Clusters hexagonal close packed

Even though qualitative bonding descriptions of metal atom clusters up to six or seven atoms can be derived and in some cases correlated with structural detail, it is clear that most structures observed for higher clusters cannot be treated thus. Nor do the structures observed correlate with those observed for borane derivatives with the same number of vertices. Much of borane chemistry is dominated by the tendency to form structures derived from the icosahedron found in elemental boron. However, elemental transition metals possess either a close-packed or body-centered cubic arrangement. In this connection, one can find the vast majority of metal polyhedra in carbonyl cluster compounds within close-packed geometries, particularly hexagonal close-packing. 

Interstitial atoms in clusters. As the size of clusters increases (and also that of their central cavity) the insertion of atoms becomes easier and easier. In particular for 12-atom clusters having a cubo-octahedral structure, the insertion of an atom having the same radius as that of the peripheral atoms is possible. Notice that this arrangement can be compared with those of the metallic cubic and hexagonal, close-packed structures. 

Any study of colloidal crystals requires the preparation of monodisperse colloidal particles that are uniform in size, shape, composition, and surface properties. Monodisperse spherical colloids of various sizes, composition, and surface properties have been prepared via numerous synthetic strategies [67]. However, the direct preparation of crystal phases from spherical particles usually leads to a rather limited set of close-packed structures (hexagonal close packed, face-centered cubic, or body-centered cubic structures). Relatively few studies exist on the preparation of monodisperse nonspherical colloids. In general, direct synthetic methods are restricted to particles with simple shapes such as rods, spheroids, or plates [68]. An alternative route for the preparation of uniform particles with a more complex structure might consist of the formation of discrete uniform aggregates of self-organized spherical particles. The use of colloidal clusters with a given number of particles, with controlled shape and dimension, could lead to colloidal crystals with unusual symmetries [69]. 

Mao et al. [478] have applied in situ STM to study Sn UPD on reconstructed and unreconstructed Au(lll) electrodes. On the unreconstructed Au(lll), Sn formed size-confined two-dimensional clusters of 1-2 nm. At more negative potential, surface alloying was observed. On the reconstructed Au(lll) surface, in turn, Sn preferably nucleated at face-centered cubic regions. The nuclei expanded toward the hexagonal close-packed regions to build up deposit domains. 

Stereochemistry of Monohydrido- and Dihydrido-Dodecanickel Carbonyl Clusters Containing a Hexagonal Close Packed Nickel Fragment... 

S. Martinengo, G. Ciani, A. Sironi, and P. Chini, Analogues of Metallic Lattices in Rhodium Carbonyl Cluster Chemistry. Synthesis and X-ray Structure of the [Rh15 ( i-CO)14(CO)13]3 and [Rh14((j,-CO)15(CO)9]4 Anions Showing a Stepwise Hexagonal Close-Packed/Body-Centered Cubic Interconversion, J. Am. Chem. Soc. 100,7096-7098 (1979). 

Rhodium clusters are prone to adopt structures having an interstitial rhodium atom within a polyhedron of rhodium atoms. The most prominent structure of this type is that for the [Rh13(CO)24H ](5 n, species, which is shown in Fig. 16-14a. This metal atom arrangement is precisely a fragment of a hexagonal close-packed array. By additions to it larger clusters are built up, as also shown in Fig. 16-14. 

In the present calculations, the geometries of the Zn clusters up to Zn were taken from Ref. (39). The larger clusters are assumed to have simple hexagonal symmetry (hex) or hexagonal close-packed synunetry (hep) with the same bond distances as in the bulk (45). We performed self consistent calculations on both neutral and ionized clusters (ASCF) in order to account properly for relaxation energies. In the case of ions the calculations were carried out using the spin polarized approach. 

Third, the notion (and reality) of such structural infractions as twins and coherent intergrowths - as is seen by Yacaman et al. in a 923-atom nanoalloy of AuPd [35] - is meaningless in our molecular bimetallic nanoparticles. In the nanoaUoys of Yacaman et al. [35, 42] and others [43], one may discern directly, by aberration-corrected electronic microscopy, thin bands of hexagonal close-packed and face-centered cubic packed sheets. In a typical molecular nanoparticle of the kind that we have studied (also by aberration-corrected electron microscopy [39]), it is directly established (in line with theoretical predictions [44]) that a single bimetallic cluster of RUj Pt does indeed possess molecular character. Furthermore, when six or more such clusters coalesce into larger entities containing ca 200 atoms they adopt the regular crystalline, and faceted state of a bulk metal. 

Figure 13-27 There are two crystal structures in which atoms are packed together as compactly as possible. The diagrams show the structures expanded to clarify the difference between them, (a) In the hexagonal close-packed structure, the first and third layers are oriented in the same direction, so that each atom in the third layer (A) Ues directly above an atom in the first layer A), (b) In the cubic close-packed structure, the first and third layers are oriented in opposite directions, so that no atom in the third layer (C) is directly above an atom in either of the first two layers A and B). In both cases, every atom is surrounded by 12 other atoms if the strucmre is extended indefinitely, so each atom has a coordination number of 12. Although it is not obvious from this figure, the cubic close-packed structure is face-centered cubic. To see this, we would have to include additional atoms and tilt the resulting cluster of atoms.

Ruthenium-copper and osmium-copper clusters are examples of bimetallic clusters in which one component is from Group VIII and the other from Group IB of the periodic table. These clusters are of particular interest because copper is virtually completely immiscible with either ruthenium or osmium in the bulk (7). Copper has the face-centered cubic structure in the metallic state, whereas ruthenium and osmium both exhibit hexagonal close-packed structures (8). 

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