Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 229

In oxide glasses with more than one type of network‐forming cation, NBOs and BOs could be equally distributed on all such cations, or could be ordered in some way. The latter is generally the case. Raman spectra of aluminosilicates have suggested that most NBOs are on Si and that most AlO₄ groups thus have four BO, when composition permits. This result has been directly confirmed by ¹⁷O NMR in Ca aluminosilicates. Comparable studies in a few borosilicates have observed B‐NBO and the predominant Si‐NBO.

For systems with more than one type of modifier cation, large differences in size or charge might be expected to lead to some kind of ordering. This can be sampled by methods that “see” the coordination environments of oxygen species directly. For example, both one‐ and two‐2‐D ¹⁷O NMR spectra have shown that chemical shift distributions in mixed Na/K and mixed Ca/Mg silicate glasses are consistent with random distributions of the two cations around the NBOs, consistent with deductions from viscosity measurements and estimated configurational entropies. In contrast, the large field‐strength difference between K⁺ and Mg²⁺ leads to a concentration of the latter around the available NBO sites. This approach has shown the considerable complexity of order/disorder patterns that can occur in the distributions of modifier‐ and charge‐compensator cations around both NBO and BO sites in a number of other mixed‐cation silicates and aluminosilicates. Decreases in this type of ordering might be expected from entropic considerations at higher temperatures in the liquids.