CdCl2 layer structures

If a crystal breaks along a plane related to the lattice structure, the plane is called a cleavage plane. 

Other compounds crystallizing with a Cdl2 structure type include MgBr2, Mgl2, Cal2, iodides of many zl-block metals, and many metal hydroxides including Mg(OH)2 (the mineral brucite) in which the [OH] ions are treated as spheres for the purposes of structural description. 

The structure of CdCl2 is related to the Cdl2 layer-structure but with the Cl ions in a cubic close-packed arrangement. Examples of compounds adopting this structure are FeCl2 and C0CI2. Other layer structures include talc and mica (see Section 14.9). 

The perovskite (CaTiOs) structure type a double oxide 

Many double oxides or fluorides such as BaTi03, SrFe03, NaNb03, KMgF3 and KZnF3 crystallize with a perovskite 

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The rhenium dichalcogenides, ReS2 and ReSe2, have a distorted CdCl2 layered structure leading to triclinic symmetry, with the Re t/-states forming the top of the... 

Figure 13.3 The CdCl2 layer structure of MgCl2. In (a) one layer is viewed from above it has three decks. The top and bottom decks consist of chlorines these have been distinguished in the Figure by giving the top-deck atoms a heavy border the middle deck consists of octahedrally coordinated magnesiums. In (b) the environment of each chlorine is shown. It is a distorted trigonal prism there are three magnesiums on one side, but on the other side there are three chlorines substantially farther away in an adjacent layer. This is not what would be expected of a collection of ions.

To take the specific example of chlorides in the third Period, sodium chloride, which is typically ionic, gives way to MgCl2, which adopts a CdCl2 layer structure (Figure 13.3, overleaf), and then to AICI3 with the layer structure described in Figure 9.20. 

A plethora of stoichiometries and structural types are found for the chalcogen compounds of the Group 5 metals. Phases approximating to the composition MX have the NiAs-type structure, whereas the MX2 compounds have layer structures related to M0S2, Cdl2, or CdCl2 types. Sometimes complex layer sequences occur in which the 6-coordinate metal atom is alternatively octahedral and trigonal prismatic. 

The molybdenite (M0S2) structure (C7) is a layer structure closely related to CdCl2 and Cdl2. Each layer of M0S2 consists of a sheet of metal atoms sandwiched between two sheets of nonmetal atoms (Fig. 1.8). These layers are stacked one upon the other along the hexagonal c-axis, the sequence of sheets of atoms being AbA BaB where the capital letters denote anions and the lower case letters cations. In this structure. 

Lanthanide and actinide trifluorides adopt a different structure called the tysonite structure (DOg). In this structure, the metal atom has the unusual eleven (5 -I- 6) coordination. Covalent AB3 halides crystallize in layer structure (DO5) typified by CrCl3 and Bil3 these structures are related respectively to CdCl2 and Cdl2. 

Tlic CdCl2 and the Cdl2 type are also layer structures (Fig. 16.9 because there exist numerous stacking variants of Cdl2, some authors prefer the term Cd(011)2 type instead of Cdlj type). The octahedra in the layer share six edges each. The structure of the layer is tire same as in an MX3 layer if the voids in the MX3 layer were occupied by M atoms. Every halogen atom is shared by three octahedra (MX6/3). The stacking variants of the layers arc ... 

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