Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 231

The distribution of network cations (e.g. Si⁴⁺, Al³⁺, B³⁺, P⁵⁺) around BO in multicomponent oxide glasses and melts is in principle relatively simple to characterize, as each BO has only two such neighbors. This is a quite important problem, though, as it defines the extent of disorder among the various network components as well as the partitioning of partial charges on the BO, which in turn affects ordering of modifier or charge‐compensator cations. Multiple‐quantum ¹⁷O NMR has allowed such distributions to be directly quantified in some systems, through direct counting of proportions of different linkages among network species.

In crystalline, framework aluminosilicates such as feldspars and zeolites, diffraction and NMR studies have generally shown that Al–O–Al linkages are “avoided”, if stoichiometry allows, leading to a high degree of ordering when Al/Si = 1, as for example in anorthite (CaAl₂Si₂O₈) and nepheline (NaAlSiO₄) in which all the oxygens are present as Si–O–Al linkages. This ordering presumably is related to the energetic and/or geometric unfavorability of bringing enough of the charge‐compensating cations close to the relatively highly charged (formally −1/2) Al–O–Al linkages. There are notable exceptions for disordered crystals formed by rapid devitrification of glasses (e.g. cordierite Mg₂Al₅Si₄O₁₈ and β‐eucryptite LiAlSiO₄), and stable tetrahedral framework compounds containing only Al–O–Al linkages are well known (e.g. CaAl₂O₄).

Triple‐quantum ¹⁷O NMR spectra of alkali aluminosilicate glasses can fully resolve Si–O–Si, Si–O–Al, and Al–O–Al sites. This has enabled more precise formulations of the thermodynamics of equilibria such as:

(4)

In terms of distributions of residual negative charges on oxygens, this reaction is analogous to that for the Al‐free system (Eq. 3), as one species on the right has a reduced net negative charge (Si–O–Si or Q ^{n + 1}) whereas the other has an enhanced charge concentration (Al–O–Al or Q ^{n −1}). In NaAlSiO₄ glass, the observation of about 10% of Al–O–Al confirmed that aluminum avoidance is not perfect, but also that this aspect of the structure is closer to ordered than to fully disordered, at least near the glass transition temperature. Further studies of other Na, Li, and Ca aluminosilicates, complemented by ²⁹Si NMR spectra, showed that the concentration of negative charge, now in the form of Al–O–Al linkages, is favored by higher field‐strength cations (as for the distribution of Q ⁿ species in Al‐free silicates) [5]. Thermodynamic modeling of the effects of Al/Si ratio on speciation predicted heats of reactions that are consistent with solution calorimetry and that were, for a few compositions, confirmed by observed increases in Al/Si disorder in glasses with higher fictive temperatures, making a larger contribution to configurational heat capacity. Subsequent extensive work on high‐pressure aluminosilicates has begun to elucidate the much more complex linkages among not only tetrahedral network species but five‐ and six‐coordinated Al and Si, where the mixing of all of these network species presumably contributes to increases in configurational entropy [16].

The same experimental approach can, in some borosilicate glass compositions, quantify the extent of mixing of boron and silicon network cations, which can be much greater than considered in early models based primarily on ¹¹B NMR data (Figure 3). In compositions with modifier oxides, the structure is further complicated by the presence of both BO₃ and BO₄ groups. As for aluminosilicates, relatively highly charged B–O–B linkages between two of the latter seem to be at least partially “avoided.” When pairs of network cations are present that can have strong nuclear dipolar couplings, notably ¹¹B and ²⁷Al, or ²⁷Al and ³¹P, double‐resonance NMR methods can reveal their relative proximities and even the correlations of species with multiple coordination environments, for example of BO₃ groups with AlO₄ [17]. These findings again can provide important constraints on mixing and contributions to order/disorder.