Diamond-type lattice

A traditional example of a Zintl phase is represented by NaTl which may be considered as a prototype of the Zintl rules. The structure of this compound (face centred cubic, cF16, a = 747.3 pm) can be described (see also 7.4.2.2.) as resulting from two interpenetrating diamond type lattices corresponding to the arrangements of the Na and T1 atoms respectively (Zintl and Dullenkopf 1932). Each T1 atom therefore is coordinated to other four T1 at a distance a)3/4 = 747.3)3/4 = 323.6pm which is shorter than that observed in elemental thallium (d = 341-346 pm in aTl, hP2-Mg type, CN = 6 + 6) and d = 336pm in /3 Tl, (cI2-W type, CN = 8). 

Very soon after the publication of the work of Rice and Freamo there appeared independently the suggestion that the stabilization of free radicals produced would be facilitated by the addition of some inert material which would dilute the substrate passing through the furnace. Many papers have been published on this technique, which is commonly known as the matrix isolation method (16, 33). However, the substances commonly used to form the matrix are molecular solids, so that the forces between the molecules are very weak and consequently radicals can be preserved only at temperatures near the boiling point of liquid nitrogen or even lower. If one could incorporate radicals into a diamond-type lattice, it might be possible to stabilize radicals sufficiently to keep them at room temperature. 

The elements themselves show a marked transition from non-metallic to metallic character. Germanium has a diamond-type lattice and a m.p. intermediate between the high values of carbon and silicon and the low ones of tin and lead. Tin exists in three solid forms ... 

In the case of semiconductors, it was first shown in this laboratory that the arrangements of the atoms in the surface monolayers of (100) and (111) germanium and silicon are not the same as those for these planes in the bulk (25). The altered arrangements were revealed by the presence of fractional order beams for the surface gratings in certain azimuths. This was later found to be the case for all crystals tested which have a diamond-type lattice, including semiconducting diamond and several of the intermetallic compounds. The surface structure of silicon was observed to be much more complex than that of germanium. In some azimuths, several fractional orders less than one-half were observed. 

The X-ray diffractogram of nanodiamond exhibits the signals characteristic for the (111)-, (220)-, and (311)-planes of a diamond type lattice at 20= 43.9°, 75.3°, and 91.5° (Table 5.2, Figure 5.19). In comparison to bulk diamond, they show another proportion of intensities and a much larger half width. In some cases even the signals for the (400)- and the (331)-plane are observed, with their intensities being rather weak, though. 

Arsenic, selenium, tin, antimony, and tellurium all exist in metallic and nonmetallic forms. For example, Sn occurs as white tin which has a metallic luster and is a good conductor of heat and electricity, and gray tin in which the atoms are covalently bonded in a diamond-type lattice. Below 18 C, the crystalline white tin slowly changes to the powdery gray tin. This transformation was first observed as blistery outbreaks (called tin pest ) on the surface of tin objects such as organ... 

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