FAST ROTATIONAL REORIENTATION OF NO3- IONS AND ITS RELATION TO THE GLASS-TRANSITION IN CA0.4K0.6(NO3)(1.4) LIQUID - A N-15 NMR SPIN-LATTICE-RELAXATION STUDY

The fast reorientational dynamics of the NO3- ions in the supercooled Ca0.4K0.6(NO3)(1.4) (CKN) liquid have been investigated in the tempera ture range 370 K less than or equal to T less than or equal to 440 K b y N-15 NMR spin-lattice-relaxation spectroscopy. The spin-lattice rela xation of the N-15 nuclides is shown to be controlled by the spin rota tion process. The correlation time tau(c) of the rapid reorientational dynamics of the NO3- ions associated with spin rotation is found to b e strongly decoupled from the shear/structural relaxation time scale i n this temperature range, varying from 10(-11.8) s at 370 K to 10(-12. 5) s at 440 K. An anomatous bifurcation in tau(c) into a slow and a fa st branch is observed at T less than or equal to 365 K and is found to be intimately related to the appearance of a bimodal spatial heteroge neity in the overall reorientation or tumbling dynamics of the NO3- io ns in the supercooled CKN liquid near glass transition. In spite of th e large decoupling in the time scale, the fast rotational reorientatio n is shown to be intimately related to the slow tumbling dynamics, res ulting in a two-step decay of the orientational correlation of the NO3 - ions. The results are found to be compatible with the predictions of the mode coupling theory and offer significant physical insight into the hierarchical nature of the dynamics associated with glass transiti on.