Atomic semiconductors are discussed easily, since all of the neighbors are equivalent. Perfect crystalline order with tetrahedral binding (fourfold coordination) requires the formation of six-member rings. Polk (1971) introduced odd-numbered rings (5 or 7), and thereby formed glasses with an otherwise tetrahedrally coordinated arrangement of atoms around these building blocks. Odd-numbered rings were confirmed in a-S (Pantelides 1987) and in a-C (Gallagher 1988).
Comparing crystalline (c) and amorphous (a) structures of the same element (e. g., Ge), one sees that first – and second-neighbor distances (2.45 vs. 2.46 and
4.2 vs. 4.00 A for c-Ge vs. a-Ge, respectively) are surprisingly similar, as is the average bond angle (109.5 vs. 108.5°).
There is, however, a spread of ±10° in the bond angle for other amorphous structure, resulting in an average coordination number of 3.7 rather than 4 for c-Ge (Etherington et al. 1982). The lower effective coordination number indicates a principal building block structure that is slightly less filled but without vacancies, which are ill-defined in amorphous structures. Hard sphere models, which would assist in defining sufficient space between the spheres as vacancies, must be used with caution since covalent structures can relax interatomic lattice spacing when relaxing bond angles (Waire et al. 1971).
Binary compounds are more difficult to arrange in such a fashion, since odd – member rings cannot be formed in an AB sequence without requiring at least one AA or BB sequence. Random network models, however, can also be made with larger even-numbered rings. Zachariasen (1932) suggested the first one for SiO2-type glasses.
Many covalent polyatomic binary compounds containing chalcogenes, easily form semiconducting glasses such as As2S3, As2Se3, or GexTey. The principal building blocks obey the 8 – N rule.
In some of the amorphous chalcogen compounds is the nearest neighbor distance shorter and the coordination number lower than in the corresponding crystalline compounds (Bienenstock 1985). The large variety of possible GexTey building blocks that still fulfill the 8 – N rule, created by replacing Ge-Ge with Ge-Te or Te-Te bonds, is the reason that glasses of a continuous composition from pure Ge to pure Te can be formed (Boolchand 1985).