Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 179

Until this point, only SRO has been discussed, along with the structural characteristics that arise directly from interatomic bonding, namely bond lengths, coordination numbers, bond angles, and coordination polyhedra. Even though LRO is by definition lacking in glasses, some ordering exists at length scales between SRO and LRO. It is known as intermediate‐ or medium‐range order (IRO or MRO).

It must be acknowledged that IRO is much harder to probe experimentally than SRO and is, therefore, much less well known. To investigate it, structural modeling may in fact be required to complement direct experimental evidence. This order can be variously characterized in terms of clustering (e.g. Greaves' MRN, cf. Figure 8a in Chapter 2.5) or free volume, for example, but the most widespread approach is to consider the rings that are defined by connected polyhedra in the CRN. Whereas a crystal contains only very few different rings (e.g. the 2‐D crystal of Figure 1 contains only six‐membered rings), the rings in a CRN have a wider and more varied distribution of sizes (Figures 2 and 4).

Although this ring‐size distribution in general cannot be directly addressed experimentally, there are some notable exceptions. A case in point is v‐SiO₂, whose Raman spectrum shows two sharp peaks, at 495 and 606 cm⁻¹, known as D‐lines, because they were initially assigned as defect modes. It is now clear, however, that they represent instead the breathing modes of highly regular four‐membered rings and planar three‐membered rings, respectively [21]. Although these modes give rise to distinctive peaks in the Raman spectrum, theoretical analysis shows that the concentrations of these highly regular rings are actually low, with fractions of oxygens in regular four‐membered rings and planar three‐membered rings estimated as 0.36% and 0.22%, respectively [22].

Likewise, borate networks exhibit different rings of BO_3/2 and BO_4/2 units, which have no (or very few) internal degrees of freedom, and thus have very clear signatures in vibrational spectra. The best known is the boroxol group, a highly planar ring of three BO_3/2 units, which was first detected in B₂O₃ glass (Chapter 10.11). Much experimental evidence from a number of techniques [3] shows that in B₂O₃ glass about 75% of boron atoms are in boroxol groups, while the remainder are in individual BO_3/2 triangles. The boroxol group is in fact so well defined that it can be regarded as a larger structural unit, a superstructural unit, as illustrated in a 2‐D representation of the structure of B₂O₃ glass (Figure 13). As a modifier is added to B₂O₃, boroxol groups become less abundant, but they are replaced by other types of borate superstructural units [19].

Figure 13 Two‐dimensional representation of the boroxol ring model for the structure of B₂O₃ glass [3]; a randomly ordered network of boroxol groups and independent BO₃ triangles. Shaded areas indicating a typical boroxol group (B₃O₆) and independent triangle (BO₃), respectively.

Another approach to IRO is to study the first peak in the diffraction pattern, the so‐called first sharp diffraction peak (FSDP), see Figure 5a. The FSDP has been treated as especially important by many workers, perhaps because it is related to the order with the longest period in real space. However, Salmon has pointed out that the longest range ordering in glasses actually gives rise to the second peak in the diffraction pattern (the so‐called principal peak) [23]. In the past, it was often popular to regard the FSDP as evidence of crystal‐like layers in the glass, because the peak position, Q₁, is similar to the position of the first (00ℓ) reflection arising from layers in a closely related crystalline phase. However, it is now clear that the FSDP arises from correlated voids in the network [24]; a more easily understood view of this idea is to regard the FSDP as arising from the approximate repetition of the walls of the three‐dimensional cages formed by the CRN [25].