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

glasses as readily as those in boron‐containing compositions, XRD and neutron diffraction demonstrate the accompanying changes in mean Ge–O distances. Also, ¹⁷O NMR can distinguish BOs, NBOs, and other species in germanates and confirms the transition from a primarily “borate‐like” mechanism at low modifier contents (mixed low‐ and high‐coordination of Ge, most or all BO in the broad sense of the term) to a “silicate‐like” mechanism (GeO₄ and “normal” NBO formation) at high modifier contents. Germanate crystals, glasses, and melts are often considered as at least rough analogs for silicates at elevated pressures. If the changes in network cation coordination with composition are as complex in silicate melts at high pressures, then highly nonlinear compositional effects on melt properties may be expected.

5 Network Cations in Aluminosilicates

      In most readily formed multicomponent oxide glasses, Al³⁺ is a network former predominantly present as AlO₄ tetrahedra. The latter are compositionally equivalent to AlO_4/2 if oxygen sharing is taken into account. Because the alumina chemical component (Al₂O₃ or AlO_3/2) has insufficient oxygen to form this species, one NBO, if present, will be converted to a BO for each added Al cation. Simple models of aluminosilicate melt structure have long assumed that, when alumina contents become large enough to balance all of modifier oxides (e.g. moles of Al₂O₃ = moles of Na₂O or CaO), NBO contents are reduced to zero and the glass or melt structure is comprised entirely of fully connected tetrahedra, by analogy with framework aluminosilicate crystals such as feldspars (e.g. NaAlSi₃O₈, CaAl₂Si₂O₈). This is a good approximation in some systems, especially those with alkali oxide modifiers only, and is supported by long‐known changes in properties with composition as well as diffraction and spectroscopic data. As alumina is added to alkali silicate melts and glasses, for example, the alkali cations are coordinated by fewer NBOs and more BOs, some of which will have partial negative formal charges, e.g. −1/4 for Si–O–Al and −1/2 for Al–O–Al. This change in role can be described as a transition from “network‐modifying” to “charge‐compensating” cation.

However, detailed spectroscopic studies, especially by ²⁷Al and ¹⁷O NMR and Raman, show that the structure can be more complex than indicated by this model, particularly in systems with modifier cations of high field strength. In Ca and Mg aluminosilicates, for example, significant concentrations of AlO₅ (typically 4–8% of Al cations) and even small amounts of AlO₆ groups are present throughout most of the glass‐forming regions [11]. Some NBOs also persist well into the peraluminous compositional range (e.g. with moles of Al₂O₃ > CaO). Trivalent modifier cations such as Y³⁺ and La³⁺ promote this shift in Al coordination, which increases even more obviously in peraluminous compositions and in aluminoborates and aluminophosphates. The mixing of these Al coordinations in the network must contribute to configurational entropy and related properties. As noted in Section 3, the distinction between “bridging” and “non‐bridging” oxygens becomes blurred as network cations increase in coordination number and their bonds to oxygen lengthen and weaken, complicating simple structure–property hypotheses. A few in‐situ X‐ray diffraction and Raman studies, and more detailed research on quenched, decompressed glasses, have clearly shown increases in Al coordination with pressure, which occurs more readily than for Si. NMR studies of glasses quenched from high‐pressure melts have shown that Al coordination increase is promoted by modifier cations with higher field strength [10].

6 Short‐range Order and Modifier Cations

      The relatively large, low‐charge cations that can serve as “network modifiers” in oxide glasses comprise much of the periodic table, so that their behavior can only be generally summarized here. Information about their local structural environments has been most commonly obtained from XAS, both XANES and EXAFS [12], from optical spectroscopy for many transition metal and rare earth cations, from Mössbauer for Fe²⁺ and Fe³⁺, and from modeling of neutron and X‐ray diffraction data. In a few cases, notably for ^6,7Li, ²³Na, ²⁵Mg, and ²⁰⁷Pb, NMR has begun to contribute as well. In a number of oxide glass systems, the possibilities of substitution of isotopes of modifier cations with different neutron scattering cross sections (e.g. ⁴⁴Ca–⁴⁰Ca) has allowed cation‐specific pair distribution functions to be derived from differential measurements, which can give unique details of ordering out to several cation shells. All of these types of data usually indicate some disorder in the first shell and, in some cases, mixes of cation coordination. Most commonly, coordination numbers are similar to those of known crystals or somewhat lower, as can be expected from the lower densities of the glass and liquid phases. Fitting of EXAFS data for some modifier cations has provided important clues about cation first neighbors and on whether these mix randomly, which can be important not only for thermodynamic models but for optical and magnetic properties. In systems with strong nuclear dipolar couplings, such as for ²³Na and ⁷Li in alkali silicate and borate glasses, detailed studies of NMR line shapes and relaxation can give estimates of mean distances among the modifier cations [13]. With these data one can discriminate between models of random, spatially homogenous distributions and of nonrandom arrangements with shorter average cation–cation separations. The latter feature is found in models in which modifiers are clustered in regions with relatively high NBO concentrations, for example, those in 2‐D “channels” thought to be important in ion transport [1].

The coordination number of a given modifier cation often depends on glass composition. For example, the coordination number of an alkali cation should increase as the fraction of coordinating oxygens that are NBO (vs. BO) decreases with increasing silica or alumina content, and thus the negative charge per oxygen is reduced. This type of change has been measured by ²³Na NMR and other methods. In cases where glass color is caused by electronic transitions in cations such as transition metals and rare earths, changing site geometry or coordination number with composition can have dramatic visible and spectroscopic consequences (Figure 6). If more than one modifier cation is present in the system, that with the higher field strength can outcompete another with a lower field strength for coordination by NBOs, displacing the latter cations into sites with higher coordination number (and/or to those with more BOs) as composition changes. This type of site ordering is probably part of the explanation for the commonly seen “mixed alkali” effects, where cation diffusion and ionic conductivity can be slowed by orders of magnitude relative to those in single‐modifier compositions. At higher temperatures, increased disorder with respect to modifier cations can be a major contributor to configurational entropies and is marked, for example, by the enhancement of the entropy of fusion of diopside (CaMgSi₂O₆) relative to those of enstatite (Mg₂Si₂O₆) and wollastonite (Ca₂Si₂O₆) [2].

Figure 6 Optical spectra for glasses in which Ni²⁺ coordination changes from primarily 4 to 5 to 6 coordinated as alkali content is decreased. The bulk glass color changes from purple to brown to green.

      Source: Modified from [14].

      The charges and sizes of network‐modifier cations, as in part captured by their field strength and reflected by their coordination numbers, can have huge effects on the network structure of oxide glasses and on both glass and melt properties. When the coordination of the network cation can readily change, as for boron and aluminum in some systems, higher field‐strength modifiers can either decrease (B) or increase (Al) the network cation