LI-7-NMR STUDY OF DIFFUSION-INDUCED SPIN-LATTICE RELAXATION IN GLASSY AND CRYSTALLINE LIALSI2O6

In order to study the effect of structural disorder on the diffusion of mobile ions, both the glass Li2O . Al2O3 . 4SiO2 and its crystalline counterpart (beta-spodumene) were investigated by Li-7-NMR. The spin-lattice relaxation (SLR) time T1 of Li-7 was measured as a function of temperature T from 150 K to about 960K in the glass (glass transition temperature) and 1300 K in the polycrystalline sample. The frequency nu was varied in the range 10 MHz to 100 MHz. Both in the glass and the crystal a diffusion-induced peak in T1(-1)(T) was observed. Comparison of the positions of the maxima and the slopes of the low-T sides of log T1(-1) vs 1/T curves shows that the Li+ jump rate in the glass is higher and the activation energy is lower than in the crystal. In both cases the nu and T dependences of the diffusional SLR rate strongly deviate from the BPP model. In particular, the nu dependence of T1(-1) on the low- T sides is sublinear or, at most, linear. The activation energies obtained from the temperature regime above the maximum and those below at various frequencies are briefly compared with corresponding values from a.c. impedance measurements on the same samples.