Cubo-octahedral crystals

As another example of utilizing platy forms of twinned crystals, we should mention a recent remarkable increase in the photosensitivity of photographic film. Photographic emulsion film consists of minute AgBr crystallites, with octahedral or cubo-octahedral Habitus. The photosensitivity is determined by the area... 

In the case of synthetic diamond, grown under high-temperature, high-pressure conditions from a high-temperature solution with metal or alloy as the solvent, diamond crystals exhibit a cubo-octahedral Tracht bounded by 100 and... 

Recently the crystal structure of (PluP CunSs] (88) has been reported to involve a cube of eight S2- anions with interpenetrating cubo-octahedral Cui2 units 198 it represents an inverted... 

In high boron-rich metal borides, there are two types of B12 units in their crystal structures one is cubo-octahedral B12 found in YBi2, as shown in Fig. 13.3.4(a) the other is icosahedral B12 in NaBi2, as shown in Fig. 13.3.4(b). These two structural types can be interconverted by small displacements of the B atoms. The arrows shown in Fig. 13.3.4(a) represent the directions of the displacements. 

Kinoshita reported a correlation between the fraction of Pt surface atoms on the (10 0) and (111) crystal faces of the platinum particles of cubo-octahedral structure with varying particle sizes and the specific catalytic activity of platinum electrocatalysts 54). For cubo-octahedral particles, which have both (1 1 1) and (10 0) faces, an optimum in mass activity at a 3.5-nm platinum particle diameter was reported. Under these conditions, the surface fraction of platinum on (1 00) and (111) faces shows a maximum according to calculations of the coordination number with changing average particle size (55). 

There are some methods to evaluate o -values experimentally [22, 86, 87]. The first method is given by Maeda et al. [86], where the a-parameter for specific CVD conditions is derived from the change in the crystal shape between an initial cubo-octahedral diamond particle of a = 3/2 and the same particle after the growth. 

The temporal evolution of twins has been investigated with an aid of computer simulation by Wild et al, [84]. Figure 6.1 shows a result of a twin evolution on a (111) face of a cubo-octahedral crystal, assuming that a =1.75. Note that time t is in arbitrary units, and the crystal size is normalized. It is seen that a small twin on a (111) face at t = 0 laterally increases the area, reaches the adjacent (100) faces, and induces a secondary nucleation on the (100) faces. According to the computer simulation, there are three types of twin evolutions, as shown in Figure 6.2. The types and characteristics of the twins shown in Figure 6.2 are summarized below. Note that the definition Tjijk here is different from that of Section 5.5. 

A more thorough study of diamond growth on cubo-octahedral cBN crystals with diameters of about 500 pm was undertaken by DC plasma CVD in Ref. [153],... 

Finally, it should be mentioned that electronic state calculations of H-terminated cBN surfaces and diamond growth are studied in Refs. [163, 164]. Also, in a recent paper [165], diamond was deposited on large cBN crystals of 200-350 pm in size that were embedded in a Cu plate. It appeared that (i) diamond nuclei were cubo-octahedral crystallites with approximately 100nm in diameter on the (111) faces of cBN, (ii) in some cases, dense carbon tubes with a diameter of lOOnm and a few micrometer in length were grown, and (iii) diamond crystals grown on Cu had deep holes in the center of the (111) faces. This article also compiled past articles on diamond growth of cBN. 

Fic. 7. Morphological forms of bacterial magnetite observed in projection, (a) Hexagonal (cubo-octahedral) (b) rectangular [note the twinned ciystal (arrow)] (c) cubic and (d) bullet shaped. In each case the crystals are oriented with the (111) faces perpendicular to the chain axis. Bars = 50 nm in all micrographs. 

Figure 2 The (a) mass- and (b) surface-averaged distribution of atoms on the (111) and (100) crystal faces and on the edges and corner sites of a cubo-octahedral cluster model. (From Ref. 4.) Mass-averaged and surface-averaged distributions are based on calculations using cubo-octahedron cluster model and represent number of different crystallographic planes divided by the (a) mass or (b) the surface area of the cluster (at the corresponding particle size). Hence (e- -c) in (a) represents edge and kink positions and (100) (111) the normal cubic crystal planes.

As discussed in section 2.8, relativistic effects on the valence electronic structure of atoms are dominated by spin-orbit splitting of (nl) states into (nlj) subshells, and stabilization of s- and p-states relative to d- and f-states. Here we examine consequences of relativistic interactions for cubo-octahedral metal-cluster complexes of the type [M6X8Xfi] where M=Mo, Nb, W and X=halogen, which have a well defined solution chemistry, and are building blocks for many interesting crystal structures. 

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