Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов

Figure 1 Oxygen linkages in crystalline and glassy CaTiSiO₅ as seen by ¹⁷O MAS (magic‐angle‐spinning) NMR. Ti–O–Ti and Si–O–Ti oxygens are abundant in both, but peaks are much narrower in the crystal because of the long‐range order, whereas the glass contains much greater local‐scale disorder. The glass also contains abundant structural groups absent from this crystal, such as Si–O–Si and Si–O–Ca oxygens, requiring a more complex structure. Spinning sidebands are marked by black dots.

      Source: Modified from [3].

Ambient‐pressure crystalline borates are known to contain both BO₄ tetrahedra and BO₃ triangles, whereas pure B₂O₃ consists only of the latter. Early X‐ray scattering studies again were fundamental to understanding the glass, which also contains only the three‐coordinated species. Subsequent evidence, from diffraction and spectroscopy, confirms this conclusion. A long controversy over how the BO₃ triangles are interconnected has largely been resolved in favor of abundant three‐membered “boroxol” rings (Figure 3), thanks to methods such as combinations of neutron and X‐ray diffraction with structural modeling and sophisticated two‐dimensional (2‐D) and multinuclear NMR. These have been applied in more complex compositions as well (Sections 3–5). Pure GeO₂ can take on both tetrahedral and octahedral structures even in ambient‐pressure crystals, and compounds such as alkali germanates often contain mixtures of GeO₄, GeO₆, and even GeO₅ groups. Diffraction and spectroscopic studies agree that pure GeO₂ glass at ambient pressure is comprised mostly or entirely of tetrahedral groups and is thus a good analog for SiO₂. In‐situ X‐ray absorption spectroscopy (XAS) and diffraction experiments on both SiO₂ and GeO₂ glasses at tens of GPa pressures indicate substantial bond lengthening for both Ge─O and Si─O and increases in Ge and Si coordination numbers, these effects taking place at much higher pressures for Si than for Ge.

Figure 2 Radial distribution curves derived in an early X‐ray scattering study of Na₂O–SiO₂ glasses, with mole fractions of Na₂O labeled. Estimated coordination numbers for Si–O (recognized by the authors to be equivalent to 4.0) and Na–O are shown under the corresponding peaks, and mean interatomic distances are shown above the curves.

      Source: Reprinted with permission from [4].

Figure 3 Two‐dimensional sketch of a mixed network oxide glass such as B₂O₃–SiO₂. “Boroxol” groups (a typical one is circled) are particularly abundant in pure B₂O₃ glass. Another aspect of the disorder is the degree of mixing of network cations, which can be determined by methods that “count” the number of different oxygen bridges, e.g. between BO₃ groups (light color) and SiO₄ units (darker color).

3 Modifying the Network: Silicates and Phosphates

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.

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