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When an oxide of a larger, lower charged cation (e.g. Na₂O or CaO) is added to liquid silica, the oxide ion (O²⁻) can interact with network oxygen bridges between SiO₄ tetrahedra to form “non‐bridging” oxygens (NBO), as symbolized by a reaction illustrated by Figure 4:

(1)

Results from even the earliest X‐ray scattering studies of alkali silicate glasses (Figure 2) supported this basic concept [4]. Likewise, many vibrational spectroscopy studies have demonstrated increasing proportions of NBOs within silicate tetrahedra with increasing modifier contents. “Network‐modifier” cations such as Na⁺ and Ca²⁺ balance the negative charge on each of the NBOs: bond‐valence considerations generally require several such cations for each NBO as seen in the corresponding crystals. At low modifier concentrations, there are not enough NBOs to fill the coordination sphere of each modifier, with typically five to eight oxygens needed. This means that without cation clustering to allow nonrandom sharing of NBOs, some BOs must also serve this coordinating role. With higher field strengths of the modifier cations (defined as the formal charge divided by the square of the mean cation–oxygen distance), this arrangement is increasingly unstable and can lead to liquid–liquid phase separation over wider ranges of composition. With very high field‐strength modifiers (e.g. La³⁺, Zr⁴⁺), or for cations with relatively high electronegativities (e.g. Sn²⁺, Pb²⁺), the structural and chemical distinctions between BO and NBO may begin to blur, in that some “modifier” cations may take on low coordination numbers and Si–O–M linkages may become relatively strong. Such cations are often described as having “intermediate” character.

Figure 4 Two‐dimensional representation of the modification of a glass network induced by addition of an alkali oxide M₂O. The SiO₄, BO₃, GeO₄ (etc.) polyhedra are symbolized by the triangles. (top) Conversion of one bridging oxygen to two non‐bridging oxygens (small, dark circles), which are charge balanced by the alkali cation (large circles). (bottom) Increase in the coordination numbers of two network cations (squares) and the formation of bridging oxygens with partial negative charges (small, light circles). Conversion between these two types of modified network, with either pressure or composition, is indicated by the vertical arrows.

Reaction (1) can also be considered as a description of chemical equilibrium among oxygen ion species, and has long played a prominent role in models of the thermodynamics of low‐silica metallurgical slags. Conventional views have suggested that “unreacted” or “free” oxide ions are probably only abundant in low‐silica glasses and/or in systems with highly electronegative modifiers such as Sn²⁺ and Pb²⁺. Oxygen ion speciation in some silicate glass compositions can be measured with methods such as XPS and ¹⁷O NMR, or estimated indirectly through techniques that provide information on silicate anionic speciation such as Raman spectroscopy and ²⁹Si NMR (see Section 7.2). Some such studies have suggested the presence of a few percent of “free” oxide ions in compositions well outside the range where they are required by stoichiometry alone, e.g. at silica contents >33.3 mol% in the MgO–SiO₂ binary. As containerless melting and quenching methods have made the formation of very low silica glasses possible (even into the “sub‐orthosilicate” range, e.g. lower silica than in Mg₂SiO₄), it has become possible to quantify more clearly this most basic aspect of silicate structure [6].

In “modified,” ambient‐pressure silicate glasses, only tiny fractions of SiO₅ groups have been detected in a few alkali silicates. However, high pressures, or high P₂O₅ contents, can lead to the formation of substantial fractions of both SiO₅ and SiO₆, which will contribute to the overall network disorder as well as to density increase. For example, both an unusual high‐pressure crystalline phase of CaSi₂O₅ and its glassy equivalent clearly have all three Si coordinations (Figure 5). A number of in‐situ, high‐pressure studies, particularly by Raman spectroscopy, have suggested that considerable structural relaxation, and reversion to lower network cation coordination, can take place on decompression of a glass, even at ambient temperature. A few of these have probably also taken samples above T_g at high pressure. The small cationic radius and high charge of P⁵⁺ makes P₂O₅ another well‐known network‐forming oxide. Over wide ranges of composition, two‐component and more complex phosphate liquids are stable and can be quenched to glasses. These have been extensively studied, particularly by vibrational spectroscopy and ³¹P MAS NMR. All phosphorus is present in PO₄ tetrahedra. As in silicate glasses, these are linked together to form chains of varying length as well as more complex structures. The roles of BO and modifier oxides in phosphate glasses are analogous to those in silicates, as are the considerations of anionic speciation discussed in Section 7.2.

Figure 5 Silicate structural groups in a high‐pressure, triclinic crystalline phase of CaSi₂O₅ and in its isochemical glass quenched from the melt at 10 GPa, showing the correspondence of signals for Si with 4, 5, and 6 oxygen neighbors in ²⁹Si MAS NMR spectra.

Source: Modified from [7].