EXAMINATION OF THE MIXED-ALKALI EFFECT IN (LI,NA) DISILICATE GLASSES BY NUCLEAR-MAGNETIC-RESONANCE AND CONDUCTIVITY MEASUREMENTS

Results from Si-29, Na-23 and Li-7 magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy, Li-7 NMR relaxation and electrical conductivity in a series of [Li-(1-x). Na-x]O-2 . 2SiO(2) (disilicate) glasses are used to investigate the mixed-alkali effect. From the Si-29 NMR spectra there is relatively little change of the network with alkali composition. Na-23 and Li-7 NMR linewidths and shifts change continuously as a function of composition, indicating that the alkali ions are intimately and uniformly mixed rather than separated into lithium and sodium-rich domains. The activation energy from electrical conductivity shows a distinct maximum at the central composition (x = 0.5), whereas the local activation energy for lithium motion determined from NMR shows only a smaller but monotonic increase as the lithium-content decreases.