A 31P MAS NMR study of glasses in the system xNa2O-(1-x)Al2O3-2SiO2-yP2O5.
The structural role of phosphorus in glass samples of variable alkali/aluminum ratio has been investigated in the range of 1 to 6 mol% P2O5, using a combination of one and two dimensional 31P MAS NMR spectroscopy. The samples have the general formula xNa2O-(1-x)Al2O3-2SiO2-yP2O5 ranging from Al-free (x = 1.0) to peraluminous (x=0.44). Spectra of Al-free compns. are consistent with the presence of Na3PO4 and Na4P2O7 complexes residing outside of the silicate network, as previously proposed in the literature. In contrast to soln. mechanisms proposed in the literature no evidence for extended metaphosphate chains (NaPO3)n was found in aluminum-bearing peralkaline glasses. Alternatively, our spectra suggest that, in addn. to Na3PO4 and Na4P2O7, phosphate tetrahedra are attached to the aluminosilicate framework through replacement of Na by tetrahedral aluminum charge balanced by sodium. The av. no. of Na replaced by Al increases with decreasing peralkalinity. At the metaluminous join all three sodium atoms of the Na3PO4 tetrahedron are replaced by Al without the need to disrupt a stable sodium aluminate complex. These soln. mechanisms are shown to be consistent with available spectroscopic and phys. property data reported in the literature. In peraluminous compns. we propose that P interacts with excess aluminum to form AlPO4 complexes. However, our spectra also imply that in mildly peraluminous glasses a fraction of phosphorus continues to interact with tetrahedral Al assocd. with Na.
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