Icosahedral clusters structure

The raw experimental spectra are shown in Fig. 15. Unlike predictions that are based on the most stable icosahedral cluster structures, the fcc-sensitive second maximum in the EXAFS-signal occurs already with appreciable intensity at an average cluster-size (N) 200, as shown in Fig. 16. The question arises why such small clusters form fcc-units, which... 

Ludwig s (2001) review discusses water clusters and water cluster models. One of the water clusters discussed by Ludwig is the icosahedral cluster developed by Chaplin (1999). A fluctuating network of water molecules, with local icosahedral symmetry, was proposed by Chaplin (1999) it contains, when complete, 280 fully hydrogen-bonded water molecules. This structure allows explanation of a number of the anomalous properties of water, including its temperature-density and pressure-viscosity behaviors, the radial distribution pattern, the change in water properties on supercooling, and the solvation properties of ions, hydrophobic molecules, carbohydrates, and macromolecules (Chaplin, 1999, 2001, 2004). 

In the same paper (Yamamoto 1996) an authoritative description is given of several interrelated topics such as super-space group determination, structure determination, indexing of diffraction patterns of quasicrystals, polygonal tiling, icosahedral tiling, structure factor calculation, description of quasicrystal structures, cluster models of quasicrystals. 

From the above discussion, it is evident that mixed cluster ions of the type Ar M exhibit strong magic numbers at values of (n + m) = 13, 19, 55, 71, and 147 in a variety of different studies. These values correspond to the completion of the first, second, and third icosahedral shells occurring at 13, 55, and 147 whereas 19 and 71 correspond to especially stable subshells formed by interpenetrating double icosahedron structures. The size and symmetry of the dopant moiety appear to be the most important factors in observing magic numbers that can be rationalized on the basis of icosahedral-like structures. The inability to observe magic numbers has been attributed to the distortion of the icosahedral structure due to size and steric factors associated with the dopant ion which destroys the delicate balance between the monomer interactions. One of the issues that has been interpreted differently involves the location of the dopant atomic/molecular... 

The icosahedral cluster model1,2 offers a structure on which large molecules can be mapped in order to investigate their interaction with water within a three-dimensional hydrogen-bonded network, and here offers new insights into the ways fullerene and polyoxomolybdate molecules interact with water in aqueous solution. 

C6o molecules in water also form colloidal clusters based on 3.4 nm diameter (carbon atoms) icosahedral arrangements of thirteen CWI molecules.5 Here the Ceo molecules are necessarily separated by water molecules to form clusters with this diameter.10 Such an arrangement is shown in Figure 2 within an expanded, but now strain-free, cluster of water icosahedral clusters. The water network is formed by tetrahedral tricyclo decamer (H20)io structures connecting groups of four Cm molecules. The modeled diameter of the cluster... 

Besides bare clusters in a vacuum (cluster beam) and clusters with passivation layers, another important experimental environment for clusters is a (solid) support. Nevertheless, this setup has been addressed in very few EA applications. Zhuang et al. [105] have used the EA method to study surface adatom cluster structures on a metal (111) surface. Miyazaki and Inoue [106] have found that n=13 clusters which are icosahedral in vacuo either form islands or form layered structures upon surface deposition, depending on the substrate-cluster interaction potential. 

Whether quasicrystalline structures are limited to alloys remains an open question. It is possible that their occurrence is much more widespread than had been previously thought. Indeed there is evidence for quasicrystallinity in both thermotropic and lyotropic liquid crystals. Diffraction patterns of decagonal symmetry have been recorded in lyotropic liquid crystals [K. Fontell, private communication], (Fig. 2.19), and there is theoretical evidence for the existence of a quasicrystalline structure within the blue phase of cholesterol (Chapters 4, 5). (The decagonal structure has quasisymmetry perpendicular to the tenfold axes, and translation symmetry along them.) Viruses crystallise in icosahedral clusters and the list continues to grow. In addition to five-fold symmetry, it has been shown that eight and ten- fold quasisymmetry is possible. ... 

Whereas microscopy observations have revealed the existence of metallic icosahedral clusters, diffraction techniques alone may provide information about their structure. Powder and microdiffraction patterns have been obtained from gold icosahedral clusters 100 to 200 A in diameter in order to determine whether their structure was rhombohedral or nondeformed fcc. The somewhat surprising result is that they always show the nondeformed fee structure. This proves that above some given size, stresses inherent in the icosahedral... 

On the other hand, Lopez et al.104 used molecular-dynamics simulations in systematically studying Cura xAux clusters with n = 13 or 14, and x ranging from 0 to n. Due to the small size of the clusters (13 or 14 atoms) it was possible to obtain a detailed description of the structure of all stoichiometries. The interatomic interactions were described with an approximate Gupta-like potential. They found that for n = 13 all clusters have an icosahedral-like structure. On the other hand, for n = 14 the pure Au and Cu clusters have different structures, and in this case it was found that all clusters, except for the pure Au one, prefers the same type of structure. This finding indicates that it is not trivial to guess the structure of binary compounds from those of the pure system. 

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