Other Perovskite-Related Structures

1 Polytypes Consisting of Close-Packed Ordered AOj Layers 

Such phases exhibit properties that range from a typical insulator or ferroelectric to a conductor or superconductor. Several single-crystal phases of the (Sr0)(SrTi03) series have been synthesized with different n-values, and their dielectric constants measured [54]. The (La0)(LaNi03) compounds have also been prepared successfirl-ly]55].Someother RP-type phases studied by various groups are detailed in Refs [5,56]. 

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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). 

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. 

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. 

Ulla and Lombardo have focused their attention on catalysis with mixed oxides usually having perovskite or perovskite-related structures, but other structures are also considered. Information on the preparation, characterization, and redox reactions of these oxides are considered. Attention is then given to many physico-chemical applications of these materials. 

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).

Titanium IV) oxide, T1O2. See titanium dioxide. Dissolves in concentrated alkali hydroxides to give titanates. Mixed metal oxides, many of commercial importance, are formed by TiOj. CaTiOj is perovskite. BaTiOa, per-ovskite related structure, is piezoelectric and is used in transducers in ultrasonic apparatus and gramophone pickups and also as a polishing compound. Other mixed oxides have the il-menite structure (e.g. FeTiOj) and the spinel structure (e.g. MgjTiO ). 

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. 

Figure 22 Structures of members of the perovskite related series /4n5n03 + 2. (a) BaZnp4, the prototype of the A 2B20i (n = 2) oxides, and (b) Ca2Nb207 and other A B Oia. oxides in = 4)...

It is likely that similar oxide structures will be found in many other systems, and systematic studies, especially involving electron microscopy, will be likely to reveal many more phases in these perovskite related systems. 

The ample diversity of properties that these compounds exhibit, is derived from the fact that over 90% of the natural metallic elements of the periodic table are known to be stable in a perovskite oxide structure and also from the possibility of synthesis of multicomponent perovskites by partial substitution of cations in positions A and B giving rise to compounds of formula (AjfA i- )(ByB i-J,)03. This accounts for the variety of reactions in which they have been used as catalysts. Other interesting characteristics of perovskites are related to the stability of mixed oxidation states or unusual oxidation states in the structure. In this respect, the studies of Michel et al. (12) on a new metallic Cu2+-Cu3+ mixed-valence Ba-La-Cu oxide greatly favored the development of perovskites exhibiting superconductivity above liquid N2 temperature (13). In addition, these isomorphic compounds, because of their controllable physical and chemical properties, were used as model systems for basic research (14). 

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. 

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. 

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