The perovskite structure

(a) The perovskite structure for compounds ABO3 (or ABX3). Large open circles represent 0 (or F) ions, the shaded circle A, and the small circles B ions, (b) The crystal structure of ReOs. 

Ideal cubic structure SrTiOa, SrZrOj, SrHfOa SrSnOa, SrFe03 BaZrOa, BaHfOa, BaSnOa BaCeOa EuTiOa, LaMnOa 

At least one form with distorted small cell (a 4 A) BaTiOa (C, T,0, R) 

Distorted multiple cells CaTiOa.NaNbOa, PbZrOa PbHfOa, LaCrOa low-PbTiOa low-NaNbOa, high-NaNbOa 

The complex oxide BaTiOs ( barium titanate ) is remarkable in having five crystalline forms, of which three are ferroelectric. The structure of the high-temperature hexagonal form, stable from 1460°C to the melting point (1612°C), has already been described. The other forms are  

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Good results are obtained with oxide-coated valve metals as anode materials. These electrically conducting ceramic coatings of p-conducting spinel-ferrite (e.g., cobalt, nickel and lithium ferrites) have very low consumption rates. Lithium ferrite has proved particularly effective because it possesses excellent adhesion on titanium and niobium [26]. In addition, doping the perovskite structure with monovalent lithium ions provides good electrical conductivity for anodic reactions. Anodes produced in this way are distributed under the trade name Lida [27]. The consumption rate in seawater is given as 10 g A ar and in fresh water is... 

Zirconates and hafnates can be prepared by firing appropriate mixtures of oxides, carbonates or nitrates. None of them are known to contain discrete [M04]" or [MOs] ions. Compounds M ZrOs usually have the perovskite structure whereas M2Zr04 frequently adopt the spinel structure. 

From a detailed analysis of the stability regions in the alloy series MR Rh3Bi y and Er(Rh, yM,)3B y the formation of the perovskite structure was discussed in terms of a stabilizing charge transfer (B as electron donor) and a corresponding characteristic VEC, 31 < VEC < 34. 

Structure of YBa2Cu307. The perovskite structure is attained by inserting O atoms between the strings of Y atoms and between the Cu04 squares. Two unit cells are shown in each direction (stereo image) r... 

Surface reconstruction has been earlier observed and reported in the literature [116]. Sequential reductive and oxidative thermal treatment usually leads to bulk transition from CoOx + La203 to LaCo03, respectively. On the other hand, the restoration of the perovskite structure is not observed under severe conditions at higher temperature. In those temperature conditions, the sintering of Co crystallites leads to irreversible redox cycle with the preferential formation of Co304 under lean conditions. 

It may be apparent from studying the perovskite structure that it is likely to exhibit quite anisotropic plastic (hardness) behavior, and it does. The primary glide plane is (110) and the glide direction is (1-10). 

Consider now the bonds to each O2- ion in the perovskite structure. First, there are two bonds to Ti4+ ions that have a character of 4/6 each, which gives a total of 4/3. However, there are four Ca2+ ions on the corners of the face of the cube where an oxide ion resides. These four bonds must add up to a valence of 2/3 so that the total valence of 2 for oxygen is satisfied. If each Ca-O bond amounts to a bond character of 1/6, four such bonds would give the required 2/3 bond to complete the valence of oxygen. From this it follows that each Ca2+ must be surrounded by 12 oxide ions so that 12(1/6) = 2, the valence of calcium. It should be apparent that the concept of electrostatic bond character is a very important tool for understanding crystal structures. 

RbCaF3 has the perovskite structure with the Ca in the center of the unit cell. What is the electrostatic bond character of each of the Ca-F bonds How many fluoride ions must surround each Ca2+ ion What is the electrostatic bond character of each Rb-F bond How many F ions surround each Rb+ ... 

The Incentive to modify our existing continuous-flow microunit to incorporate the square pulse capability was provided by our work on perovskite-type oxides as oxidation-reduction catalysts. In earlier work, it had been inferred that oxygen vacancies in the perovskite structure played an important role in catalytic activity (3). Pursuing this idea with perovskites of the type Lai-xSrxFeg 51 10 503, our experiments were hampered by hysteresis effects which we assumed to be due to the response of the catalyst s oxygen stoichiometry to the reaction conditions. 

The principles described above apply equally well to oxides with more complex formulas. In these materials, however, there are generally a number of different cations or anions present. Generally, only one of the ionic species will be affected by the defect forming reaction while (ideally) others will remain unaltered. The reactant, on the other hand, can be introduced into any of the suitable ion sites. This leads to a certain amount of complexity in writing the defect equations that apply. The simplest way to bypass this difficulty is to decompose the complex oxide into its major components and treat these separately. Two examples, using the perovskite structure, can illustrate this. 

The perovskite structure, ABO3 (where A represents a large cation and B a medium-size cation) is adopted by many solids and solid solutions between them can readily be prepared. Vacancy-containing systems with the perovskite structure are of interest as electrolytes in solid-state batteries and fuel cells. Typical representatives of this type of material can be made by introducing a higher valence cation into the A sites or a lower valance cation into the B sites. 

The ion Co corresponds to Co4+ and the number of Co4+ ions is then equal to the number of dopant Sr2+ ions. The reaction can be formally considered to be the formation of a solid solution between the perovskite structure phases LaCo03 (Co3+) and SrCo03 (Co4+). The formula is formally Laj Sr.Coii.Co4 103. 

The Brouwer diagram approach can be illustrated with reference to the perovskite structure oxide system BaYbvPr VC>3, which has been explored as a potential cathode material for use in solid oxide fuel cells. The parent phase... 

The use of this approach can be illustrated by the perovskite structure proton conductor BaYo.2Zro.gO3 g- This material has been investigated for possible use in solid oxide fuel cells, hydrogen sensors and pumps, and as catalysts. It is similar to the BaPr03 oxide described above. The parent phase is Ba2+Zr4+03, and doping with... 

In the early 1990s, Balachandran et al. (51,64,65) of the Argonne National Laboratory, in collaboration with Amoco (now part of BP), investigated the partial oxidation of methane using membrane materials consisting of Sr-Fe-Co-O mixed oxides with the perovskite structure, which have high oxygen permeabilities. In their experiments (51,66), the membrane tubes, which were... 

Lanthanum chromite is a p-type conductor so divalent ions, which act as electron acceptors on the trivalent (La3+ or Cr3+) sites, are used to increase the conductivity. As discussed above, the most common dopants are calcium and strontium on the lanthanum site. Although there is considerable scatter in the conductivities reported by different researchers due to differences in microstrucure and morpohology, the increase in conductivity with calcium doping is typically higher than that with strontium doping [4], The increase in conductivity at 700°C in air with calcium additions is shown in Figure 4.1 [1, 2, 28-44], One of the advantages of the perovskite structure is that it... 

This structure is closely related to the perovskite structure. 

The perovskite structure and its variant and derivative structures, and superstructures, are adopted by many compounds with a formula 1 1 3 (and also with more complex compositions). The ideal, cubic perovskite structure is not very common, even the mineral CaTi03 is slightly distorted (an undistorted example is given by SrTi03). 

Another way of representing the perovskite structure is to move the origin of the coordination axes such that the Ca2+ ions are now at the centre of the cubic cell, Figure 9. The latter representation allows a better understanding of the structure of a superconductor such as... 

The perovskite structure is capable of high anion conductivity when oxide vacancies are introduced, as in, for example, Lai (Sr Co03 (/2 or in the perovskite-related superconductor phases, La2Cu04 and YBa2Cu307. The oxide ion transport number is not unity since such materials are often electronic conductors as well, due to the presence of... 

The perovskite structure is stable to relatively large amounts of dopant ions on either A or B sites. Oxygen vacancies are introduced into the lattice, either through transition-metal redox processes or by doping on the A or B sites with lower valence cations. 

Finally, a number of other mixed oxides that do not have the perovskite structure have also been examined. For example, niobium titanates with the rutile structure,tetragonal tungsten bronze... 

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