The amorphous structures

During the irradiation of diamond by carbon atoms, amorphization of the crystal structure may occur and two specific amorphous forms of carbon may appear: the diamondlike amorphous carbon which will be denoted by $ta-C$ and the graphitelike amorphous carbon named $a-C$. These two structures can be distinguish clearly by their macroscopic and microscopic properties.

Generally we can characterize the amorphous structures by the high degree of short range order and absence of long range order. On one hand, in amorphous structures, the bond length, the number of nearest neighbor atoms and the angle between two bonds are close to those in crystalline structure, which gives rise to short range order. For instance, in the $ta-C$ structure, the mean number of nearest neighbor atoms (the coordination number) is $z=3.5-3.8$, the mean bond length is 1.50-1.53 Å, and the mean bond angle 110-115$^{\circ}$. The way the amorphous diamond is prepared strongly affects the properties of the structure, therefore the data that characterize these properties cannot be specified exactly. They lie in a range that will define the structure in consideration (see Table 2.2).

On the other hand, there is no periodicity in the amorphous structure, i.e. one cannot built the entire lattice from a unit cell by means of appropriate translations. This lack of long range order breaks symmetries and gives isotropic characteristics to the structure.

From the energetic point of view, atoms in an amorphous crystal are not bonded ideally, they are subject to significant stresses and distortions. The energy of an amorphous solid is thus higher than that of a pure crystal.