Alkali and Alkaline Earth Aluminosilicate Glasses

It is generally assumed that most, if not all, of the aluminum in these glasses will occur in aluminum-oxygen tetrahedra, so long as the total concentration of alkali and/or alkaline earth oxides equals or exceeds that of alumina. These tetrahedra substitute directly into the network for silicon-oxygen tetrahedra. It follows that alumina, which does not readily form a glass by itself, can, however, easily replace silica in the vitreous network. Oxides which act in this manner are said to be intermediate in behavior between glass formers and modifier oxides. 

Since aluminum oxide only provides 1.5 oxygens per aluminum-oxygen tetrahedron, the oxygen provided by the alkali or alkaline earth oxide is needed to complete the requirement of 2.0 oxygens per tetrahedron for fully linked tetrahedra, i.e., Q4 species. Since the 

The terms on the right side of this equation represent the number of oxygens in the aluminum Q4, silicon Q4, and silicon Q3 units, respectively. Solving this equation, we find that the number of Q3 units is given by the expression  

This equation thus predicts that the concentration of Q3 units will be zero if X = 10, which is consistent with our contention that the network will be free of NBO when the concentration of modifier oxides exactly equals that of the alumina. The connectivity number of this glass will be 4.0 since all of the tetrahedra are fully-linked Q4 species. Similar calculations can be made for any glass in the ternary, so long as the composition contains more modifier oxide than alumina. One must remember, however, to use the appropriate Q units required by the alumina-free glass (see Table 5.4). 

The structures of these glasses are not well defined. MAS-NMR and Raman studies indicate that the aluminum and gallium ions primarily occupy tetrahedral sites, with lesser concentrations of these ions in 5- and 6-fold coordination, while the other trivalent cations occupy octahedral sites. The silicon tetrahedra appear to be broadly distributed among Q4, Q3, Q2 and Qi species. 

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