Bonding, tetrahedral

The tetrahedrally bonded materials, such as Si and Ge, possess only positional disorder however, materials of this type exhibit high density of defect states (DOS). It is only with the addition of elements such as hydrogen and/or a halogen, typically fluorine, that the DOS is reduced to a point such that electronic device appHcations emerge. These materials contain up to - 10 atomic % hydrogen, commonly called hydrogenated amorphous siHcon (i -Si H). 

FIGURE 6.2 The amide or peptide bond planes are joined by the tetrahedral bonds of the ff-carbon. The rotation parameters are p and Ip. The conformation shown corresponds to clockwise rotation as viewed from Starting from 0°, a rotation of 180° in the clockwise direction ( + 180°) is equivalent to a rotation of 180° in the counterclockwise direction (—180°). (truing G s)... 

The concept of hybridization explains how carbon forms four equivalent tetrahedral bonds but not why it does so. The shape of the hybrid orbital suggests the answer. When an 5 orbital hybridizes rvith three p orbitals, the resultant sp3 hybrid orbitals are unsyimmetrical about the nucleus. One of the two... 

Like alcohols, ethers have nearly the same geometry as water. The R-O-R bonds have an approximately tetrahedral bond angle (112° in dimethyl ether), and the oxygen atom is 5p3-hybridized. 

Furthermore, spz bonding is connected with tetrahedral bond angles (as in Figure 16-11). These expectations are consistent with the experimentally determined structure of diamond, shown in Figure 17-2. 

Free or Restricted Rotation.—Each of these tetrahedral bond eigen-... 

Throughout this paper the word eigenfunction will be used to denote a singleelectron eigenfunction, such as one of the four tetrahedral bond eigenfunctions of a carbon atom. 

Pig. 1. Polar graph showing the dependence on and q> of a tetrahedral bond orbital. The value of y> in cross-section is shown the function is cylindrically symmetrical about the midline of this cross-section. 

In this discussion of the transition elements we have considered only the orbitals (n— )d ns np. It seems probable that in some metals use is made also of the nd orbitals in bond formation. In gray tin, with the diamond structure, the four orbitals 5s5p3 are used with four outer electrons in the formation of tetrahedral bonds, the 4d shell being filled with ten electrons. The structure of white tin, in which each atom has six nearest neighbors (four at 3.016A and two at 3.17.5A), becomes reasonable if it is assumed that one of the 4d electrons is promoted to the 5d shell, and that six bonds are formed with use of the orbitals 4dSs5p35d. 

After rising at copper and zinc, the curve of metallic radii approaches those of the normal covalent radii and tetrahedral covalent radii (which themselves differ for arsenic, selenium, and bromine because of the difference in character of the bond orbitals, which approximate p orbitals for normal covalent bonds and sp3 orbitals for tetrahedral bonds). The bond orbitals for gallium are expected to be composed of 0.22 d orbital, one s orbital, and 2.22 p orbitals, and hence to be only slightly stronger than tetrahedral bonds, as is indicated by the fact that R(l) is smaller than the tetrahedral radius. 

It consists of discrete pentagonal dodecahedra each of which forms eight hydrogen bonds with eight surrounding dodecahedra. In addition, there are six water molecules in positions 0, etc., each of which forms four hydrogen bonds with molecules of the four surrounding dodecahedra in such a way that a complete tetrahedrally-bonded framework of 46 water molecules per unit cube is achieved. In this... 

Mercuric sulfide (HgS) is dimorphic. The more common form, cinnabar (red a-form), has a distorted RS, trigonal structure which is unique among the monosulfides, for the crystal is built of helical chains in which Hg has two nearest neighbors at 2.36 A, two more at 3.10 A, and two at 3.30 A. Bulk a-HgS is a large-gap semiconductor (2.1 eV), transparent in the red and near IR bands. The rare, black mineral metacinnabarite is the 3-HgS polymorph with a ZB structure, in which Hg forms tetrahedral bonds. Upon heating, 3-HgS is converted to the stable a-form. The ZB structure of HgS is stabilized under a few percent admixture of transition metals, which replace Hg ions in the lattice. 

It is important to stress that nitrogen incorporation in a-C H films always result, at least above a certain level, in a strong decrease in the tetrahedrally bonded carbon atom fraction. Raman spectroscopy also gives support to this observation, because the increase in the size of graphitic clusters only can proceed with also increasing sp fraction. 

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