Crystalline system Hexagonal

Silvery-white, brittle metallic element crystal system-hexagonal, rhombo-hedral also, exists in two unstable allotropic forms— a yellow modification and a dark-grey lustrous amorphous powder—both of which revert to crystalline form hardness 3.0 to 3.5 Mohs density 6.697g/cm3 melting point 630.5°C boiling point 1635°C electrical resistivity 39.1 microhm-cm at 0°C magnetic susceptibifity —0.87 x 10 emu/g. 

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],... 

FIGURE 21.6 Ternary phase diagram of the sodium octanoate-decanol-water system at 25°C. There are two isotropic solution phases, micellar and reversed micellar (rev mic), and three liquid crystalline phases, hexagonal (hex), lamellar (lam), and reversed hexagonal (rev hex) (from Ref. 17). 

Figure 2. Ternary phase diagram of the system didodecyldimethylammonium bromide / water / hexene at 25°C. The nomenclature is cub cubic phase Lamj and Lam2 lamellar phases l.c. liquid crystalline, inverted hexagonal phase L2 microemulsion phase, with curvature toward water. (Courtesy of K. Fontell).

Flow of hexagonal liquid crystalline systems is presumably a function of the alignment of the rod-like aggregates along their long axis in the direction of flow.f " The shear thinning flow process can be accompanied by an apparent yield stress. Viscoelastic behavior has also been reported. ... 

There seems to be even less structural similarity for many other metal halides when the crystalline systems are compared with the molecules in the vapor phase. Aluminum trichloride, for example, crystallizes in a hexagonal layer structure. Upon melting and then evaporation at relatively low temperatures, dimeric molecules are formed. At higher temperatures, they dissociate into monomers (Figure 9-60) [9-60]. The coordination number decreases from six to four and then to three in this process. 

Primitive three-dimensional lattices have been classified into seven crystalline systems triclinic, monoclinic, orthorombic, tetragonal, cubic, trigonal, and hexagonal. They are different in the relative lengths of the basis vectors as well as in the angles they form. An additional seven nonprimitive lattices, belonging to the same crystalline systems, are added to the seven primitive lattices, which thus completes the set of all conceivable lattices in ordinary space. These 14 different types of lattices are known as Bravais lattices (Figure 3). 

In the case of PUs based on MDI and EG, the X-ray measurements evidenced a periodicity of 15.7 0.1 A with a quantitatively similar arrangement [59, 256], but belonging to the hexagonal lattice crystalline system. 

All the above disperse systems contain self-assembly structures (i) micelles (spherical, rod-shaped, lamellar) (ii) liquid crystalline phases (hexagonal, cubic or lamellar) (iii) liposomes (multilamellar bilayers) or vesicles (single bilayers). They also contain thickeners (polymers or particulate dispersions) to control their rheology. All these self-assembly systems involve an interface whose property determines the structures produced and their properties. 

As another example, let us take a look at copper-zinc alloys. Table 2.1 shows quite different characteristics of the two elements in particular, they crystallize in two different crystalline systems (centered cubic faces for copper and hexagonal compact for zinc). Although the 15% rule is respected (there is a difference of 4%), these two metals are not miscible in all proportions, which shows that this rule is not sufficient, alone, to ensure total solubility. In addition, we can see that zinc is more soluble in copper than vice versa. This is attributable to the fact that the valences of the two metals are different - the valence of zinc is 2, whereas that of copper is 1. 

Since the discovery by researchers at Mobil of a new family of crystalline mesoporous materials (1), a large effort has been expended on synthesis, characterization, and catalytic evalrration (2). MCM-41 is a one-dimerrsiorral, hexagonal structure. MCM-48 is a cubic structine with two, norrintersecting pore systems (3). MCM-50 is a layered stractme with silica sheets between the layers (4). Many scientists also looked into other mesoporous materials, of note the HMS (Hexagonal Molecular Sieve) family (5) and SBA-15 (acronym derived from Santa Barbara University) (6), bnt to date few materials have been both catalytically significant and inexpensive to synthesize. 

---