Short-range ordered 7 -type units

The structure of crystalline FeO belongs to the NaCl type. When iron(II) oxide is prepared under normal conditions, the composition of the product (wustite) is always Fei 50. In order to retain overall electric neutrality, part of the Fe2+ is oxidized to Fe3+, and the chemical formula becomes Fc Fe. O. Since the radius of Fe3+ is small, the Fe3+ cations tend to occupy the tetrahedral holes to form a short-range ordered Fe40io cluster, which is called the Koch cluster of Fei 0, as shown in Fig. 10.1.3. The Koch clusters are distributed randomly in the crystal structure. To satisfy charge neutrality, the formation of a Koch cluster must be accompanied by the presence of six Fe2+ vacancies, one of which is located at the center of the cluster, and the remaining five are distributed randomly at the centers of the edges of the cubic unit cell. 

Table 2 Categories of chalcogen-based glasses" types of building unit (Figure 9) can be distinguished, giving rise to so-called short-range order.

In fact, a more complicated pattern of interaction is likely to give rise to different types of short-range order. The symmetries of the Bragg-Williams approximation can be readily broken if, instead of the chain molecules, the pairs of chains corresponding to the interaction described in Eq. (2.21) are regarded as statistical units. This leads to the Bethe-Peierls approximation or to the quasi-chemical approximation of Fowler and Guggenheim defect polymer crystal appears then to be a sub-... 

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