Magnetic and electronic properties of LixCoO2 single crystals

Measurements of electrical resistivity (rho), dc magnetization (M), and specific heat (C) have been performed on layered oxide LixCoO2 (0.25 <= x <= 0.99) using single-crystal specimens. The rho versus temperature (T) curve for x=0.90 and 0.99 is found to be insulating but a metallic behavior is observed for 0.25 <= x <= 0.71. At T-S similar to 155 K, a sharp anomaly is observed in the rho-T, M-T, and C/T-T curves for x=0.66 with thermal hysteresis, indicating the first-order character of the transition. The transition at T-S similar to 155 K is observed for the wide range of x=0.46-0.71. It is found that the M-T curve measured after rapid cool becomes different from that after slow cool below T-F, which is similar to 130 K for x=0.46-0.71. T-F is found to agree with the temperature at which the motional narrowing in the Li-7 NMR linewidth is observed, indicating that the Li ions stop diffusing and order at the regular site below T-F. The ordering of Li ions below T-F similar to 130 K is likely to be triggered and stabilized by the charge ordering in CoO2 layers below T-S.