Symmetry crystallographic distortion

Extended-Hiickel calculations have been carried out [185] for systems such as IrCl4(NO)2-, based on a slightly distorted square pyramid of C4v symmetry (crystallographically studied 5-coordinate systems do not have a planar base but exhibit this slight distortion). Figure 2.104 shows how the... 

Surface Reconstructions in W. In earlier sections, we have noted that the pair functional formalism is not always appropriate as the basis for an energetic description of metals. One case where this is evident is in the context of the surface reconstructions seen on certain bcc transition metals. In particular, the (001) surface of both W and Mo exhibits fascinating reconstructions, and as in the case of the Au(OOl) surface, the symmetry breaking distortions leave few vestiges of their crystallographic ancestry. 

The crystal stmcture of the calcium fluoroapatite has two different crystallographic sites for the Ca " ion. The Ca(I) site has a threefold axis of symmetry and is coordinated to six oxygen ions at the vertices of a distorted trigonal prism. The Ca(Il) ions are located at the corners of equilateral... 

The extraordinary bonding properties in 16 can also be seen in a diagram of 16 (Fig. 34). In 16, an almost perfect five-numbered axis is attained, distorted only by the central Ga2 unit. This is the first time that a symmetry close to the five-numbered symmetry is observed for molecular metalloid clusters. A few solid-state modifications with five-numbered symmetry have, however, been found for compounds involving Group 13 elements. Because there is no crystallographic space group with a five-numbered axis, these compounds are summarized under the collective name quasi-crystals .83 For a better understanding of quasi-crystals,... 

Both show a greater number of vibrational modes than predicted for pure Tj symmetry, but show good correspondence with the expected distribution for a distorted D2d cube (24,43). This finding Is In agreement with the x-ray crystallographic results which Indicate that the 4Fe-4S clusters In these two proteins, as well as In a number of model compounds, have a compressed tetragonal structure with 4 short and 8 long Fe-S bonds (41,45). 

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