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:
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.