Perovskite layered compounds

This method of approach, when applied to the above mentioned compounds leads to a Na2Ta306F5 structure in which n = 3 (n denotes the number of layers) if the perovskite positions remain vacant, as shown in Fig. 36 (a). The Na4Ta5OioF9 phase corresponds to n = 2.5, which leads to the proposed structure consisting of two types of alternating layers, characterized by n = 2 and n = 3, as shown in Fig. 36 (b). The central positions of the perovskite layers remain vacant in this structure as well. 

If the perovskite layers have a thickness of n unit cells (instead of one), compounds of the general formula A +, B X3 +, are formed, as in Sr3Ti207 and K3Mn2Cl7. Crystallographic and magnetic properties of perovskites and perovskite derivatives have been recently tabulated by Goodenough and Longo (446). 

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. 

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... 

Compounds Containing Perovskite Layers. A second class of layered oxides have structures related to the three-dimensional perovskite lattice and include the Auriv-illius phases, the Ruddlesden-Popper phases and the Dion-Jacobson phases. The general composition can be written Ma[A iB 03 +i] where A is an alkaline or rare earth metal, and B is niobium or titanium. In the Aurivillius phases = Bi2 02 +, whereas M is an alkali metal cation in the ion-exchangeable Ruddlesden-Popper a = 2) and Dion-Jacobson a = 1) phases. The relationships between the three structure types is shown in Figure 14. The intercalation chemistry of the Dion - Jacobson phases was the first to be studied. 

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