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2.1.2 Chemical Order and Disorder
ОглавлениеFor a given x, the arrangement of A and B atoms on the vertices of a network determines the nature of the chemical order present. Often, for networks having compound‐like compositions, ACBV (where C and V are integers), the arrangement of two types of atoms is well defined: every A atom is coordinated by a number, V, of B atoms. Similarly each B atom is coordinated by another number, C, of A atoms. Such networks with well‐defined chemical arrangement are called chemically ordered. In these networks, linear bonds exist only between dissimilar atoms (i.e. heteropolar bonds). A prime example of such networks is that of silica (SiO2): each silicon is coordinated by four oxygens (V = 4) and each O is linked to two silicons (C = 2).
Not all glasses can be chemically ordered as this state can be achieved only for some definite stoichiometries. Noncompound‐like compositions are thus necessarily disordered. A signature of chemical disorder is the presence of homopolar bonds (between similar atoms). Even compound‐like compositions can be chemically disordered in covalent systems because of the presence of a significant fraction of such homopolar bonds: an example is GeSe2 [15].