Relaxation mechanisms and effects of motion in albite (NaAlSi3O8) liquid and glass: A high temperature NMR study

The nuclear magnetic relaxation of ²³Na and ²⁹Si in albite glass and liquid has been studied from 800 K to 1400 K. The dominant spin-lattice relaxation mechanism for ²³Na is found to be nuclear quadrupole interaction arising from the Na⁺ diffusion. The activation energy of the Na diffusion is found to be 71±3 kJ/mol, in close agreement with the results on electrical conductivity and on Na self-diffusion from radio-tracer experiments. The correlation time of the Na motion is estimated to be about 8.5×10^?11 s near the melting point (～1390 K). Both nuclear dipole-dipole interaction and chemical shift anisotropy interaction are large enough to contribute to the ²⁹Si relaxation. However, calculations based on a simplified model which employ single correlation time and exponential correlation function, with interactions typical of crystalline silicates, cannot completely account for the experimental data. NMR relaxation data also reveal that the Si motion is correlated to the Na motion and that the Si is relatively immobile. Several possible motions of SiO₄ tetrahedra that can cause ²⁹Si relaxation are suggested. The motion responsible for ²⁹Si relaxation differs from that which is responsible for viscosity: the apparent activation energy for the former is much lower. Measurements of spin-spin relaxation times and linewidths are also presented and the significance of their temperature dependence is discussed.