Layer-lattice properties

The common structural element in the crystal lattice of fluoroaluminates is the hexafluoroaluminate octahedron, AIF. The differing stmctural features of the fluoroaluminates confer distinct physical properties to the species as compared to aluminum trifluoride. For example, in A1F. all corners are shared and the crystal becomes a giant molecule of very high melting point (13). In KAIF, all four equatorial atoms of each octahedron are shared and a layer lattice results. When the ratio of fluorine to aluminum is 6, as in cryoHte, Na AlF, the AIFp ions are separate and bound in position by the balancing metal ions. Fluorine atoms may be shared between octahedrons. When opposite corners of each octahedron are shared with a corner of each neighboring octahedron, an infinite chain is formed as, for example, in TI AIF [33897-68-6]. More complex relations exist in chioUte, wherein one-third of the hexafluoroaluminate octahedra share four corners each and two-thirds share only two corners (14). 

The molecular and bulk properties of the halogens, as distinct from their atomic and nuclear properties, were summarized in Table 17.4 and have to some extent already been briefly discussed. The high volatility and relatively low enthalpy of vaporization reflect the diatomic molecular structure of these elements. In the solid state the molecules align to give a layer lattice p2 has two modifications (a low-temperature, a-form and a higher-temperature, yS-form) neither of which resembles the orthorhombic layer lattice of the isostructural CI2, Br2 and I2. The layer lattice is illustrated below for I2 the I-I distance of 271.5 pm is appreciably longer than in gaseous I2 (266.6 pm) and the closest interatomic approach between the molecules is 350 pm within the layer and 427 pm between layers (cf the van der Waals radius of 215 pm). These values are... 

There are few systematic guidelines which can be used to predict the properties of AB2 metal hydride electrodes. Alloy formulation is primarily an empirical process where the composition is designed to provide a bulk hydride-forming phase (or phases) which form, in situ, a corrosion— resistance surface of semipassivating oxide (hydroxide) layers. Lattice expansion is usually reduced relative to the ABS hydrides because of a lower VH. Pressure-composition isotherms of complex AB2 electrode materials indicate nonideal behaviour. 

D. Surface Properties of Native S-Layer Lattices from Bacillaceae... 

E. Surface Properties of Glutaraldehyde-Treated S-Layer Lattices... 

The anisotropy in the physicochemical surface properties and differences in the surface topography of S-layer lattices allowed the determination of the orientation of the monolayers with respect to different surfaces and interfaces. Since in S-layers used for crystallization studies the outer surface is more hydrophobic than the inner one, the protein lattices were generally oriented with their outer face against the air-water interface [120,121]. Crystallization studies with the S-layer protein fromB. coagulansE i Nl at different lipid monolayers [122] revealed that the S-layer lattice is attached to lipid monolay-... 

In reconstitution experiments, the self-assembly of the pore-forming protein a-hemolysin of Staphylococcus aureus (aHL) [181-183] was examined in plain and S-layer-supported lipid bilayers. Staphylococcal aHL formed lytic pores when added to the lipid-exposed side of the DPhPC bilayer with or without an attached S-layer from B coagulans E38/vl. The assembly of aHL pores was slower at S-layer-supported compared to unsupported folded membranes. No assembly could be detected upon adding aHL monomers to the S-layer face of the composite membrane. Therefore, the intrinsic molecular sieving properties of the S-layer lattice did not allow passage of aHL monomers through the S-layer pores to the lipid bilayer [142]. 

The physical properties of samples of Cr(OMe)3 prepared by a variety of methods have been investigated.736 Good crystals could not be obtained but limited crystallography suggested a layered lattice with chromium occupying octahedral sites. A useful summary of earliei preparations and attempted preparations of chromium methoxides can be found in this paper. 

There is an ill-defined boundary between molecular and polymeric covalent substances. It is often possible to recognise discrete molecules in a solid-state structure, but closer scrutiny may reveal intermolecular attractions which are rather stronger than would be consistent with Van der Waals interactions. For example, in crystalline iodine each I atom has as its nearest neighbour another I atom at a distance of 272 pm, a little longer than the I-I distance in the gas-phase molecule (267 pm). However, each I atom has two next-nearest neighbours at 350 and 397 pm. The Van der Waals radius of the I atom is about 215 pm at 430 pm, the optimum balance is struck between the London attraction between two I atoms and their mutual repulsion, in the absence of any other source of bonding. There is therefore some reason to believe that the intermolecular interaction amounts to a degree of polymerisation, and the structure can be viewed as a two-dimensional layer lattice. The shortest I-I distance between layers is 427 pm, consistent with the Van der Waals radius. Elemental iodine behaves in most respects - in its volatility and solubility, for example - as a molecular solid, but it does exhibit incipient metallic properties. 

Beside the chemical composition, the crystalline structure of the mineral has an important effect on the adsorption ability of its surface. This is due to the fact that lattice bindings are usually not equivalent and space disproportions occur, so that fission surface areas have specific properties. Typical examples are layer lattices of graphite or talc where the main valences proceed in the layer plains whereas these are interconnected with feeble valences. Fission areas of such minerals are hydrophobic. The effect of the structure on adsorption properties of a mineral surface increases with increasing adsorption density and with decreasing force of the adsorption binding of the solid phase5. A crystalline lattice contains structural defects (which include physical and chemical surface imperfections and deficiencies in the volume phase) which can influence the chemical reactivity of a crystal surface. 

Rhodium trichloride has a layer lattice isostructural with A1C13 and is exceedingly inert. However, when RhCl3-3H20 is dehydrated in dry HQ at 180°C, the red product is much more reactive and dissolves in water or THF this property is lost on heating at 300°C. 

However, Jamison has intensively studied the relationship between the crystal and electronic structures of layer-lattice solid lubricants and their frictional properties, and has shown that other aspects of its electron distribution give a particularly favourable structure to molybdenum disulphide. In its structure the molybdenum atoms in one layer do not lie directly above or below the molybdenum atoms in an adjacent layer, but are opposite holes in that layer. The sulphur atoms are directly opposite other sulphur atoms, but do not have any unpaired electrons to provide strong bonding. It is this lack of electronic interactions which leads to the high interlamellar spacing, and low interlamellar attraction. 

Acid-treated clay minerals were employed as cracking catalysts in the first commercial process, the Houdry process, widely used in the early petroleum industries to produce high-octane gasoline. The Houdry process catalysts had been discussed extensively by many investigators (2) but were eventually completely replaced by synthetic silica-alumina or zeolite catalysts. Recently, the need for new catalytic materials has revived special interest in the layer lattice silicates because of their ion-exchange properties and their expandable layer structures. 

Fluorotetrasilisic mica is a synthetic swelling layer lattice silicate. Because of the cation-exchange properties and the lack of acid sites on the sili-... 

Because several spatial stacking arrangements are possible there are several kaolin minerals, each with the same chemical composition, namely Al2Si205(0H)4, but with different properties. Nacrite, dickite, kaolinite, halloysite, and livesite are well recognized species. No positive evidence has so far been published linking other trivalent cations with a single layer lattice structure, but it has been suggested that iron(iii) can replace aluminium in part in the kaolin lattice. 

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