Clusters stacking

In all the two ring-systems discussed above, each elementary repeat unit is constituted of one molecule only, similar to pure discotic materials, and as such differ from true polycatenar materials. Indeed, for the latter, the internal structure of the columns is completely changed upon the elongation of the rigid cores, and the repeat unit is constituted of several molecular species which aggregate together into clusters, these clusters stacking into columns. These systems will be discussed below. 

Reductions of [PtClg] " in an atmosphere of CO provide a series of clusters, [Pt3(CO)6] ( = 1-6,10) consisting of stacks of Pt3 triangles in slightly twisted columns Pt-Pt = 266 pm in triangles, 303-309 pm between triangular planes (Fig. 27.12). A feature of these and other Pt clusters is that they mostly have electron counts lower than predicted by the usual electron counting rules. In the series just mentioned for instance, = 1 and n — 2 have electron counts of 44 and 86 whereas 48 and 90 would... 

Platinum forms a wide variety of clusters and these are of interest as models for metallic platinum surfaces and catalysis on such surfaces. Many of these clusters are based on stacked triangles23,571 or bicapped pentagonal prisms.571 Many smaller clusters are also derived from triplatinum building blocks.16... 

In 1996 Stack and co-workers reported an unusual 3 1 (copper 02 stoichiometry) reaction between a mononuclear copper(I) complex of a A-permethylated (lR,2R)-cyclohexanediamine ligand with dioxygen. The end product of this reaction, stable at only low temperatures (X-ray structure at —40 °C) is a discrete, mixed-valence trinuclear copper cluster (1), with two Cu11 and a Cu111 center (Cu-Cu 2.641 and 2.704 A).27 Its spectroscopic and magnetic behavior were also investigated in detail. The relevance of this synthetic complex to the reduction of 02 at the trinuclear active sites of multicopper oxidases4-8 was discussed. Once formed, it exhibits moderate thermal stability, decomposed by a non-first-order process in about 3h at —10 °C. In the presence of trace water, the major isolated product was the bis(/i-hydroxo)dicopper(II) dimer (2). 

A third effect that has been encountered [185] is a structure that is made from a mixture of different kinds of clusters which are all finite stacks and vary by the number of their members. At the first glance such a material looks as if the zero-sum rule were violated - but for each individual cluster it is not. Figure 8.45 shows a sketch of the probability of different clusters in such a structure. In the study [ 185] the parallel fit using three finite stacks (solos, duos, trios of lamellae) yielded a peculiar coupling of cluster fractions according to the relation... 

Structure determinations of LiZn2Mo308 and Zn3Mo308 (18) showed that these compounds are isomorphous but the pattern of oxide-layer stacking is different from that of Zn2Mo308. Otherwise the structures are closely related and all contain the cluster units with the same connectivity, [Mo30l+06/203/3]n . 

Some of the major questions that semiconductor characterization techniques aim to address are the concentration and mobility of carriers and their level of compensation, the chemical nature and local structure of electrically-active dopants and their energy separations from the VB or CB, the existence of polytypes, the overall crystalline quality or perfection, the existence of stacking faults or dislocations, and the effects of annealing upon activation of electrically-active dopants. For semiconductor alloys, that are extensively used to tailor optoelectronic properties such as the wavelength of light emission, the question of whether the solid-solutions are ideal or exhibit preferential clustering of component atoms is important. The next... 

Gold has been used for many years as a minority carrier lifeline controller in Si. As such, it is introduced in a controlled manner, usually by diffusion into transistor structures to decrease the carrier lifetime in the base region in order to increase the switching speed (Ravi, 1981). Conversely, the uncontrolled presence of Au is clearly deleterious to the performance of devices, both because of the increased recombination within the structure and the increase of pipe defects, which can cause shorting of the device. These pipe defects consist of clusters of metallic impurities at dislocations bounding epitaxial stacking faults. 

Figure 4.8 Structure of a Willis 2 2 2 structure (a) an empty Og cube (b) the stacking of four Og cubes in the UO2 structure (c) an Og cube containing a < 111 > interstitial oxygen ion (1d) an Og cube containing a < 110> interstitial oxygen ion and (e) the 2 2 2 cluster.

More recently, Adams and coworkers have provided a very interesting case of heteronuclear clusters that are very active for the hydrogenation of alkynes [4, 54, 55]. The high-nuclearity layer-segregated Pt-Ru complex [Pt3Ru6(CO)21(jU3-H)(/ -H)j], consisting of three stacked triangular layers of metal atoms with an... 

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