Twinned clusters

Cation vacancies and interstitials, (111) twins and stacking faults, grain boundaries, microstrains, misfit dislocation network at C03O4/C0O interface Dislocations and (100) stacking faults intergrowth of e and P phases. Cations vacancies and superstructure (110) stacking faults and twins Clusters of point defects (110) twins surface steps, dislocations, spinel microinclusions, planar defects stabilized by impurities. 

These defects are generally detrimental to electronic, thermal and mechanical properties of diamond films, although in some cases they may be neutral or beneficial. The frequently occurring defects appear to be stacking faults (twinned clusters with many re-entrant surfaces/comers), which are deemed to play a major role in enhancing diamond growth rates.t ... 

Initiation of growth may also proceed by formation of metastable structures when nucleation is inhibited. Multiply twinned structures have been observed for a number of metals. Their presence indicates an icosohedral or decahedral precursor cluster which has decomposed to a multiply twinned crystal at a critical size [117, 118], Another example of metastable intermediate structures was reported by Dietterle et al. 

The water and A1 contents decrease while the amount of M cations per unit cell of zeolite increases. For Li and Na,the morphology consists of clusters of polycrystalline aggregates. Better outlined single crystals are observed for K and Rb and additional pronounced twinning appears for (Cs)ZSM-5 (Figure 10). 

Particles of face-centered cubic metals of diameter 5 nm of more have been studied extensively by high resolution electron microscopy, diffraction and other methods. It has been shown that such particles are usually multiply twinned, often conforming approximately the idealized models of decahedral and icosahedral particles consisting of clusters of five or twenty tetrahedrally... 

If the clusters exhibit either two- or three-dimensional aromaticity as discussed in Chapter 1.1.3, three orbitals of each bare metal vertex atom are required for the internal orbitals (two twin internal orbitals and one unique internal orbital). 

There has been considerable work in the literature on the structure of very small particles and clusters. Interest in this field has been primarily due to Ino s (1966) early experimental studies of normally fee metals prepared by vapour condensation which showed that a sizable portion of the particles exhibited non-crystallographic structures. These non-crystallographic atomic clusters or polycrystalline nuclei have been observed to consist of pentagonal bi-pyramid or icosahedra form of twinned structures and are known as multiply twinned particles (MTPs). EM studies of supported transition metal catalyst systems have indicated that MTPs sinter faster in catalytic reactions leading to the loss of surface area and are not beneficial to catalysis (Gai 1992). We describe the structure and the role of MTPs in catalysis in the following sections. 

Fig. 5.19 A helium field ion image of a tungsten tip electroplated with a thick layer of platinum. Many lattice defects such as grain boundaries, twin boundaries, vacancies and vacancy clusters can be seen. From the work of Rendulic ...

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. 

L. Pauling, Icosahedral quasicrystals are twins of cubic crystals containing large icosa-hedral clusters of atoms The 1012-atom primitive cubic structure of Al6CuLi3, the C-phase of Al37Cu3Li2iMg3, and GaMg2Zn3. Proc. Natl. Acad. Sd. (USA) 85, 3666-3669 (1988). 

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