Cube structure distortions

There are two modifications of Agl at ordinary temperatures, (3-AgI has the wurtzite (2 2PT) structure and y-Agl has the zinc blende (3 2PT) structure. For both of these structures Ag+ and I ions have CN 4 (tetrahedral). Above 145.8°C, a-Agl is formed with a bcc (3 2PTOT) structure for I ions. For a bcc structure all P, T, and O layers are filled by I ions for a-Agl. There are secondary interstitial sites for a bcc structure—four distorted tetrahedral sites (T ) in each face of the cube and distorted octahedral sites (O ) in the centers of the edges (12) and in the centers of the faces (6) of the cube. The T sites are shown as squares in each face of the bcc cube in Figure 7.28. The bcc cell... 

Whereas CoSij and NiSij crystallize with the cubic fluorite structure, this structure is distorted in -FeSij to give Fe two Fe neighbours, the metal atoms being arranged in squares of side 2-97 A (compare 2 52 A in the 12-coordinated metal). The metal atom also has 8 Si neighbours at the vertices of a deformed cube. This distortion of the fluorite structure represents a partial transition towards the CuAlj structure (p. 1046), in which the coordination group of Cu is a square antiprism and there are chains of bonded Cu atoms. (AC 1971 B27 1209). 

The theorem is illustrated by the perovskites BaTiC>3, SrTiC>3 and CaTiC>3, in which the size of the cubic cell is defined by the Ti03 framework (Fig. 2.4(b)). The Sr2+ ion fits almost exactly into the central cavity of the cube but, when it is replaced by the larger Ba2+ ion, the framework expands, leaving the Ti4+ ion in a cavity that is too large for it. The structure distorts by Ti4+ moving away from the centre of its octahedron (Fig. 2.4(a)). Ca2+ is, however, smaller than the... 

Structure.Bacterial ferredoxin shows a very similar band pattern (Fig. 22, third spectrum), indicating a similar distortion of the cube. The spectrum of HiPIP (high-potential iron protein) from C. vinosum (Fig. 22, second spectrum) appears to be simpler and was interpreted as indicating a relatively undistorted cube structure, but at higher resolution the spectrum has been found to show band splittings, which are more pro-... 

Although the cube structures 6 and 7 appear to be highly symmetric, closer inspection reveals that the cages show no symmetry at all. A strong distortion of the cube is indicated by the broad range of the Ti-O-Si angles from 137.13° to 165.09°. 

In view of the facile oxidation of 10.13a-c it is not surprising that some metathetical reactions with metal halides result in redox behaviour. Interestingly, lithium halides disrupt the dimeric structures of 10.13a or 10.13c to give distorted cubes of the type 10.14, in which a molecule of the lithium halide is entrapped by a Ei2[E(N Bu)3] monomer. Similar structures are found for the MeEi, EiN3 and EiOCH=CH2 adducts of 10.13a. In the EiN3 adduct, the terminal... 

SbCl3 in pentane at 150° under a pressure of H2/CO gave black crystals of [Sb4 Co(CO)3 4] which was found to have a cubane like structure with Sb and Co at alternate vertices of a grossly distorted cube (Fig. 13.24). ... 

The other tetrahalides can all readily be made by direct reactions of the elements. Crystalline SeCU, TeCU and -SeBr4 are isotypic and the structural unit is a cubane-like tetramer of the same general type as [Me3Pt(/Z3-Cl)]4 (p. 1168). This is illustrated schematically for TeCU in Fig. 16.13d each Te is displaced outwards along a threefold axis and thus has a distorted octahedral environment. This can be visualized as resulting from repulsions due to the Te lone-pairs directed towards the cube centre and, in the limit, would result in the separation into... 

The most symmetrical structure possible is the cube Oh but, except in extended ionic lattices such as those of CsCl and CaF2, it appears that inter-ligand repulsions are nearly always (but see p. 1275) reduced by distorting the cube, the two most important resultant structures being the square antiprism D4h and the dodecahedron Did (Fig. 19.10). 

If one-fourth of the fluorine ions are removed and the others are replaced by oxygen ions, calcium being replaced by (Mn, Fe), a structure is obtained which approximates that of bixbyite, which differs from it only in small displacements of the ions. This similarity is shown by the fact that the highly distorted octahedra have corners which are nearly at six of the eight corners of a cube, the six being chosen differently for the 8e and the 24 e octahedra, as is seen from Fig. 4 and 5. This analogy was, indeed, pointed out by Zachariasen for his incorrect structure. As a matter of fact the "ideal structure, with u = 0 and... 

The crystal structures of many organolithium compounds have been determined.44 Phenyllithium has been crystallized as an ether solvate. The structure is tetrameric with lithium and carbon atoms at alternating corners of a highly distorted cube. The lithium atoms form a tetrahedron and the carbons are associated with the faces of the tetrahedron. Each carbon is 2.33 A from the three neighboring lithium atoms and an ether molecule is coordinated to each lithium atom. Figures 7.2a and b show, respectively, the Li-C cluster and the complete array of atoms, except for hydrogen 45 Section 6.2 of Part A provides additional information on the structure of organolithium compounds. 

With an increase in size of the active metals, the interlayer interstitials between the triacontahedral and the penultimate icosidodecahedral shells appear to be occupied by smaller electronegative components, with variable occupancies. These interlayer interstitials are actually the centers of cubes and correspond to the Wyckoff 8c (1/4 V4 A) special position in 1/1 ACs. Strictly speaking, occupation at this site means that the structure is no longer YCd6-type but, for convenience, they are still referred to as Tsai-type phases. According to Piao and coworkers [94], occupation of these cube centers has strong correlation with the orientations of the innermost tetrahedra and distortions of the dodecahedra. 

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