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The relatively large, low‐charge cations that can serve as “network modifiers” in oxide glasses comprise much of the periodic table, so that their behavior can only be generally summarized here. Information about their local structural environments has been most commonly obtained from XAS, both XANES and EXAFS [12], from optical spectroscopy for many transition metal and rare earth cations, from Mössbauer for Fe²⁺ and Fe³⁺, and from modeling of neutron and X‐ray diffraction data. In a few cases, notably for ^6,7Li, ²³Na, ²⁵Mg, and ²⁰⁷Pb, NMR has begun to contribute as well. In a number of oxide glass systems, the possibilities of substitution of isotopes of modifier cations with different neutron scattering cross sections (e.g. ⁴⁴Ca–⁴⁰Ca) has allowed cation‐specific pair distribution functions to be derived from differential measurements, which can give unique details of ordering out to several cation shells. All of these types of data usually indicate some disorder in the first shell and, in some cases, mixes of cation coordination. Most commonly, coordination numbers are similar to those of known crystals or somewhat lower, as can be expected from the lower densities of the glass and liquid phases. Fitting of EXAFS data for some modifier cations has provided important clues about cation first neighbors and on whether these mix randomly, which can be important not only for thermodynamic models but for optical and magnetic properties. In systems with strong nuclear dipolar couplings, such as for ²³Na and ⁷Li in alkali silicate and borate glasses, detailed studies of NMR line shapes and relaxation can give estimates of mean distances among the modifier cations [13]. With these data one can discriminate between models of random, spatially homogenous distributions and of nonrandom arrangements with shorter average cation–cation separations. The latter feature is found in models in which modifiers are clustered in regions with relatively high NBO concentrations, for example, those in 2‐D “channels” thought to be important in ion transport [1].

The coordination number of a given modifier cation often depends on glass composition. For example, the coordination number of an alkali cation should increase as the fraction of coordinating oxygens that are NBO (vs. BO) decreases with increasing silica or alumina content, and thus the negative charge per oxygen is reduced. This type of change has been measured by ²³Na NMR and other methods. In cases where glass color is caused by electronic transitions in cations such as transition metals and rare earths, changing site geometry or coordination number with composition can have dramatic visible and spectroscopic consequences (Figure 6). If more than one modifier cation is present in the system, that with the higher field strength can outcompete another with a lower field strength for coordination by NBOs, displacing the latter cations into sites with higher coordination number (and/or to those with more BOs) as composition changes. This type of site ordering is probably part of the explanation for the commonly seen “mixed alkali” effects, where cation diffusion and ionic conductivity can be slowed by orders of magnitude relative to those in single‐modifier compositions. At higher temperatures, increased disorder with respect to modifier cations can be a major contributor to configurational entropies and is marked, for example, by the enhancement of the entropy of fusion of diopside (CaMgSi₂O₆) relative to those of enstatite (Mg₂Si₂O₆) and wollastonite (Ca₂Si₂O₆) [2].

Figure 6 Optical spectra for glasses in which Ni²⁺ coordination changes from primarily 4 to 5 to 6 coordinated as alkali content is decreased. The bulk glass color changes from purple to brown to green.

Source: Modified from [14].

The charges and sizes of network‐modifier cations, as in part captured by their field strength and reflected by their coordination numbers, can have huge effects on the network structure of oxide glasses and on both glass and melt properties. When the coordination of the network cation can readily change, as for boron and aluminum in some systems, higher field‐strength modifiers can either decrease (B) or increase (Al) the network cation coordination, again probably through a process of formation of and/or competition for NBOs. These effects are much less well known, but could be expected in germanates and in high‐pressure silicates.