NA-ION TRANSPORT IN BORATE AND GERMANATE GLASSES AND LIQUIDS - A NA-23 AND B-11 NMR SPIN-LATTICE-RELAXATION STUDY

The temperature dependence of the spin-lattice relaxation times (T-1) of Na-23 and B-11 nuclides has been studied in a number of Na-borate a nd -germanate glasses and liquids in order to understand the effects o f the type of network, concentration of the modifying cation, and netw ork former-modifier interaction on the transport of Na cations. Our re sults indicate that at low alkali contents the dynamics of both the ne twork former and the modifier are strongly coupled to the glass transi tion and mixed alkali effect on spin-lattice relaxation is absent. Thi s can be explained by the lack of any percolation pathway at low alkal i concentrations for the long-range diffusion of the Na cations. Event ually, such continuous pathways for diffusion form with increasing Na concentration (Na2O greater than or equal to 15-20 mol %) and a mixed alkali effect becomes observable, indicating decoupling of the dynamic s of the network-former and -modifier ions. Simulation of the <(T)over bar (1)> data with a barrier height distribution model also indicates such a transition from homogeneous to percolative transport with incr easing Na2O content. These phenomena are less pronounced in germanates than in berates, indicating a stronger coupling between the Na ions a nd the network in the latter.