Perovskite structures

Perovskite structure for ABX3 compounds. The quadravalent A cation is located in the center (darker sphere), octahedrally coordinated by the six divalent anions on the faces. The divalent cations sit on the comers, octahedrally coordinated by the six anions on the faces. 

The inverse spinel structure has one of the cations on the tetrahedral site and the A cation and the other B cation on the octahedral sites. The tetrahedrally coordinated cation shares an ligand with one of the two octahedrally coordinated cations. This feature is responsible for the interesting magnetic properties of the ferrites, a class of ceramic magnets that will be discussed in more detail in Chapter 25. 

Ferroelectric perovskites have their Ciuie temperatrrre scattered over the wide temperatrrre range (e g. -260°C for KTa03 and 490 C for PbTi03). Ferroelectric phases exhibit mostly tetragonal, rhombohedral or orthorhombic crystallographic symmetry. Paraelectric phase is of the cubic m3m symmetry. For ferroelectric characteristics of perovskite crystals see Tables 7.11 and 7.12. 

In the almost ideal perovskite structure of SrTi03, the TiOg octahedra are perfect with 90° angles and six equal Ti—O bonds at 1.952 A Each Sr atom is surrounded by twelve equidistant oxygens at 2.761 A [2]. 

The perovskite structure is thus a superstructure with a Re03-type framework built up by the incorporation of A cations into the BOg octahedra. The significance and role of the Re03-type framework as a host structure for deriving numerous structures of metal oxides has been emphasized by Raveau [38]. 

The crystal chemistry of perovskites was first developed by Goldschmidt [3], who prepared and studied not only a large number of the synthetic perovskites with different compositions (including BaTi03) but also went on to establish the principles for the synthesis of similar compounds, which remain valid to the present day. 

For an ideal perovskite, f is unity. In practice, perovskite-structured compotmds are found to adopt the ideal cubic arrangement for 0.97 f 1.03, but a great many elements can form perovskite structures for lower f-values. The concept of the tolerance factor for the perovskite arrangement of interpenetrating dodecahedra and octahedra indicates how far from ideal packing the ionic sizes can move but still be tolerated by the perovskite structure. The f = 1 condition represents the closepacking in the perovskite stmcture. For f 1, the size of the unit cell is governed by the B-site ion, and as a result the A-site ions have too much room for vibration. 

As a matter of fact, the tolerance factor is a rather complex crystaUo-chemical parameter, which can reflect the structural distortion, force constants of binding, rotation and tilt of the BOg octahedrons. These in turn affect the dielectric properties, transition temperature, temperature coefficient of the dielectric constant of material, and even the dielectric loss behavior in a perovskite dielectric. 

Since each is coordinated with 2 Th+ and 4 Ca L the total bond strength becomes (4/3) + (4/6) = 2, which is the same as the anion oxide valency. Flence, Pauling s rule is satisfied. 

Other examples for compounds having perovskite structure are BaTi03, SrTiOg, SrSnOj, and CaZrOg. 

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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... 

Ternary alkali-metal halide oxides are known and have the expected structures. Thus Na3C10 and the yellow K3BrO have the aqti-perovskite structure (p. 963) whereas Na4Br20, Na4l20 and K4Br20 have the tetragonal anti-K2NiF4 structure. 

Chlorates and bromates feature the expected pyramidal ions X03 with angles close to the tetrahedral (106-107°). With iodates the interatomic angles at iodine are rather less (97-105°) and there are three short I-O distances (177-190 pm) and three somewhat longer distances (251-300 pm) leading to distorted perovskite structures (p. 963) with pseudo-sixfold coordination of iodine and piezoelectric properties (p. 58). In Sr(I03)2.H20 the coordination number of iodine rises to 7 and this increases still further to 8 (square antiprism) in Ce(I03)4 and Zr(I03)4. 

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. 

The relatively high cost and lack of domestic supply of noble metals has spurred considerable efforts toward the development of nonnoble metal catalysts for automobile exhaust control. A very large number of base metal oxides and mixtures of oxides have been considered, especially the transition metals, such as copper, chromium, nickel, manganese, cobalt vanadium, and iron. Particularly prominent are the copper chromites, which are mixtures of the oxides of copper and chromium, with various promoters added. These materials are active in the oxidation of CO and hydrocarbons, as well as in the reduction of NO in the presence of CO (55-59). Rare earth oxides, such as lanthanum cobaltate and lanthanum lead manganite with Perovskite structure, have been investigated for CO oxidation, but have not been tested and shown to be sufficiently active under realistic and demanding conditions (60-63). Hopcalities are out-... 

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. 

The sharp peak around OV in Fig. 4 may be caused by the insertion of protons into the empty A sites of the poorly crystalline WO3 with a distorted perovskite structure. Amorphous... 

With respect to CO oxidation an activity order similar to that described above for CH4 combustion has been obtained. A specific activity enhancement is observed for Lai Co 1-973 that has provided a 10% conversion of CO already at 393 K, 60 K below the temperature required by LalMnl-973. This behavior is in line with literature reports on CO oxidation over lanthanum metallates with perovskite structures [17] indicating LaCoOs as the most active system. As in the case of CH4 combustion, calcination at 1373 K of LalMnl has resulted in a significant decrease of the catalytic activity. Indeed the activity of LalMnl-1373 is similar to those of Mn-substituted hexaaluminates calcined at 1573 K. Dififerently from the results of CH4 combustion tests no stability problems have been evidenced under reaction conditions for LalMnl-1373 possibly due to the low temperature range of CO oxidation experiments. Similar apparent activation energies have been calculated for all the investigated systems, ranging from 13 to 15 Kcal/mole, i.e almost 10 Kcal/mole lower than those calculated for CH4 oxidation. 

The magnetic properties of the new solid solution series SrFe Rui 3 3, (0 < X < 0.5) with distorted perovskite structure, where iron substitutes exclusively as Fe(in) thereby causing oxygen deficiency, has also been studied by Greenwood s group [147] using both u and Fe Mossbauer spectroscopy. Iron substitution was found to have little effect on the magnetic behavior of Ru(IV) provided that X remains small (x < 0.2). 

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). 

Zener, C. 1951. Interaction between the (/-shells in the transition metals. II. Ferromagnetic compounds of manganese with perovskite structure. Physical Review 82 403-405. 

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. 

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