16 | Other important ionic structure types

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CsCl

The CsCl structure is composed of a primitive cubic arrangement of anions with a single cation at the center of the unit cell at (0.5, 0.5, 0.5). The coordination number of the cation is 8 and the coordination number of the anion is 8. The CsCl structure is a common structure for alkali halides. The structure is similar to that of the elemental structure type bcc but is a binary phase and the lattice is primitive not body-centered. The space group is \(Pm\bar{3}m\). The CsCl structure type is it’s own anti-structure. Swapping the positions of the cation and anion just results in the same exact structure with the exact coordination chemistry of the cations and anions; unit cell is just shifted by (0.5, 0.5, 0.5).

Formula: CsCl
Space group: \(Pm\bar{3}m\)
Lattice: Cubic-P
Cell: \(a = 4.127\,\text{Å}\)
Z: 1   V: \(70.29\,\text{Å}^3\)

Fig. 43 Crystal structure of the CsCl structure type. Cs is shown in yellow and Cl is shown in green.

Compounds that adopt the CsCl structure include CsCl, CsBr, CsI, TlBr, and TlI.

Perovskite

Perovskite is a ternary mineral oxide with the formula CaTiO₃. There are an enormous number of compounds with the perovskite structure that are isostructures or near isostructures of the CaTiO₃. Apart from the most radioactive elements, almost every elements on the periodic table can be incorporated into a stoichiometric perovskite structure. The structure of perovskite does not has a eutactic arrangement of anions not is the arrangement of anions analogous any of th simple packings of spheres.

Perovskite can be derived from first consideration of the rock salt structure type, Fig. 44 with the generic formula BX. By removing 3/4 of the cation sites in rock salt a vacancy ordered structure with the generic forumla BX₄ is obtained where the remaining B-site cations are octahedral (CN = 6) and 3/4 of the anion sites are two coordinate, and the other 1/4 of the anion sites are not coordinated to any cations. If the uncoordinated anion sites are swapped for an different (A-site) cation, the result is the aristotype perovskite structure with the generic formula ABX₃.

Fig. 44 structural relationships between the rock salt and perovskite structures. The B-site cation is shown in red, the A-site cation in grey, and the X-site anion in red.

The coordination number of the A-site cation in perovskite is 12. While these bonding interactions are often not drawn in depictions of the perovskite structure for clarity they are certainly important for the overall stability of the perovskite structure. In Fig. 44 image an alternative setting of the perovskite unit cell where the B-site cations are at the vertices of the cell and the A-site in in the middle at (0.5, 0.5, 0.5). In this setting the X-site anions would be positioned at the 12 edges of the cubic unit cell. These 12 X-site anions are the 12 nearest neighbors of the A-site.

The perovskite structure is perhaps best identified by the 3D bond network of vertex sharing B-site octahedra at the vertices of a primitive cubic lattice. In many cases the structures of compounds with perovskite structure type or structures very similar to perovskite are slightly distorted via a tilting of the octahedra. However, provided the bond network topology remains the same, these structures are still considered to be perovskite structures even though they may not be truly isostructural to the CaTiO₃.

Formula: SrTiO₃
Space group: \(Pm\bar{3}m\)
Lattice: Cubic-P
Cell: \(a = 3.905\,\text{Å}\)
Z: 1   V: \(59.55\,\text{Å}^3\)

Fig. 45 Crystal structure of the ideal perovskite structure type exemplified by SrTiO₃. The B-site cation is shown in red, the A-site cation in grey, and the X-site anion in red. A unit cell with the A-sites at the vertices is shown at the center of the model

Some compounds that adopt the perovskite structure include CaTiO₃, BaTiO₃, SrTiO₃ (aristotype), LaAlO₃, RbVF₃, CsSnCl₃, K₂NaAlF₆, CsPbBr₃, and (CH₃NH₃)PbI₃.