Expanding layer lattice

FIG. 16 Fomation of a Langmuir lipid monolayer at the air/subphase interface and the subsequent crystallization of S-layer protein, (a) Amphiphilic lipid molecules are placed on the air/subphase interface between two barriers. Upon compression between the barriers, increase in surface pressure can be determined by a Wilhelmy plate system, (b) Depending on the final area, a liquid-expanded or liquid-condensed lipid monolayer is formed, (c) S-layer subunits injected in the subphase crystallized into a coherent S-layer lattice beneath the spread lipid monolayer and the adjacent air/subphase interface. 

The pressure inside the heated chamber may also vary as a result of the local density changes produced by thermal expansion or phase changes resulting from the heating. For example NaCl may expand, melt, and thereby increase the local pressure, while pyrophyllite, a layer-lattice-type aluminum silicate, may transform into a denser assembly of coesite and kyanite, thereby reducing the local pressure. It follows that experimental results in high-pressure, high-temperature work must be interpreted with care. 

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. 

The results seen in Tables X and XI show that the reaction always proceeds at a considerable rate when the interlayer spacing of Cu " -TSM is expanded. This means that only a part of the metal ions is distributed on the rim of interlayer space, and the observations give a guide for improvement of the catalytic activity of metal ion-exchanged layer lattice silicates. 

The difference in strength of inter-iayer bonding between expanding and non-expanding double layer lattice silicates may be easily observed by the following demonstration ... 

The decomposition of magnesium hydroxide has been evaluated as a process of nucleation and growth of MgO crystals within the brucite matrix. The formation of a defect layer of hydroxide structure, which suddenly recrystallizes to the cubic MgO structure when the fracture stress is exceeded in the defect layer, has been postulated (Freund et al., 1975 Guilliat et al., 1970 Garn et al., 1978 Lpnvik, 1978). The MgO crystals formed have an expanded cubic lattice that upon increasing calcination temperature gradually decreases in size until the equilibrium unit cell dimension is reached. 

Another important facet of nitrogen availability in acid soils is the pH dependence of ammonium-ion fixation between the lattices of expanding layer-silicate minerals. Such fixation generally decreases with increasing soil pH. Although the mechanism for this pH effect is incompletely understood, the decrease may be due to islands of hydroxy aluminium and hydroxy iron polymers, which prevent the complete collapse of mineral lattices and hence decrease NH4 fixation. 

Structure of CsM there is an expanded graphite lattice, in the o direction, with alkali layers in each vacancy of the C lattice. In CS4M, CagM, C4gM, each second, third, or fourth layer vacancy of the C lattice is expanded by an intercalated alkali layer. 

The layer charge in the 2 1 type lattice expanding layered aluminosilicates is known to result from isomorphous substitutions of tetrahedral Si" or octahedral Fe " and Mg " by cations of lower charge [1]. 

In some Hquid crystal phases with the positional order just described, there is additional positional order in the two directions parallel to the planes. A snapshot of the molecules at any one time reveals that the molecular centers have a higher density around points which form a two-dimensional lattice, and that these positions are the same from layer to layer. The symmetry of this lattice can be either triangular or rectangular, and again a positional distribution function, can be defined. This function can be expanded in a two-dimensional Fourier series, with the coefficients in front of the two... 

Smectites (Montmorillonites). Smectites are the 2 1 clay minerals that carry a lattice charge and characteristically expand when solvated with water and alcohols, notably ethylene glycol and glycerol. In earUer Uterature, the term montmorillonite was used for both the group (now smectite) and the particular member of the group in which Mg is a significant substituent for Al in the octahedral layer. Typical formulas are shown in Table 2. Less common smectites include volkhonskoite [12286-87-2] hich. contains Cr " medmontite [12419-74-8], Cu " andpimeUte [12420-74-5], (12). 

Due to the relatively weak forces between the layers of MMT, water and other polar molecules can enter between the unit layers, causing the lattice to expand in the thickness direction. The charge deficiency on the sheet surface is typically balanced by exchangeable cations adsorbed between the unit layers and around their edges because of the substitution of ions of different valence. 

We have thus far discussed only the most simple and best known types of interlayering, those between expandable and mica structures. It is possible however, that several types of 2 1 lattice can coexist in the same structure. For example di- and tri-octahedral forms of various types of each species. Because of their similarity under X-ray investigation, it is almost impossible to detect their presence in a mixed layered structure. 

A model system to study the effects of tensile strain is Cu on Ru(0001). Cu has a 5.5% smaller lattice parameter than Ru. Each Cu layer grown on Ru(0001) presents a specific pattern of surface reconstruction due to the layer-dependent relaxation of the strain [69]. The first Cu layer is pseu-domorphic with Ru(0001) [70], e.g. it is laterally expanded by 5.5% from a nearest neighbor distance (nnd) of 2.55 A in Cu(lll) to 2.70 A. The Cu atoms occupy hep sites (i.e. the continuation of the Ru lattice) with a Cu-Ru distance at the interface of 2.10 A as determined by LEED [71]. 

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