Hexagonal crystal form

Formula AICI3 MW 133.31 Structure and bonding an electron-deficient compound, a Lewis acid, occurs as dimer AI2CI6 in hexagonal crystal form. Above 300 °C, dissociation to monomer AICI3 begins completely dissociates to AICI3 at 1,100°C. 

Whittaker, A. G. Wolten, G. M. 1972 Carbon A suggested new hexagonal crystal form. Science, Wash. 178, 54-56. 

The differences between synthetic and naturally-occurring molybdenum disulphide are considered later. Unless otherwise specified, information in this book relates to the hexagonal crystal form obtained from natural molybdenite. 

Their crystal structures have been mentioned briefly in connection with intercalation in Section 14.2. All five compounds can be obtained in the layered hexagonal crystal form, and most are also found in rhombohedral or trigonal form. The compounds of the Group 6 metals, molybdenum and tungsten, as well as niobium diselenide, have a hexagonal form similar to that of molybdenum disulphide, in which the metal atoms in one layer are displaced sideways from those in the layers immediately above and below. This structure results in the widest interlamellar spacing, the easiest interlamellar shear, and the lowest friction. 

FIGURE 9.11 The w = j plane of the difference Patterson map for the K2HgI4 heavy atom derivative of the hexagonal crystal form of the protein canavalin. The space group is P6, so w = is a Harker section. The derivative crystal contained two major K2HgI4 substitution sites and one minor substitution site per asymmetric unit. The Patterson peaks corresponding to those sites are marked with crosses. Note that the Patterson peak corresponding to the minor site cannot be discriminated from noise peaks in the Patterson map as is often the case. 

Figure 10.18 (a) A 0.4° oscillation photograph of the hexagonal crystal form of cow pea mosaic virus obtained at LURE. At the start of the exposure the crystal had been rotated 6.4°... 

In the 1000-1500°C range frequently used in chemical laboratories, a hexagonal crystal form of PajOj is obtained. Black Pa02 (fluorite structure) is formed by hydrogen reduction of the higher oxide at temperatures above 1550°C. The oxides with O/M ratios between 2.0 and 2.5 can be obtained by selective oxidation of Pa02 or reduction of Pa205. As in the case of ThO, a PaO film has been reported to form on the surface of Pa metal but it is not likely that a bulk form of pure PaO can be prepared. A number of complex (ternary) oxides have been reported for Pa in addition to hydrated oxides and a hydrated peroxide of variable composition. 

Ramsdell Numbers. A shorthand notation for labelling polytypes (q.v.) of crystal structures The numbers are of the form /iR or nH were n is the number of layers in the unit cell (q.v.) necessary for the arrangement of the layers to repeat itself. R, H stand for rhombohedral or hexagonal crystal form respectively. Originally devised to classify silicon carbide polytypes, the system has been applied to other carbides and nitrides. (L.S. Ramsdell, Amer. Miner 32, 147, p64). 

M49 Corundum AI2O3. Bottom left specimen from Madras in India with the hexagonal crystal form, typical for corundum. Besides a specimen of ruby, which is corundum, red colored by chromium. From Froland in Norway and Roland Eriksson s collection. 

Solid-state absorption spectra of CfCl and CfCli (hexagonal crystal form)... 

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