Layer structures metal halides

Structure of A and B. Graphite lattice expanded in the o direction with a layer of metal halide in each layer vacancy. 

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. 

These have been known for many years.1052-1054 Chromium(III) is approximately octahedral ( ie(f = 3.69-4.1 BM) the compounds have a layer structure. In the chloride, r(Cr—Cl) is 5.76 A between layers and 3.46 A within layers. The iodide is isomorphous with the chloride and the bromide has a similar but distinct structure. All may be prepared by the direct halogenation of the metal. Other methods are available, e.g. CrCl3 may be prepared by heating Cr203-xH20 in CCU vapour at 650 °C.1055 The anhydrous halides are insoluble in water, however reducing agents such as zinc catalyze dissolution. The trichloride reacts with liquid ammonia to form ammine complexes. 

Extreme diversity is exhibited in the coupling of metal halide tetra-hedra to form polymeric halogenocuprate(I) and halogenoargentate(I) ions, there being an abundance of different types of infinite chains, layers, and three-dimensional arrays. Because the object of this article is to focus on variations in metal coordination number, and, in particular, on trends associated with the nature of the cation, structural description will be limited to those types of polymeric anion most frequently encountered hitherto in crystalline halogenocuprates(I) and halogenoargentates(I). 

This brings us to a class of compounds too often overlooked in the discussion of simple ionic compounds the transition metal halides. In general, these compounds (except fluorides) crystallize in structures that are hard to reconcile with the structures of simple ionic compounds seen previously (Figs. 4.1-4.3). For example, consider the cadmium iodide structure (Fig. 7.8). It is true that the cadmium atoms occupy octahedral holes in a hexagonal closest packed structure of iodine atoms, but in a definite layered structure that can be described accurately only in terms of covalent bonding and infinite layer molecules. 

Layered structures are extremely prevalent among transition metal halides. Examples of compounds adopting the cadmium iodide structure or the related cadmium chloride structure (Fig. 7.9) are ... 

Fig. 16.68 Thu. structure of ZrC], a reduced metal halide system containing infinite melal-mctal bonds, showing double metal atom layers alternating with double chlorine atom layers. [From Corbett. J. D. An. Chem. Res. 1981. 14. 239. Reproduced with permission.]...

There seems to be even less structural similarity for many other metal halides as the crystalline systems are compared with the molecules in the vapor phase. Aluminum trichloride, e.g., crystallizes in a hexagonal layer structure. Upon melting, and then, upon evaporation at relatively low temperatures, dimeric molecules are formed. At higher temperatures they dissociate into monomers (Figure 9-58) [107], The coordination number decreases from 6 to 4 and then to 3 in this process. However, at closer scrutiny, even the dimeric aluminum trichloride molecules can be derived from the crystal structure. Figure 9-59 shows another representation of crystalline aluminum trichloride which facilitates the identification of the dimeric units. A further example is chromium dichloride illustrated in Figure 9-60. The small oligomers in its vapor have structures [108] that are closely related to the solid structure [109], Correlation between the molecular composition of the vapor and their source crystal has been established for some metal halides [110],... 

Sesquichlorides of a unique type are formed by Y, Gd, and Tb. In Gd2Cl3 or [Gd4+(C1 )6] , there are infinite chains of metal atoms in octahedra sharing opposite edges (Fig. 19-8) chlorine atoms are located over triangles formed by three Gd atoms. These halides can be further reduced at 800°C to GdCl and TbCl. The latter materials are graphitelike platelets. They have a layer structure like ZrCl and ScCl where close-packed double layers of metal atoms alternate with double layers of... 

Layer misfit was first recognised in the structures of two metal chloride-graphite intercalation compounds. Intercalates of graphite with many other metal halides, including fluorides and bromides, have since been reported, but they seem to be of the CC type and are not discussed here. 

This situation, clear in the case of more ionic structures, is less stringent in graphite intercalates where, presumably, there is electron transfer to (in the case of alkali-metal intercalates) or from (in the case of metal halide intercalates) the half-filled conduction bands of the graphite layers (produced by overlap of the 7t orbitals). Similarly, the periodicity requirements are less stringent for the alternating composite layers of layer silicates with complex intralayer and interlayer charge balance. 

The Tb halides are similar to the sdver hahdes in that they are sensitive to light. The yellow compound TII has a curious orthorhombic layer structure, which is transformed to a red metastable cubic form (CsCl type) at 4.7kbar or 175 °C, becoming a metallic conductor at about 160kbar. If a small quantity of a Tb halide is added to an aqueous solution of an alkali halide, a blue Luminescence is emitted furthermore, TlCl doped KCl behaves as a thalhum alkali halide phosphor. In both cases, TlCb is believed to be the active species. ... 

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