Spin relaxation and ultra-slow Li motion in an aluminosilicate glass ceramic.

The ion dynamics in a lithium aluminosilicate glass ceramic was studied using stimulated-echo 7Li-NMR. For temperatures 300 K<T<450 K the hopping correlation times, tau(C), were determined from the decay of the quadrupolar spin-alignment amplitude. The decay times were thermally activated with an energy barrier of 0.61 eV, in agreement with conductivity measurements. For T<300 K the temperature dependence of the decay times, now corresponding to T1Q, was much weaker. The T1Q times followed the same trend as the independently measured spin-lattice relaxation times, T1, but were a factor of about three shorter than those. The theoretical ratio of T1/T1Q is shown to be 25/8 in the slow-motion regime if quadrupolar spin relaxation prevails. This result explains the present observation and similar ones made for several Li ion conductors. The relaxation of the octupolar spin-alignment order is discussed.