Structural change in Li and Na aluminophosphate glasses: evidence of a "structural mixed alkali effect"

The short- and long-range structure of a series of single and mixed aluminophosphate glasses with the general composition [xNa(2)O (46 - x)Li(2)O], [yAl(2)O(3) (54 - y)P(2)O(5)] is analyzed using (31)P and (27)Al magic-angle spinning (MAS) NMR as well as small-angle X-ray scattering. These series of glasses allow analyzing both the effect of alumina incorporation in these glasses, for small alumina content (y = 0, 4, 8), and the structural changes associated with the so-called mixed alkali effect (x = 0, 11.5, 23, 34.5, 46). Our results indicate that aluminum is mainly octahedrally coordinated in these glasses and that there is most likely some segregation of the Al(OP)(6) species. In the pure phosphate glasses, we observe a "classical" continuous variation of the structural properties with the relative alkali content, but in the aluminophosphate, both local and long-range structural results reveal for the first time some nonlinear change as a function of the relative alkali content.