Glass-Forming Building Blocks

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).

Updated: July 30, 2015 — 11:59 pm