Cuboctahedral clusters

Only large clusters usually adopt the face-centered cubic structure of metallic platinum. A novel cuboctahedral cluster [Pt15Hx(CO)8(PBut3)6] has been reported by Spencer et al.512 and the first octahedral cluster [Pt6(CO)6(/i-dppm)3]2+ was only reported recently.573... 

There are many ways in which these square antiprism and cuboctahedral defect clusters can be arranged. A nonstoichiometric composition can be achieved by a random distribution of varying numbers of clusters throughout the crystal matrix. This appears to occur in Ca0.94Y0.06F2.06> which contains statistically distributed cuboctahedral clusters. 

Fig. 5. The MES f-factors versus site coordination numbers calculated for cub-octahedral Auj3 and AUjs as well as several larger magic-number and non-magic-number cuboctahedral cluster sizes. The symbols are defined as follows ...

FIGURE 12 Average coordination number as a function of platinum cluster diameter for cuboctahedral clusters. 

G. Ciani, A. Magni, A. Sironi, and S. Martinengo, Synthesis and X-Ray Characterization of the High-nuclearity [Rh17( i3-CO)3( j,-CO)15(CO)12]3 Anion containing a Tetra-capped Twinned Cuboctahedral Cluster, J. Chem. Soc., Chem. Comm. 1981,1280-1281. 

Figure 21. Radial distribution functions calculated using a Fourier transform of scattering patterns produced with a Debye equation. Top Cuboctahedron (cluster with both octahedral 111 and cnbe 100 faces) and icosahedron (multiply twinned hep structure) clusters of the same size. Center Cuboctahedra of different sizes. Bottom Experimental and simulated cluster RDF of a Pt colloid. The fit is a 90 10 mixture of 55 and 147 cuboctahedral clusters, respectively. After Casanove et al. (1997).

Keywords superheated crystals, molecular dynamics, defects, relaxation, cuboctahedral clusters... 

The coordination number of icosahedral and cuboctahedral clusters of 13 atoms is five and is larger than the values obtained firom EXAFS measurements for the samples calcined at 250°C and 280°C. This discrepancy suggests that smaller gold clusters composed of less than 10 atoms may also exist in fi sh active catalysts. 

These deviations in adsorption energies from extrapolated surface atom values relate to changes in the electronic stracture of these clusters. They are due to the high fraction of surface atoms over bulk atoms. We will illustrate this for the Mjj and M55 cuboctahedral clusters. 

Consistent with the AF2-RF3 systems, all contain MgX37 polyhedral (cuboctahedral) clusters. Lattice parameters reported for these phases are listed in table 2. Qiao and Xing (1991) report ternary phase diagrams for a few RClj-MClj-MgClj and RCl3-MCl2-LiCl systems, R = La-Nd, M = Ca, Sr, derived by combining limited experimental data and computer calculations. 

Flowever, the 13-atom core may be either a cuboc-tahedron cut out of the fee crystalline solid, or an icosahedron (Figure 4). Either way, the total number of atoms in complete-shell clusters would be 13, 55, 147, 309, 561 these cluster sizes are, indeed, particularly abundant in the mass spectra. Figure 3 reveals several additional magic numbers. They indicate closure of subshells that occurs whenever a facet of the cluster is covered by a layer of atoms. A cuboctahedral cluster has 14 facets, the icosahedral one has 20, leading to distinct subshell closures. For argon through xenon in this size range, only the... 

Because the cuboctahedral clusters are the most widespread and the largest scale ones, we will therefore consider them in detail. All eight normal anion positions (Fp) in the fluorite unit cell containing such a cluster are vacant. Twelve atoms F form the cubocta-hedron (8 12 0). The position in the centre of the unit cell could be occupied by one additional fluorine atom F" (8 12 1). The anion cluster is surrounded by an octahedron of R " " cations located in face centres (Figure 14.12). The coordination polyhedron of R " " cations is a square antiprism (coordination number equal to 8). 

The supercluster model [42,43] as a development of the cluster model considers not only anion cuboctahedral clusters but surrounding cations as well. 

Practically all the structures of the ordered phases studied by now, in fluoride systems with RE elements, belong to the class of phases with ordered cuboctahedral clusters. The position in the centre of cuboctahedron can be occupied by an anion (8 12 1) or can be vacant (8 12 0). In oxyfluoride ordered phases, the position in the centre of cuboctahedron is usually vacant, as for instance in PbZr3Fe04 [60], PbgY6p320 [61], Ba2.iBio.9(0,F)6.g [59]. 

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