LITHIUM, SODIUM AND POTASSIUM-TRANSPORT IN FAST ION CONDUCTING GLASSES - TRENDS AND MODELS

The ionic conductivities, glass transition temperatures (T(g)) and densities were examined for an extensive number of glasses in the system R2O-RCl-B2O3 (R = Li, Na, K). In general, increases in RCI resulted in systematic and sharp increases in alkali ion conductivity and decreases in T(g) and density. Linear decreases in the activation energy for conduction and T(g) on chlorine addition are traced to major changes in the network structure. Substitution of chlorine for oxygen serves to dilate the structure, thereby lowering the strain energy associated with migration of ions between near equivalent sites. The conductivity for a given glass composition decreases as the cation radius increases in the order lithium, sodium and potassium. Although there are large differences in ion size, the 0.81 eV activation energy for potassium ion motion in the diborate glass is only marginally different from that of sodium (0.77 eV) and lithium (0.74 eV). The larger alkali ions induce a dilation in the glass structure which compensates, at least in part, for the larger bottleneck size required for the larger ions in their motion through the structure. Chlorine additions produce more complex effects in the potassium glasses and are less efficient in enhancing the ionic conductivity. These observations are discussed with respect to the competing roles of excess volume and stiffness of the structure in controlling transport in these glasses.