NMR-STUDY OF LICO1-XCRXO2 AND LI-1-XNAXCOO2 (X = 0 AND 0.05)

The Li-7 and Li-6 nuclear spin-lattice relaxation rates, (T-7(1))(-1) and (T-6(1))(-1), have been measured for LiCoO2 as a function of tempe rature. Comparison between Li-7- and Li-6- rich samples shows that the ratio (T-6(1))(-1)/(T-7(1))(-1) = 0.06 is considerably larger than th e value expected for quadrupolar relaxation processes. Furthermore (T- 7(1))(-1) decreases significantly as 7Li is replaced by Li-6. These ob servations indicate that (T-7(1)) is dominated by the nuclear magnetic dipole-dipole interaction between the Li-7 nuclei. The substitution o f Cr for Co has no effect on (T-7(1))(-1) in the range below 200 K, wh ereas the substitution of Na for Li enhances (T-7(1))(-1). The substit ution effect supports the view that the two-dimensional Li ion motion is dominant in LiCoO2. The weak dependence of (T-7(1))-1 upon temperat ure and frequency suggests the presence of a broad distribution of bar rier heights for ionic motion in LiCoO2.