NMR Study of LiCo_1-xCr_xO₂ and Li_1-xNa_xCoO₂ ( x=0 and 0.05)

抄録

The ⁷Li and ⁶Li nuclear spin-lattice relaxation rates, (⁷T₁)^-1 and (⁶T₁)^-1, have been measured for LiCoO₂ as a function of temperature. Comparison between ⁷Li- and ⁶Li-rich samples shows that the ratio (⁶T₁)^-1/(⁷T₁)^-1=0.06 is considerably larger than the value expected for quadrupolar relaxation processes. Furthermore (⁷T₁)^-1 decreases significantly as ⁷Li is replaced by ⁶Li. These observations indicate that (⁷T₁)^-1 is dominated by the nuclear magnetic dipole-dipole interaction between the ⁷Li nuclei. The substitution of Cr for Co has no effect on (⁷T₁)^-1 in the range below 200, K, whereas the substitution of Na for Li enhances (⁷T₁)^-1. The substitution effect supports the view that the two-dimensional Li ion motion is dominant in LiCoO₂. The weak dependence of (⁷T₁)^-1 upon temperature and frequency suggests the presence of a broad distribution of barrier heights for ionic motion in LiCoO₂.