Cubic Perovskite-Type Structure

6 Crystal Structure and Structural Disorder of Oxide Ions in Cathode Materials, Lao.6Sro.4Co03 3 and Lao.6Sro.4Coo.8Feo.203 3, with a Cubic Perovskite-Type Structure [12,13] 

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Figure 28. Crystal structure of TisAlN (CaTi03, perovskite type) and Ti2AlN (Cr2AlC, H phase type structure). In both structures the metal atoms are close packed. In the cubic perovskite type structure the positions of the close-packed metal atoms correspond to those of the CU3 Au structure (space group Pmim) with the stacking sequence ABC, ABC. In the hexagonal Cr2AlC type structure of Ti2AlN the stacking sequence of the close-packed metal atoms is AB, AB.

Rietveld analysis of the neutron powder diffraetion patterns of Lao,64(Tio,92Nbo.o8)02.99 at 769 K, 1281 K, and 1631 K was performed assuming a tetragonal P4/mww strueture a = b k, a, c k. 2op. subseript p denotes pseudo-cubic perovskite-type structure). The refined unit-cell and structural parameters and R factors are summarized in Table 6.2 [11]. The unit-cell parameters increase with temperature, as does the unit-cell volume due to thermal expansion. Figure 6.7(a) shows the crystal structure of the material... 

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... 

Several other research groups (89-91) picked up on this discovery and obtained Ba/K/Bi/O materials with an onset transition temperature of 34 K. The structure of this oxide superconductor is cubic, perovskite-type, having unit cell dimensions, 4.288-4.293 A. This system appears to be a three-dimensional superconductor and has... 

Table 4 Numbers and stacking schemes of close-packed [AF3] layers in trigonal and hexagonal fluoride perovskites , elpasolites , and derivatives and typical compounds (cubic perovskite included, structure types in italics, space groups in parentheses)...

Ferroelectrics of the perovskite-type structure CTi03 with different cations C = Ba, Ca, Pb, Sr are of fundamental importance in the comprehension of the mechanism governing ferroelectricity, because the simple perovskite structure gives the opportunity to study the onset of ferroelectricity from a highly symmetric cubic phase. 

At a later time, it was found that those compounds with a perovskite-type structure can be stable if Ta > 0.90, and 0.51 A< < 1.1 A [40-42]. If Ta > r , then the oxygen ions in layers AO3 are separated and the size of octahedral voids are increased, so that cations with > 0.41 r can be placed. If Ta < r , the ions A are found smaller than the voids with Za = 12, and a parameter of the cubic unit cell is defined primarily by the value of 2(rB + ro). Thus, in a perovskite structure the sizes of positions with Za = 12 and Zb = 6 are interconnected. Therefore, the ABO3 perovskite structure has a great versatility to adapt to unmatched equilibrium A—O and B—O bond lengths. 

Figure 12 Crystal structures of AMXs-type compounds, (a-d) The cubic perovskite type of structure, e.g., CsTmCl3 the so-called hexagonal perovskite, e.g., CsScojiCls the stuffed PuBrs or FelJSs type of structure, e.g., KTmIs the NH4CdCl3 type of structure, e.g., RbTm ...

Yashima, M. and Tsuji, T. (2007) Structural investigation of the cubic perovskite-type doped lanthanum cobaltite Lao.5Sro.4Co03 5 at 1531K possible diffusion path of oxygen ions in an electrode material. /. Appl. 

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