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The conceptual CRN and DRPHS models for glass structure [15, 16] are based on single‐type polyhedra or atoms, respectively, and predict extended range order to be homogeneous everywhere. Because these models are constructed statically, not dynamically, DFs are also excluded. A geometric consequence of introducing more than one size of polyhedron or atom type is microsegregation in the packing of the minority component, first as clusters, which then coalesce into channels above the percolation limit (~20%). This evolution was originally predicted from the 2‐D modified random network (MRN) model [9] and later from 3‐D MD simulated structures [2].

Interestingly, a similar clustering is also evident in computer‐simulated metallic glass structures, such as the metalloid P in the RMC model for Ni_0.8P_0.2 metallic glass, where this microsegregation can be seen threading through the close‐packed Ni structure ([12] Figure 1).

For MRN structures (Figure 8), channels are defined by nonbridging oxygens (NBOs) resulting in silicons, for example, being surrounding by mixtures of BOs and NBOs, which can be readily identified by ²⁹Si MAS NMR [1, 28]. In aluminosilicate glasses, aluminums occupy tetrahedral sites AlO₄ ⁻¹ [1] where the extra charge is compensated for by adjacent modifier cations like alkalis or alkaline earths. For fully compensated compositions, like those of feldspars, glasses, silicon, and aluminum tetrahedra are corner‐shared via BOs, adopting a CRN‐like geometry that is categorized as a compensated continuous random network (CCRN) [28]. Alkali channels have also been predicted in aluminosilicate melts and glasses, like the nepheline family (Na_xK_1−xAlSiO₄) (Figure 8), confirmed by MD modeling, which also reproduces the way the viscosity changes with composition [27].

Figure 8 Microsegregation in network glasses. (a) Modified random network (MRN) used to model oxide glasses [9]. Cation modifier channels clearly seen percolating through the two‐dimensional network, 3‐D microsegregation later confirmed with MD methods [2]. (b) Isosurfaces delineating K⁺ conducting pathways in MD simulated K₂Si₂O₅ disilicate glass, with cutaway showing adjacent interweaving modified silicate network. (c) Alkali channels in MD simulated aluminosilicate Na_0.25K_0.75AlSiO₄[27]. (d) Ag⁺¹ channels in the superionic glass (AgI)_0.6–(Ag₂O–2B₂O₃)_0.4 separated from anion borohalide pockets.

Source: (a) Reproduced from [9] © (1985) Elsevier; (b) image courtesy Z. Zhou; (c) reproduced from [27] © (2017) Nature Publications; (d) reproduced from [10] © 2001 Institute of Physics.

Modifier channels were envisaged from the start as supporting ionic diffusion, such as that of alkali ions migrating through oxide glasses [28]. The use of isosurfaces to delineate channels (Figure 8) helps visualize the separation of mobile ions from the surrounding network. The presence of well‐defined channels explains the additional FSDP observed, for example, in modified silicate glasses around 0.8 Å⁻¹, which can be attributed to correlations between alkalis with a quasiperiodicity of about 8 Å [1]. Other evidence comes from EXAFS and MAS NMR experiments.

There is now much support for the idea of reconciling structural heterogeneity with transport properties [28]. For example, the mixed alkali effect, where the mixing of different mobile alkalis in the same glass drastically affects the mobility of both, can be understood in terms of the segregation of alkalis packing within channels, diluting the diffusivity of each ([2], Chapter 4.6). The huge fall in the average ionic mobility with alkali mixing reduces the ionic conductivity of glass and increases its corrosion resistance and other related transport properties [28].

Fast‐ion conducting glasses based on silver salts, like AgI dissolved in oxide and sulfide matrices, have unusually high ionic conductivities at ambient temperature, exceeding those of binary alkali silicates and borosilicates by three or four decades, making them attractive as possible electrolytes for solid‐state batteries. For these systems RMC modeling has been interpreted in terms of the MRN and the high diffusivity of Ag⁺¹ along percolating channels that are approximately one‐dimensional (Figure 8). Like for conventional modified oxide glasses, the FSDP is often structured for fast‐ion conducting glasses with Ag–Ag and anion–anion components, the former with a quasiperiodicity approaching 10 Å. This is illustrated for (AgI)_0.6–(Ag₂O–2B₂O₃)_0.4 glass in Figure 8 where the topology of ion transport channels can be clearly seen. As the AgI content increases, extending the composition range (and with it the ionic conductivity), the halide component “pushes apart” the remaining network, which also becomes more disordered. The FSDP is dominated by Ag–Ag quasiperiodicity so that this is the feature that correlates with superionic diffusivity within glass‐forming compositions [10].

In these examples, structural heterogeneity is manifest both in the IRO related to the FSDP and topologically over LRO and beyond to embody the extended structure of glass.