Double-layer stack

Nordstrandite. Tlie x-ray diffraction pattern of an aluininum tiiliydroxide wliich differed from the patterns of gibbsite and bayerite was pubhshed (4) prior to the material, named nordstrandite, being found in nature. Tlie nordstrandite structure is also assumed to consist of double layers of hydroxyl ions and aluininum occupies two-tliirds of the octaliedral interstices. Two double layers are stacked with gibbsite sequence followed by two double layers in bayerite sequence. 

A number of theories have been put forth to explain the mechanism of polytype formation (30—36), such as the generation of steps by screw dislocations on single-crystal surfaces that could account for the large number of polytypes formed (30,35,36). The growth of crystals via the vapor phase is beheved to occur by surface nucleation and ledge movement by face specific reactions (37). The soHd-state transformation from one polytype to another is beheved to occur by a layer-displacement mechanism (38) caused by nucleation and expansion of stacking faults in close-packed double layers of Si and C. 

The appearance of a flake of clay reflects its internal structure, which is something like an untidy stack of papers (Fig. 14.45). Sheets of tetrahedral silicate units or octahedral units of aluminum or magnesium oxides are separated by layers of water molecules that serve to bind the layers of the flake together. Each flake of clay is surrounded by a double layer of ions that separates the... 

Both C-H N motifs operate edge-edge, and between opposite enantiomers of 9. The molecules of 9 within each layer may be enantiomerically pure (as for this case), or may be a mixture of enantiomers (as for the trifluoromethylben-zene compound, Section 3.3). However, in all cases, the layers stack edge-edge, with the double C-H N weak hydrogen bonds [31] operating only between opposite enantiomers of the host molecules. 

The crystal structure of [XeF5] AgF4] consists of alternate stacks of double layers of square-pyramidal XeFlj" cations and approximately square-planar AgF anions (site symmetry D2h, Ag-F = 190.2 pm). The XeFj- ion has C4v symmetry with Xe-F(ax) = 185.2, Xe-F(eq) = 182.6 pm, and F(ax)-Xe-F(eq) = 77.7°. Each cation lying on a 4-axis interacts with one bridging F ligand of each of four anions at 263.7 pm, as illustrated below ... 

Figure 3.16 Schematic of different lamellar phases (a) stacked lamellar phase, (b) vesicle and (c) l 3 phase the grey area consists of a surfactant double layer similar to those in (a).

In addition, structural similarities can often be determined from careful interpretation of XRD powder patterns. The powder patterns of offretite and erionite look quite different, but are easily understood in terms of the crystallographic consequences of a change in the ordered layer stacking sequence (11), cf. Figure 4. In offretite, the layers are stacked in an AAB sequence, while in erionite, they are ordered in an AABAAC arrangement that doubles one of the crystallographic unit cell parameters. The doubled c-parameter is readily deduced from an analysis of the XRD powder pattern of erionite. Another framework structure effect, isomorphous substitution, can result in changing unit cell sizes, observed as shifts in XRD line positions for such systems as X and... 

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