Perovskite-Related Structures

The parent perovskite-type structure (Fig. 4.13A ) is composed of corner-linked BOe octahedra surrounding large A cations and is conveniently idealized to cubic symmetry (Fig. 4.27a). (The real structures have lower symmetry than the idealized structures, mainly due to temperature-sensitive distortions of the BOft octahedra.) In the phases related to Ca2Nb2C 7 the parent structure is broken into slabs parallel to 110 planes. The formula of each slab is A B 03 +2, where n is the number of 

Similar 110 faults arise in a number of other systems as a result of composition change. For example, the reaction of the perovskite SrTiC 3 with Nb205 results in crystals containing randomly distributed 110 defects. The incorporation of Nb205 gives each crystal a composition Sr(Nbi,Ti j and each Nb5+ ion 

Many examples of Ruddleston-Popper phases have been synthesized. The structure of the first member of the series, corresponding to n = 1, is adopted by a number of compounds, including the important phase La2Cu04 (Section 4.3.3) and is often referred to as the K2NiF4 structure. In practice, synthesis of A +1B C)3 +i phases frequently results in disordered materials in which random or partly ordered regions of 100 faults occur, and particular efforts have to be made to produce perfectly ordered crystals. 

If the pair of A atoms at the boundaries of the perovskite-like sheets in the Ruddleston-Popper phases are replaced with just one A atom, the series of phases takes the formula A (A iB 03 +i), where A and A are large ions, typically a (+1/+3) pairing, and B is a medium-sized transition-metal ion, typically Nb5+. These materials are called Dion-Jacobson phases. The majority of examples synthesized to date are of the n = 2 phase, typified by KLaNb207, CsBiNb207, and so on (Fig. 4.29). A few examples of the n = 3 phase are also known, including CsCa2Nb3Oio- 

If the A layers in the above series are replaced by a layer of composition Bi202, a series of phases is formed called Aurivillius phases, with a general formula (Bi202)(A 1B 03 +i), where A is a large cation, B a medium-sized cation, and the index n mns from 1 to oo. The structure of the Bi202 layer is similar to that of 

In the brownmillerite structure, the oxygen vacancies order in such a way that half the iron of CaFe02.5 are octahedrally coordinated and half are tetrahedrally coordinated. This ordering is an expression of the tetrahedral-site stability of Fe ions. In a system like Lai yCayFe03 x y/2, there is a tendency to form intergrowths of perovskite and brownmillerite structures the oxygen vacancies do not remain randomly distributed. Mossbauer spectra at 4.2 K for this complex system exhibit three sextets they have been interpreted in terms of 2 Fe and one Fe , or of an Fe and the disproportiona- 

The system Bai xSr,Fe03 y has also been extensively studied The structural, electrical and magnetic properties show a progressive evolution with x from those for BaFe03 y to those for SrFe03 y. 

---

Modular structures are those that can be considered to be built from slabs of one or more parent structures. Slabs can be sections from just one parent phase, as in many perovskite-related structures and CS phases, or they can come from two or more parent structures, as in the mica-pyroxene intergrowths. Some of these crystals possess enormous unit cells, of some hundreds of nanometers in length. In many materials the slab thicknesses may vary widely, in which case the slab boundaries will not fall on a regular lattice and form planar defects. 

CUPRATE HIGH-TEMPERATURE SUPERCONDUCTORS 8.6.1 Perovskite-Related Structures and Series... 

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... 

The high oxygen diffusivity, even at room temperature, of the 90 K high-temperature superconductor YBayCusOy s (0 < 5 < 1) is one of its remarkable properties. The large concentration of anion vacancies of the perovskite-related structure (i.e. A3B3O9 2) is responsible for the high mobility of this phase. 

Low-Dimensional Oxides, Materials with 2H-Perovskite Related Structures... 

Some Anion-deficient Perovskite-related Structures.—In this Section we will consider some anion-deficient perovskite phases which can be written as There has... 

These phases seem to form a natural bridge between the perovskite related structures described in Section 2 and classical point-defect containing non-stoicheiometric compounds, and as such would seem to merit further study. 

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... 

X-Rav and Compositional Analyses. Single crystal x-ray precession photographs of YBC crystals from the Pt and Au crucible runs confirmed their perovskite-related structure. Crystal composition was determined by electron microprobe analysis. The Y/Ba/Cu ratios were 1/2/3 in each crystal within the accuracy of the microprobe measurements. However, the crystals grown in Au crucibles contained approximately 0.7 mole% Au. 

Table 5. Structural data for materials with perovskite-related structures...

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. 

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. 

---