Mixed-Alkali Effects in Aluminophosphate Glasses: A Re-examination of the System [xNa2O(1 - x)Li2O]0.46[yAl2O3(1 - y)P2O5]0.54

Mixed-alkali effects in the system[xNa(2)O(1-x)-Li2O](0.46)[yAl(2)O(3)(1 - y)P2O5](0.54) (0 <= x <= 1; y = 0, 0.08, 0.16) have been examined using dc-conductivity measurements, thermal analysis, Raman spectroscopy, and various complementary solid-state NMR approaches. In contrast to the results of an earlier study reporting results on mixed-alkali aluminophosphate glasses with very similar chemical compositions (Faivre, A.; Viviani, D.; Phalippou, J. Solid State Ionics 2005, 176, 325), ionic conductivities and T-g values exhibit composition dependences typical of normal mixed-alkali effect behavior. The P-31 MAS NMR spectra have been quantitatively analyzed in terms of Q(m)((n)) units, where n denotes the number of P-O-P linkages and m is the number of Al atoms connected to P. The data suggest that the number of P-O-Al linkages is maximized, and that no Al-O-Al linkages occur. Na-23 triple-quantum MAS NMR studies indicate a monotonic trend in isotropic chemical shifts, consistent with intimate Na/Li mixing. In agreement with this conclusion, Na-23 spin echo decay measurements reveal a linear dependence of dipolar second moments M-2((NaNa)-Na-23-Na-23) on atomic number concentrations, indicating the absence of like-cation clustering, both in the pseudobinary phosphate and in the ternary aluminophosphate systems. Information about the cation dynamics can be obtained from Al-27-MAS NMR first-order quadrupolar spinning sideband patterns involving the vertical bar +/- 1/2 ><->vertical bar +/-(3/2 > Zeeman transitions. These data give clear evidence for a reduced cationic mobility in the mixed-alkali glasses. No significant differences in general behavior are observed between the binary alkali phosphate and the ternary alkali aluminophosphate systems.