suited. The interaction between structure, properties, and stoicheiometry is significant at all levels, but here we will discuss only the narrow areas where intergrowth is a dominant structural feature. We will not, therefore, consider solid solutions typified by the Pb(Zr Tii )03 ferroelectrics, and neither will we discuss the structurally complex but stoicheiometric phases related to hexagonal BaTiOj, which includes BaNiOj, which has a simple two-layer repeat in the c-direc-tion, the nine layer BaRuOj, the twelve layer Ba4Re2CoOj2, and the twenty-four layer Sr5Re20ig phase. The crystal chemistry of these phases is treated in detail by Muller and Roy. The materials we shall discuss are the two series of phases A B 0 +2 and A + B 02n+, and the bismuth titanates. Some of the anion deficient perovskites, ABO -x, will be considered in Section 5. 

In some crystals a particular lattice parameter is determined by more than one set of bonds. For example, layer compounds are composed of a sequence of different layers, each of which will have its lattice translations determined by the lengths of the bonds within the layer. In general, the lattice parameters predicted for one layer will be different from those predicted for the others, so some accommodation is needed if the layers are to coexist in the same crystal. There are then three possibilities (1) the incommensuration between the layers may be so severe that the compound cannot form, (2) each layer may keep its own lattice spacing and so form an incommensurate structure or (3) the bonds in some layers will stretch and in others will compress so as to ensure that the lattice parameters of all layers are the same. The second solution is found in structures such as cannizzarite (Fig. 2.9) where the bonding between the incommensurate layers is weak and the third is found in perovskite-related structures (e.g. La2Ni04, Fig. 2.10) where the interlayer bonding is strong. 

Lamellar perovskites of the general formula MI(A 1B 03 +i) are also known and have been tried as catalysts for reactions such as oxidative coupling of methane. [Barrault et al. (1992)]. One example of this type is CsCa2Nb3Oi0 which consists of blocks built up from three perovskite layers interleaved with Cs+ cations. Other perovskite-related structures have been discussed by Baran (1990). 

Very recently, however, a perovskite-related structure has been shown to exhibit fairly good photocatalytic activity. Domen et al. (1996) prepared and characterized a layered perovskite series with the general formula Ki La Ca2 Nb30io. They also retrieved previous data obtained with layered perovskite-type niobates AV . Nb 03 -1 (A = K, Rb or Cs B = La, Ca, Pb and others n = 2 or 3) which were found to possess high photocatalytic activity for H2 evolution from aqueous methanol solutions. 

Exfoliation of the perovskite related layer structures is more difficult than for the clays and acid phosphates discussed earlier but can be achieved by intercalation of large bulky amines. Treacy etal. reported that the layered perovskite HCa2Nb30io could be made to form unilamellar sheets by first intercalation of an amine polyether. Spontaneous exfoliation of the layers occurs on subsequent exposure of the intercalated phase to a suitable solvent. Exfoliation techniques have been extended to other systems using tetra(n-butyl)-ammonium hydroxide (TBAOH) by Mallouk and others. A number of examples of the protonated layered perovskite phases that intercalate bases have been exfoliated. The Dion-Jacobson phases typically exfoliate to form plates but others including Ruddlesden-Popper tantalates curl to form tubular scrolls . Part of the interest in these single layer dispersions arises from their use as building blocks in the layer-by-layer self-assembly of thin films. Single layers derived from exfoliated perovskites can be attached to or alternately stacked with polycationic layers to produce thin films. Tiled monolayer structures and multilayer perovskite heterostructures result from the self-assembly. 

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