NMR-STUDY OF THE DYNAMICS OF NETWORK-GLASS-FORMING SYSTEMS - XNA2O.(1-X)B2O3

The temperature. dependence of the B-11 NMR line shapes and longitudinal-relaxation times up to 1000-degrees-C have been investigated in glassy and liquid sodium borates with compositions 0.09Na2O.0.91B2O3, 0.20Na2O.0.80B2O3, 0.33Na2O.0.67B2O3. The B-11 longitudinal-relaxation data are interpreted in terms of multiexponential quadrupolar relaxation, and the results are described by a reorientational correlation time tau(c). A crossover between two distinct orientational relaxation processes is observed at a temperature well above the glass-transition temperature T(g). Above the crossover temperature, the characteristics of the process are roughly similar to those of macroscopic viscosity. On the other hand, below the crossover temperature, the process is clearly distinct from that of the primary process, and is described as the secondary process. The temperature dependence of tau(c) of both processes is non-Arrhenius in the temperature range studied. The value of tau(c) corresponding to the secondary process is approximately 10(-7) s at T(g), and this process persists below T(g).