Octahedral faces

Crystal Morphology. Size, shape, color, and impurities are dependent on the conditions of synthesis (14—17). Lower temperatures favor dark colored, less pure crystals higher temperatures promote paler, purer crystals. Low pressures (5 GPa) and temperatures favor the development of cube faces, whereas higher pressures and temperatures produce octahedral faces. Nucleation and growth rates increase rapidly as the process pressure is raised above the diamond—graphite equiUbrium pressure. 

In ordinary diamond (2inc-blende stmcture) the wrinkled sheets He in the (111) or octahedral face planes of the crystal and are stacked in an ABCABC sequence. In real crystals, this ABCABC sequence continues indefinitely, but deviations do occur. For example, two crystals may grow face-to-face as mirror images the mirror is called a twinning plane and the sequence of sheets crossing the mirror mns ABCABCCBACBA. Many unusual sequences may exist in real crystals, but they are not easy to study. 

Siace these masses of polycrystaUine diamond possess extensive diamond-to-diamond bonding, they have, ia contrast to siagle-crystal diamond, excellent crack resistance, siace any crack that begias ia oae crystal oa an easy cracking plane (parallel to an octahedral face) is halted by neighboring crystals that are unfavorably oriented for their propagation. 

A similar type of isomerism occurs for [Ma3b3] octahedral complexes since each trio of donor atoms can occupy either adjacent positions at the comers of an octahedral face (/hcial) or positions around the meridian of the octahedron (meridional). (Fig. 19.12.) Geometrical isomers differ in a variety of physical properties, amongst which dipole moment and visible/ultraviolet spectra are often diagnostically important. 

At the initial growth stage, crystals of adamantane show only cubic and octahedral faces. The effects of this unusual stmcture on physical properties are interesting [5]. 

Figure 4 shows the calculated energy barrier for Mn and Co hopping through an octahedral face (F) into a nearest neighbor tetrahedron in the Li/vacancy layer at Xu = 0 and 1/2. This path is the first half Oh of the open Oh Td Oh path pictured... 

The spin structures of hematite are shown in Figure 6.8. Above Tm, the Fe ions are antiferromagnetically coupled across the shared octahedral faces along the c-axis. 

The approach adopted is to view the molecule in three dimensions, imagining each atom or group to be placed at a vertex of hn appropriate polyhedron. In organic chemistry this is usually the tetrahedron with carbon at the centre. Table 3.3 (p. 18) shows the polyhedra normally encountered in organic and inorganic chemistry. It also includes for each polyhedron the polyhedral symbols to denote shape and coordination number. It is to be noted that these polyhedra are often presented in a highly formalised fashion. An octahedron is often represented with the apices rather than the octahedral faces depicted, thus ... 

The hexagonal lattice constant c in the direction of finite or infinite 1-dimensional linking in the compounds mentioned is an integer multiple of the distance between opposite octahedral faces (about 2.5 A). 

The hardness of sodium chloride is nearly 2, and it is a little harder than gypsum. The hardness of a cubic face of the crystal is less than that of an octahedral face and the hardness of a dodecahedral is intermediate between the two. 

The structure shown in Figure 19 was first described for the [CeMoi2042]8 anion.85 It contained two unprecedented features, an icosahedrally coordinated heteroatom, and face-shared pairs of MoOe octahedra. Other examples of octahedral face-sharing have since been observed, but this feature remains uncommon owing to the high coulombic repulsion between... 

Amwen W CO)6 has three C, axes, lour C axes (through the octahedral faces), and six C2 axes (through the octahedral edges)... 

Pd7(CO)7(PMe3)7] has been isolated from the reaction of (23) with CO.115 This cluster has a monocapped octahedral structure with one phosphine attached to each Pd atom. Four of the carbonyl ligands bridge the octahedral faces, while the other carbonyls edge bridge (Pd—Pd 2.79(2) A). 

Birch, Farnell, and Flint (141) studied the effect of sulfur sensitization on the location of latent image centers on grains that had both cubic and octahedral faces. In the emulsions without deliberate chemical sensitization, the latent image centers formed preferentially on the cubic faces, but progressive sulfur sensitization caused a gradual shift in location from the cubic to the octahedral faces. Based on the assumption that the latent image centers formed at the sulfide center sites, they concluded that silver sulfide formed predominantly on the octahedral faces. 

The thermodynamic stability of the binuclear site has been demonstrated by the spontaneous assembly of [Fe20(02CR)2L2] (13) from ferric salts in the presence of water, an alkyl carboxylate salt, and a tridentate nitrogen donor ligand L that can cap an octahedral face on iron (8). Suitable ligands include tris(pyrazolyl)borates and 1,4,7-triazacyclononanes. Structure (13) is in essence a portion of the basic ferric acetate structure. The complexes are excellent physical and structural models of the diiron sites and model some aspects of reactivity including redox activity and interconversion of the oxo and hydroxo bridge. 

Figure 5.19. The structure of AlClybhLO. (a) A projection on the hexagonal base of the cell with heights of atoms shown, (b) A packing drawing viewed along the c axis. The Cl atoms are largest spheres. A1 atoms (small, black) are covered by the triangular arrangements of oxygens (octahedral faces).

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