Perovskite cubic

In Figure 2.18a, the comer-sharing octahedra that form the skeleton of the ideal close-packed perovskite cubic structure is shown, in which the center of the cube is occupied by the A cation. Alternatively, this structure can be viewed with the B cation placed in the center the cube (see Figure 2.18b) [8],... 

An orthohombic domain (0) for 0 < x < 0.1, which corresponds to a monoclinic distortion of the perovskite cubic cell leading to the following relations ... 

Perovskite Cubic close-packed 12 6 6 ABO3 1/4 oct. (B) CaTi03, SrTi03, SrSn03, SrZr03. SrHf03, BaTi03... 

Methane oxidative coupling by definite compounds( e.g. perovskite, cubic or monoclinic structure,...) obtained by low temperature processes... 

We investigate the catalytic properties of several rare earth definite compounds ( e.g. perovskite, cubic or monoclinic structure,...). We notice that the nature of the rare earth oxygen environment is in relation with the selectivity. Coulomblan energy computations show clearly these... 

Oxygen Octahedra. An important group of ferroelectrics is that known as the perovskites. The perfect perovskite stmcture is a simple cubic one as shown in Figure 2, having the general formula ABO, where A is a monovalent or divalent metal such as Na, K, Rb, Ca, Sr, Ba, or Pb, and B is a tetra- or pentavalent cation such as Ti, Sn, Zr, Nb, Ta, or W. The first perovskite ferroelectric to be discovered was barium titanate [12047-27-7] and it is the most thoroughly investigated ferroelectric material (10). 

Fig. 2. Cubic (m3m) prototype stmcture of perovskite-type ABO compounds where , A , B and O, O.

Lead zirconate [12060-01 -4] PbZrO, mol wt 346.41, has two colorless crystal stmctures a cubic perovskite form above 230°C (Curie point) and a pseudotetragonal or orthorhombic form below 230°C. It is insoluble in water and aqueous alkaUes, but soluble in strong mineral acids. Lead zirconate is usually prepared by heating together the oxides of lead and zirconium in the proper proportion. It readily forms soHd solutions with other compounds with the ABO stmcture, such as barium zirconate or lead titanate. Mixed lead titanate-zirconates have particularly high piezoelectric properties. They are used in high power acoustic-radiating transducers, hydrophones, and specialty instmments (146). 

Elemental composition, ionic charge, and oxidation state are the dominant considerations in inorganic nomenclature. Coimectivity, ie, which atoms are linked by bonds to which other atoms, has not generally been considered to be important, and indeed, in some types of compounds, such as cluster compounds, it caimot be appHed unambiguously. However, when it is necessary to indicate coimectivity, itaUcized symbols for the connected atoms are used, as in trioxodinitrate(A/,A/), O2N—NO . The nomenclature that has been presented appHes to isolated molecules (or ions). Eor substances in the soHd state, which may have more than one crystal stmcture, with individual connectivities, two devices are used. The name of a mineral that exemplifies a particular crystal stmcture, eg, mtile or perovskite, may be appended. Alternatively, the crystal stmcture symmetry, eg, rhombic or triclinic, may be cited, or the stmcture may be stated in a phrase, eg, face-centered cubic. 

The ternary Ge halides, MGeX3 (M = Rb, Cs X = Cl, Br, I) are polymorphic with various distorted perovskite-like (p. 963) structures which reflect the influence of the nonbonding pair of electrons on the Ge" centre. Thus, at room temperature, rhombohedral CsGel3 has three Ge-I at 275 pm and three at 327 pm whereas in the high-temperature cubic form (above 277°C) there are six Ge-I distances at 320 pm as a result of position changes of the Ge atoms (reversible order-disorder transition). Again, RbGel3 has a lemon-yellow, orthorhombic form below —92° an intermediate, bordeaux-red orthorhombic perovskite form (—92° to —52°) a black rhombohedral form (—52° to —29°) and... 

Figure 21.3 Two representations of the structure of perovskite, CaTi03, 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 Re03 (p. 1047).

Geometry requires a value of t = 1 for the ideal cubic structure. In fact, this structure occurs if 0.89 < t < 1. Distorted perovskites occur if 0.8 < t < 0.89. With values less than... 

Figure 11.6 Views of perovskite crystal structure. Top—conventional cubic unit cell white circles = oxygen black circle = transition metal gray circles = alkali or alkaline earth metal. Bottom—extended unit cell to show the cage formed by the oxygen octa-hedra. Adapted from Bragg et al. (1965).

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