Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 224

Pure SiO₂, GeO₂, and B₂O₃ readily form glasses on cooling from the melt, and epitomize the “network‐forming” oxides. The Si⁴⁺, Ge⁴⁺, and B³⁺ cations all have high valences, relatively high electronegativities, and are small enough to be stable in four‐ and/or three‐coordination with oxygen. The strongly bonded and interconnected structure that results contributes to high liquid viscosities, slow diffusion, and crystal growth rates, and thus good glass‐forming ability.

Of the three pure‐oxide glass formers, silica is by far the best studied because of its many important technological applications, although many of its properties are anomalous with respect to multicomponent silicate glasses. The structures of the multiple (low pressure) crystalline forms of silica are all comprised of three‐dimensional networks of corner‐shared tetrahedra linked by Si─O─Si “bridging oxygens” (BO). Early X‐ray scattering results confirmed this coordination in the glass and yielded mean Si─O bond distances similar to those in the crystals (Figure 2) [4], as have many subsequent diffraction and spectroscopic studies. The short‐range disorder is most obvious in distributions of Si─O─Si bond angles, which have been determined from methods such as X‐ray and neutron scattering, vibrational spectroscopy, and ²⁹Si and ¹⁷O NMR. This disorder is in turn related to distributions in the sizes and connections among the rings that can be mapped in structural models. The lack of other major sources of disorder is probably why the entropy difference between the liquid and crystal is the lowest known for any oxide [5].

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).