Nuclear magnetic resonance studies of 7Li ionic motion in alkali silicate and borate glasses

Abstract

Nuclear magnetic resonance (NMR) studies of ⁷Li ionic motion in four lithium and lithium‐potassium silicate glasses of the R₂O·4SiO₂ and R₂O·2SiO₂ composition were made before and after heat treatment, and a newly developed equation was used for the analysis of the data. Other data from alkali borate glasses of the 3R₂O·7B₂O₃ composition were reanalyzed using the equation, which eliminates the problem of a distribution of ionic relaxation times. It is shown how heterogeneities in the form of barriers to alkali ion motion in the glasses can be detected and studied by NMR. Although all glasses with higher alkali contents appear to be homogeneous, both the borates and silicates with lower alkali contents appear to be heterogeneous. These heterogeneities appear to be on a smaller scale in the borates than in the silicates. In addition to long‐range ⁷Li ionic motion observed in homogeneous glasses (and intermediate‐range motion in heterogeneous glasses), there is evidence of short‐range motion within severely restricted regions in nearly all glasses studied. The data show that the long‐range ⁷Li ionic motion in glasses proceeds by an interstitial process. The ⁷Li NMR data of several crystallized lithium silicate glasses in which polycrystalline lithium disilicate was formed reveal that the local atomic structure in the crystal differs from that of the glasses. A large difference found between the activation energies for ⁷Li long‐range motion in glassy (0.648 eV) and crystalline (2.19 eV) lithium disilicate supports this conclusion. Changes of the ⁷Li activation energy in lithium borate glasses upon partial substitution of lithium‐7 by other alkali species follow a trend in accord with an explanation of the mixed‐alkali effect in glass previously proposed by the authors.