Network Modifier Effects in Multiple Network Former Glasses: NMR Results on the System 70 SiO2-7.5 P2O5-(22.5-x)Al2O3-xNa(2)O (0 & LE; x & LE; 17.5)

Themodification of the glass structure in the system SiO2-P2O5-Al2O3 by Na2O has been studied along the composition line 70SiO(2)-7.5P(2)O(5)-(22.5 - x)Al2O3-xNa(2)O (0 & LE; x & LE; 17.5) using Ramanspectroscopy and multinuclear solid state NMR of Si-29, P-31, Na-23, and Al-27 nuclei. Spectraldeconvolutions have been proposed based on P-31/Na-23 and P-31/Al-27 double resonance and P-31 double-quantum filtering experiments, leading to a detailed pictureof the short- and medium-range order in this glass system. The structureof these glasses is dominated by a significant bonding preferencebetween formally anionic four-coordinate aluminum Al-4 speciesand formally cationic phosphate P-4 units (Al-O-Plinkages). In the notation used here, the superscript denotes thenumber of bridging oxygen atoms bound to the network forming unit.The gradual replacement of alumina by sodium oxide initially (I) decreasesthe amount of higher coordinate aluminum and increases the numberof Si-O-Al linkages, which are charge balanced by thesodium ions introduced by Na2O. Higher levels of x (II) lead to a partial depolymerization of Al-O-Plinkages and the formation of anionic phosphate species (P-2 and P-1) strongly interacting with the sodium ions. WhileAl-O-P linkages continue to be present for these anionicunits, (III) some P-O-P linkages are also formed around x = 15.0, leading to the partial crystallization of sodiumpyrophosphate at x = 17.5. There is no evidence forany modification of the silica component, which remains of the Si-4 type throughout the glass composition range. The resultsof the present study highlight the dual role of the sodium ions inthe modification of the network structure of this three-componentnetwork former system in a quantifiable manner.