Diamond structural types

E = valence electron number per formula unit N = average bond number per semi-metal atom in a chosen defect-free reference structure N = additional electrons to be introduced upon formation of one defect in the reference structure, e.g. a defect in the diamond structure type localizes four electrons to form four lone electron pairs in the neighborhood of the defect Ec = electrons located in electron-poor clusters which, for example, may be determined according to Wade-Mingos rules or according to the 18e rule Ec = electron number in electron-poor cluster(s) composed of semi-metal atoms Ec = electron number in electron-poor cluster(s) composed of metal atoms). By definition, Zintl phases do not contain metal-centered occupied electronic states for these cases (1) may be rewritten as ... 

Usually tills is not tire case and Table 5.2 shows values for Eg, jig and ptp for a number of semiconductors having the diamond structure. It will generally be observed from this table that the mobilities of electrons are greater than those of positive holes making these materials n-type semiconductors. 

It is well known that block copolymers and graft copolymers composed of incompatible sequences form the self-assemblies (the microphase separations). These morphologies of the microphase separation are governed by Molau s law [1] in the solid state. Nowadays, not only the three basic morphologies but also novel morphologies, such as ordered bicontinuous double diamond structure, are reported [2-6]. The applications of the microphase separation are also investigated [7-12]. As one of the applications of the microphase separation of AB diblock copolymers, it is possible to synthesize coreshell type polymer microspheres upon crosslinking the spherical microdomains [13-16]. 

Of the numerous ternary and polynary diamond-like compounds we deal only with those that can be considered as superstructures of zinc blende. A superstructure is a structure that, while having the same structural principle, has an enlarged unit cell. When the unit cell of zinc blende is doubled in one direction (c axis), different kinds of atoms can occupy the doubled number of atomic positions. All the structure types listed in Fig. 12.8 have the tetrahedral coordination of all atoms in common, except for the variants with certain vacant positions. 

In addition to the types of compounds discussed so far, the group IVA elements also form several other interesting compounds. Silicon has enough nonmetallic character that it reacts with many metals to form binary silicides. Some of these compounds can be considered as alloys of silicon and the metal that result in formulas such as Mo3Si and TiSi2. The presence of Si22 ions is indicated by a Si-Si distance that is virtually identical to that found in the element, which has the diamond structure. Calcium carbide contains the C22-, so it is an acetylide that is analogous to the silicon compounds. 

Equiatomic tetrahedral structure types. (Carborundum structure types). To this group pertain the diamond-type structure, the wurtzite (h) and the sphalerite (c) types, and the large family of SiC polytypes (such as he, hcc, hccc, hcchc,. .. (hcc)5(hccc)(hcc)5hc. .. (hchcc)17(hcc)2,. .. (hcc)43hc...). 

C (diamond), cF8-C, structural type Face-centred cubic, space group Fd3m, N. 227. 

The diamond structure, see Fig. 7.14 below, is a 3D network in which every atom is surrounded tetrahedrally by four neighbours. The eight atoms in the unit cell may be considered as forming two interpenetrating face-centred cubic networks. If the two networks are occupied by different atoms, the derivative cF8-ZnS (sphalerite) type structure is obtained. As a further derivative structure, the tI16-FeCuS2 type structure can be mentioned. These are all examples of a family of tetrahedral structures which have been described by Parthe (1964). 

C (graph ite), hP4, structural type. In comparison with the tetrahedral structure of C diamond a very different structure is adopted by carbon in graphite. 

Simple binary tetrahedral structures and polytypes (ZnS-sphalerite, cF8-ZnS and ZnS-wurtzMe, hP4-ZnO, structural types). The sphalerite- and wurtzite-type structures (together with C diamond) are well-known examples of the... 

Here it is worth paying attention to the existence, now proved, of a cubic form of ice [2] with a diamond structure corresponding to the wurtzite-type structure of ordinary hexagonal ice. It is also of interest that this type of ice seems to be unstable at temperatures of above — 100°, and the enormous difference bel/veen its thermal stability and that of ordinary ice is a problem wbb-n requires explanation and whose solution may throw light on t.be fundamental nature of the hydrogen bond in ie < The existence of tetrahedral bonding suggests... 

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