Perovskite CaTiOj

The Perovskite Structure, ABXS Systems. Cubic Pm3m (Space Group 221) A cubic structure was assigned to the mineral perovskite, CaTiOj, but this particular compound was later found to actually possess orthorhombic symmetry. Today, however, we refer to the perovskite structure in its idealized form as having cubic symmetry and it is normally represented by a simple unit cell (Figure 10). 

Biissem (B12) determined the approximate crystal structure of C4AF. Subsequent determinations or refinements were reported for preparations with. Y = 0 (C F) (B13,C4,B11),. v = 0.285 and 0..36 (C2) and, v = 0.5 (C3). Fig. 1.8 shows the structure for the compositional range with 0.33 <. Y < 0.7. It is built from layers of corner-sharing octahedra similar to those in perovskite (CaTiOj), alternating with layers composed of chains of tetrahedra, together with Ca " ions. The layers are perpendicular to the h axis and the chains are parallel to c. The composition of an individual octahedral layer in the ac cross-section of the unit cell is MiO. and that of an individual chain is where M and T denote octahedral and... 

For each of the crystai structures iisted here, indicate whether it is a metaiiic. Group i8, network covaient, ionic, or moiecuiar soiid and identify the Bravais iattice. a. Perovskite (CaTiOj)... 

Titanium is relatively abundant and widely distributed in the earth s crust In igneous rocks titanium forms the acidic component of basic magmas and the basic component of acidic magmas. In the first case titanates are present, the most important of which are ilmenite FeTiOj and perovskite CaTiOj. In the second case oxides are formed. Among different minerals with the formula TiOj, rutile (Figure M28) is the most important Other variants, with other crystal structures but the same composition TiOj, are brookite and anatase. 

Some oxide superconductors adopt a crystal stmcture similar to that of perovskite (CaTiOj). The unit cell is cubic with a Ti ion in each corner, a Ca ion in the body center, and 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 ) ... 

FIGURE 14.16. Idealized structure of (a) cubic perovskite (CaTiOj) and (b) orthorhombic brownmillerite (CajFejOj) lattice with ordered oxygen vacancies (N) along the cubic [110] direction. 

The LaPdjB structure with perovskite CaTiOj-type is shown on the left-hand side of fig. 45, This structure can be considered as a filled-up Cu3Au-type. The binary RPd3 compounds, also CeRhj and ScRh3, all crystallize with the CujAu-type. Boron atoms can be inserted in the T octahedra of the binary RT3 compounds, giving rise to a cominuous solid solution of composition RTB, with 0 x < 1 when T = Pd. For some of the Rh-containing compounds a binary CU3AU phase is unknown. In CePdj and EuPdj the addition of boron causes a change in the valence state of the rare earth ion (Ce -+Ce and Eu +— Eu +). 

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