A P-31 MAS NMR-STUDY OF GLASSES IN THE SYSTEM XNA(2)O-(1-X)AL2O3-2SIO(2)-YP(2)O(5)

The structural role of phosphorus in glass samples of variable alkali/ aluminium ratio has been investigated in the range of 1 to 6 mol% P2O5 , using a combination of one and two dimensional P-31 MAS NMR spectros copy. The samples have the general formula xNa(2)O-(1-x)Al2O3-2SiO(2)- yP(2)O(5) ranging from Al-free (x=1.0) to peraluminous (x = 0.44). Spe ctra of Al-free compositions are consistent with the presence of Na3PO 4 and Na4P2O7 complexes residing outside of the silicate network, as p reviously proposed in the literature. In contrast to solution mechanis ms proposed in the literature no evidence for extended metaphosphate c hains (NaPO3)(n) was found in aluminium-bearing peralkaline glasses. A lternatively, our spectra suggest that, in addition to Na3PO4 and Na4P 2O7, phosphate tetrahedra are attached to the aluminosilicate framewor k through replacement of Na by tetrahedral aluminium charge balanced b y sodium. The average number of Na replaced by Al increases with decre asing peralkalinity. At the metaluminous join all three sodium atoms o f the Na3PO4 tetrahedron are replaced by Al without the need to disrup t a stable sodium aluminate complex. These solution mechanisms are sho wn to be consistent with available spectroscopic and physical property data reported in the literature, In peraluminous compositions we prop ose that P interacts with excess aluminium to form AlPO4 complexes. Ho wever, our spectra also imply that in mildly peraluminous glasses a fr action of phosphorus continues to interact with tetrahedral Al associa ted with Na. (C) 1998 Elsevier Science B.V.