Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 230

In silicate and phosphate glasses, the best‐known aspect of network order/disorder, as affected by the modifier cations, involves the distributions of the NBOs. One can sample these distributions by counting the proportions of “Q ⁿ ” species, defined as tetrahedral groups with n BOs and 4‐n NBOs. Such measurements were originally done by Raman spectroscopy [2]. At least in some simple systems such as alkali silicates and phosphates, these proportions can often be readily quantified by ²⁹Si or ³¹P NMR; such results can be used to evaluate cross sections for vibrational spectroscopy as well. In both approaches, peak fitting and associated assumptions about line shapes are usually required and can lead to some ambiguity. An early and important question was whether the number of Q ⁿ species present in a glass was the minimum derived from stoichiometry (in general two, but only one at special compositions), or whether entropy induced a greater variety. The clear detection of Q ², Q ³, and Q ⁴ species in glasses such as Na₂Si₂O₅, which could contain only Q ³ as in the crystal, confirmed the latter view. Simple equilibria among Q ⁿ species can be defined for n = 1, 2, 3:

(3)

Apparent equilibrium constants k_n for such reactions have been evaluated from Raman and NMR data on glasses, including 2‐D ²⁹Si NMR on Ca and Mg silicates [15]. Higher field‐strength modifiers push these reactions to the right (increasing k_n), presumably favoring the concentration of more NBOs around some Si sites and thus better local charge balance for small, highly charged modifiers. More Q ⁴ species are also generated, which correlates with higher thermodynamic activities of silica as deduced from phase diagrams [5]. In the range of observed speciation, greater k_n values lead to greater contributions to the configurational entropy if models of random mixing of the Q ⁿ species are considered. But enhanced ordering of NBOs around higher field‐strength cations could counter this effect to some degree and, in the extreme, could lead to cation clustering and even incipient phase separation. Shifts of such equilibria have been measured with both in‐situ high‐temperature Raman spectroscopy and NMR on glasses with increasing fictive temperatures, the results from the two methods often agreeing well. Estimated enthalpies of reaction are usually positive, but are less so for higher field‐strength cations. If a random model is assumed (which can in some cases be tested by NMR methods yielding spatial correlations of different species), mixing of observed Q ⁿ populations can contribute a substantial fraction of the calorimetrically determined entropy differences between glasses and crystals.

Complementary to these results for glasses in the normal, high‐silica, glass‐forming range are the recent findings for orthosilicate (e.g. Mg₂SiO₄) and even “sub‐orthosilicate” glasses formed by quenching in laser‐heated, gas‐levitated, containerless melting systems. Here, significant concentrations of Q ¹ species can be observed by Raman and ²⁹Si NMR, requiring as well the presence of nonstoichiometric “free oxide” ions. Direct evidence for the latter can be seen in ¹⁷O NMR spectra [6].