Perovskite, CaTiO structures

The perovskite (CaTiOs) structure type a double oxide... 

As an example, we consider oxygen atom 2s functions in the perovskite CaTiOs structure. The oxygen atoms occupy Wyckoff position c of the space group 0 with the site-symmetry 04 The 2s-functions of an oxygen atom transform over Uig irrep of the point group 04. Thus, the induced representation in q basis c,aig) is three-dimensional at each k point (d ) = a4g,np = l,n,q = 3). 

Figure 1.42 The perovskite crystal structure of CaTiOs. From W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

BaTi03 is isostructural with the mineral perovskite (CaTiO ) and so is referred to as a perovskite . The generalized perovskite structure AB03, is visualized as based on a cubic close-packed assembly (see Fig. 2.1) of composition A03 with the A-ion cordinated with 12 oxygen ions and the B-ion in the octahedral interstices (see Fig. 2.39). A consideration of the geometry shows that for a prefect fit the following relationship between the ionic radii holds [12]. 

As has been mentioned, ternary compounds having polyatomic ions such as NO3-, C032-, NFLt+, and so on often have the same types of structures as binary compounds in which a polyatomic ion occupies a lattice site as a unit. The mineral perovskite, CaTiOs, calcium titanate, however, is a somewhat different type of ternary compound that has the structure shown in Figure 3.8(a). Most ternary compounds are oxides, and the general formula ABO3 corresponds to many compounds because A = Ca, Sr, Ba, and so forth, and B = Ti, Zr, Al,... 

Several members of the MM O3 class of ternary metal oxides adopt the perovskite-type (CaTiOs) structure and are sought as worthy target materials possessing ferroelectric properties see Ferroelectricity) Among the more widely investigated members of this class are BaTiOs and SrTiOs. Clearly, use of these materials as potential memory device... 

Barium titanate, which has many novel properties, is a mixed oxide ceramic. It has the same structure as the mineral perovskite, CaTiOs (Fig. 22.13), except, of course, that Ba replaces Ca. Perovskites typically have two metal atoms for every three O atoms, giving them the general formula ABO3, where A stands for a metal atom at the center of the unit cube and B stands for an atom of a different metal at the cube corners. 

Fig. 10.15. Idealized structure of (a) cubic perovskite (CaTiOs) and b) orthorhombic brownmillerite (Ca2Fe20s) lattice with ordered oxygen vacancies ( ) along the cubic [110] direction.

Figure 21.3 Two representations of the structure of perovskite, CaTiOs, showing (a) the octahedral coordination of Ti, and (b) the twelve-fold coordination of Ca by oxygen. Note the relation of (b) to the cubic structure of ReOs (p. 1047).

Some high-temperature superconductors adopt a crystal structure similar to that of perovskite (CaTiOs). The unit cell is cubic with a Ti" " ion in each comer, a Ca " ion in the body center, and 0 ions at the midpoint of each edge, (a) Is this unit cell simple, body-centered, or face-centered (b) If the unit cell edge length is 3.84 A, what is the density of perovskite (in g/cm ) ... 

Redfern, S.A.T. (1996) High-temperature structural phase transitions in perovskite (CaTiOs)./. Phys. Condens. Mater., 8, 8267-8285. 

The special electric, magnetic, optical, superconductive and catalytic properties of perovskite-typed oxides make this group of materials attracting and widely used. Perovskites were named according to the similarity of their structure with the CaTiOs compoimd. The... 

Ordering of vacancies also plays a key role in selective oxidation catalysis over perovskite-based catalysts such as CaMnOs oxides. CaMnOs has a CaTiOs (AMO3) perovskite structure which is made up of cations coordinated to 12 0 anions. They, in turn, are connected to corner-sharing MoOe octahedra. CaMnOs was used as a model catalyst on a laboratory scale by Thomas et al (1982) in propene oxidation to benzene and 2-methyl propene to paraxylene. In such reactions the compounds are found to undergo reduction to form anion-deficient metastable phases of the type CaMnOs-x where 0 < x < 0.5, forming several distinct phases. 

FIGURE 1.44 The perovskite structure of compounds ABX3, such as CaTiOs. Ca, green sphere Ti, silver spheres 0, red spheres. 

FIGURE 10.6 The A-type unit cell of the perovskite structure for compounds ABOs, such as CaTiOs. 

The parent perovskite structure shown in Fig. 10.4 consists of alternating layers of composition AO and BO2, as for example in BaTiOs (23759) and CaTiOs (62149). It is also possible to have several AO layers between each BO2 layer providing each AO layer is sheared by half a unit cell from the adjacent AO layers, as shown for La2Ni04 in Fig. 12.1. This permits a wide range of structures with an even wider range of compositions to be prepared. Which compositions are possible depend on how well the structure can accommodate the bonding requirements of the atoms A and B. 

Perovskite structure the structure of the mineral CaTiOs Platinum metals the six platinum metals are ruthenium, rhodium, palladium, osmium, iridium, and platinnm Radiolysis decomposition of a compound by ionizing radiation such as X- or y-rays or a-particles Rutile structure the structure of the mineral rutile, a form of Ti02... 

Raman spectra as a function of temperature are shown in Fig. 21.6b for the C2B4S2 SL. Other superlattices exhibit similar temperature evolution of Raman spectra. These data were used to determine Tc using the same approach as described in the previous section, based on the fact that cubic centrosymmetric perovskite-type crystals have no first-order Raman active modes in the paraelectric phase. The temperature evolution of Raman spectra has indicated that all SLs remain in the tetragonal ferroelectric phase with out-of-plane polarization in the entire temperature range below T. The Tc determination is illustrated in Fig. 21.7 for three of the SLs studied SIBICI, S2B4C2, and S1B3C1. Again, the normalized intensities of the TO2 and TO4 phonon peaks (marked by arrows in Fig. 21.6b) were used. In the three-component SLs studied, a structural asymmetry is introduced by the presence of the three different layers, BaTiOs, SrTiOs, and CaTiOs, in each period. Therefore, the phonon peaks should not disappear from the spectra completely upon transition to the paraelectric phase at T. Raman intensity should rather drop to some small but non-zero value. However, this inversion symmetry breakdown appears to have a small effect in terms of atomic displacement patterns associated with phonons, and this residual above-Tc Raman intensity appears too small to be detected. Therefore, the observed temperature evolution of Raman intensities shows a behavior similar to that of symmetric two-component superlattices. 

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