Compounds with the Perovskite Structure

In these compounds, the blocks R of expressions (9) and (10) have m - 1, i.e., the rock-salt blocks are monolayers (AX). Important materials with the perovskite structure are represented in the scheme of Table 1, where they are compared to each other and to the general structural type discussed in the previous section. 

In the system Ba(Pb1.xBix)Os, the compound with x = 0.25 must be considered the first discovered ceramic material showing high-temperature superconductivity (7). Structure determinations have been carried out over the entire range of composition (8)-(ll) and the refined parameters are presented in Table 2. Superconductivity in this system exists only for values of x between 0.05 and 0.35. The value of the critical temperature increases with x, reaches a maximum value Tc 13K for x = 0.25, and then decreases. For x 0.35, the material becomes a semiconductor. 

The compound NCu02 (N = Ca0 86Sr014) is an insulator, but its structure, which is a simple defect perovskite made of layers (CuOa) sandwiched between layers (N), can be considered as the parent structure of a large family of superconductors. The sequence. ..(Cu0 )OiC(N)e o(Cu02)OjC. .. is in fact one of the building blocks of many compounds considered in this review. The refined parameters for NCu02 are given in Table 3. 

In this structure the (CuOz) layers are flat (z(Cu) - z(0) - 0.0). The coordination of the Cu atoms is square-planar while the atoms N = Ca0 ggSrQ are eight-coordinated and the coordination polyhedron is a square prism. In this compound, the copper is formally 2+. 

Compounds with n 2 have never been prepared, however. The refined parameters of BaYM2Og are given in Table 4, and the structure of Ba2YCu3Ox will be discussed in a separate section. 

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Table 5.6 Some compounds with the perovskite structure. 

Figure 8.31. A. Relationship between the " i nuclear quadrupole coupling constants of ATiOs-type titanates and their structural shear strain i(( defined in equation (8.3). B. Relationship between the Ti isotropic chemical shifts (ppm) and the mean Ti-O bond lengths (A) for a series of ATi03 compounds with the perovskite structure. Note that compounds with the ilmenite stmcture do not fit this relationship and are not included here. From Padro et al. (2000), by permission of the copyright owner.

A third point to be noted from table 8.07 is that among the compounds with the perovskite structure are many titanates niobates 4 stannates , etc., which would normally be regarded as inorganic salts. Structurally, however, there is no justification for this view. We shall later find that in the true salts of inorganic acids finite complex anions have a discrete existence in the crystal structure in calcium carbonate, for example, anions C032 are clearly recognizable and the structure as a whole is built up of these anions and of Ca2+ cations arranged in a manner very... 

The most numerous and most interesting compounds with the perovskite structure are oxides. Some hydrides, carbides, halides, and nitrides also crystallize with this structure (4). This review will refer only to the study of oxides and their behavior in the gas-solid interface and in heterogeneous catalysis. It will not cover, however, electric, magnetic, and optical properties of perovskites. Comprehensive studies on these... 

The perovskite family is one of the most important representative among a large variety of inorganic compoimds. The majority of chemical elements from the periodic table can form ABX3 compounds with the perovskite structure (Goodenough and Longo, 1970 Goodenough, 1971 Fesenko, 1972 Reller, 1993 Woodward,... 

Liebermann RC, Jones LEA, Ringwood AE (1977) Elasticity of aluminate, titanate, stannate and germanate compounds with the perovskite structure. Phys Earth Planet Inter 14(2) 165... 

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. 

Barium titanate is one example of a ferroelectric material. Other oxides with the perovskite structure are also ferroelectric (e.g., lead titanate and lithium niobate). One important set of such compounds, used in many transducer applications, is the mixed oxides PZT (PbZri-Ji/Ds). These, like barium titanate, have small ions in Oe cages which are easily displaced. Other ferroelectric solids include hydrogen-bonded solids, such as KH2PO4 and Rochelle salt (NaKC4H406.4H20), salts with anions which possess dipole moments, such as NaNOz, and copolymers of poly vinylidene fluoride. It has even been proposed that ferroelectric mechanisms are involved in some biological processes such as brain memory and voltagedependent ion channels concerned with impulse conduction in nerve and muscle cells. 

Neither element shows any simple aqueous chemistry in the M(IV) state, as the oxides M02 are insoluble in water at all pH values. Reaction of Sn02 in molten KOH gives the octahedral hydroxanion [Sn(OH)6]2-, in contrast to the normal tetrahedral silicates and germinates, but in parallel with isoelectronic compounds such as Te(OH)6 also found in period 5. Other stannates are mixed oxides without discrete oxoanions (e.g. CaSn03 with the perovskite structure). 

Perovskites — are a very large family of compounds with the crystal structure similar to that of (Ca, Sr) TiC>3 mineral, named after the Russian mineralogist T. A. Per-ovski (1792-1856). This structure is often called an inorganic chameleon due to the diversity of chemical compositions, structural modifications, and properties. Although most members of this family are oxidic compounds, some nitrides, carbides, halides, and hydrides also have this structure. 

Kosova N.V., Devyatkina E.T., Avvakumov E.G., Gainutdinov 1.1., Rogachev A.Yu., Pavlyukhin Yu.T., Isupova L.A., Sadykov V.A. Mechanochemical synthesis of dicalcium ferrite with the perovskite structure. Neorg. Materialy. 1998 34 385-90. Schmidt M. And Kaczmarek W.A. Synthesis of SrFeOj 5 from mechanically activated reactants. 1. Alloys and Compounds. 1999 283 117-21. 

Other oxygen ion conductors that have potential use as solid electrolytes in electrochemical devices are stabilized bismuth and cerium oxides and oxide compounds with the perovskite and pyrochlore crystal structures. The ionic conductivity and related properties of these compounds in comparison with those of the standard yttria-stabilized zirconia (YSZ) electrolyte are briefly described in this section. Many of the powder preparation and ceramic fabrication techniques described above for zirconia-based electrolytes can be adapted to these alternative conductors and are not discussed further. 

In 1987 J. G. Bednorz and K. A. Muller were awarded the Nobel prize for physics for their discovery of the high temperature superconductivity exhibited by compounds with a perovskite structure. 

The luminescence from octahedral uranate groups has also been reported for other uranium-doped oxidic compounds (see e.g. Ref. 7). Like in uranium-doped compounds with ordered perovskite structure isolated UOg octahedra are present in several other host lattices. In this type of compounds e.g. Y3Li3Te20i2-U" LigWOs-LT I and Mg3TeOg—, the luminescence properties of the octahedral uranate group are similar to the properties which have been observed for uranium-doped ordered per-ovskites. Due to symmetry lowering the vibrational structure in the luminescence spectra is more complicated, and also the luminescence decay time is shorter than in ordered perovskite systems (c.f. Sect. 2.1). 

A still more extreme example shows that the perovskite structure can even occur with some of the A sites unoccupied. Sodium tungsten bronze has the ideal composition NaWOs, with the perovskite structure, but this compound shows very variable composition and colour, and is better represented by the formula Na WOg with 1 > x > 0. In the sodium-poor varieties the structure remains essentially unaltered but some of the sites normally occupied by sodium are vacant. To preserve neutrality one tungsten ion is converted from W5 to W6+ for every site so unoccupied, and this change in ionization gives rise to the characteristic alteration in colour and explains its association with the sodium content. In the extreme case, when no sodium is present, we have W03, the structure of which is closely related to that of A1F3. We have already shown how this structure, in its turn, is related to that of perovskite. 

The influence of geometrical considerations is similarly reflected in the common occurrence of solid solution among compounds with the perovskite, ilmenite and spinel structures, to which we have already referred. In these structures solid solution takes place particularly readily because in each case the structure is basically a rigid framework of large ions with small cations accommodated in the interstices. So long... 

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