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The structure of oxide and chalcogenide glasses is usually described by the Zachariasen–Warren random network model, thus termed because it was proposed by Zachariasen [1] in 1932 and subsequently supported by early XRD studies of glass structure made by Warren and coworkers (Chapter 10.11, [2]).

This model is easily understood with reference to the structure of a 2‐D A₂O₃ oxide glass (Figure 2), where the local structure is very well defined in terms of rigid triangular AO₃ units within which there is almost no variation in the bond lengths and angles. The way in which the units connect together to form a noncrystalline structure is described by a set of rules for glass formation, postulated by Zachariasen:

1 An oxygen atom (O) is not linked to more than two network‐forming cations (A).

2 The coordination number, nAO, of oxygen around the network‐forming cations must be small, i.e. 3 or 4 (where the coordination number of an atom simply means the number of other atoms that are within a certain distance from it).

3 The oxygen polyhedra should share corners with each other, not edges or faces.

4 For a three‐dimensional network, at least three corners of each polyhedron must be shared.

For the 2‐D illustration of Figure 2, the coordination number for the triangular AO₃ units is three, thus satisfying the second rule. Two AO₃ units are connected to each other by the sharing of a common oxygen atom, so that the first rule is satisfied. The third rule is also satisfied, since each pair of connected units shares only one common oxygen, not two (edge‐sharing), or three (face‐sharing). Even though this example is a 2‐D structure, it also satisfies the fourth rule, since the AO₃ units are three‐connected. In fact, it is well established that real glasses such as B₂O₃ [3] and As₂O₃ [4] can form three‐dimensional structures based on three‐connected structural units.

Figure 2 Two‐dimensional representation of a random network for a composition A₂O₃ [1]. Small dark spheres: A atoms; large light spheres: oxygens.

It should be emphasized that the random nature of the structure does not arise from disorder within the basic structural units because the distributions of bond lengths and bond angles within them can be very narrow, as in a crystal structure. Instead, there is a wide distribution of bond angles (e.g. A–Ô–A in Figure 2) leading to a distribution in sizes and shapes of the rings formed by the connections between the AO₃ units. The noncrystalline nature of the structure arises from this wide distribution of bond angles.

As will become clear later, not all real network glasses obey all of Zachariasen's rules. Nevertheless, these rules provide a basic philosophical framework and a valuable starting point from which the structure of real glasses can be described.

A comparison of the crystalline structure of Figure 1 with the random network of Figure 2 shows the similarity and difference between the two. Both types of structure have short‐range order (SRO) when distances similar to the bond lengths are considered. In fact, SRO arises naturally from the finite sizes of atoms and their mutual bonding so that it exists in all condensed phases, be they liquid, glassy, or crystalline. The fundamental difference is that only crystals exhibit long‐range order (LRO), extending over distances that are large compared to interatomic spacings (Figure 1), whereas glasses (Figure 2) and liquids are by definition isotropic because their structure is on average the same in any direction.