Magnetic and Electronic Properties of Li$_x$CoO$_2$ Single Crystals

TL;DR

This study investigates the electrical, magnetic, and thermal properties of Li$_x$CoO$_2$ single crystals across various compositions, revealing a temperature-driven phase transition, Li ion ordering, and their correlation with charge ordering in CoO$_2$ layers.

Contribution

It provides detailed experimental insights into the phase transitions and Li ion ordering in Li$_x$CoO$_2$ single crystals, highlighting the interplay between charge and ion ordering.

Findings

Insulating behavior for x=0.90 and 0.99; metallic for 0.25≤x≤0.71.

First-order transition at T_S≈155 K observed across a range of x.

Li ions order below T_F≈130 K, correlated with charge ordering in CoO$_2$ layers.

Abstract

Measurements of electrical resistivity ($\rho$), DC magnetization ($M$) and specific heat ($C$) have been performed on layered oxide Li$_x$CoO$_2$ (0.25$\leq$$x$$\leq$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$\leq$$x$$\leq$0.71. At $T_{\rm S}$$\sim$155 K, a sharp anomaly is observed in the $\rho$$-$$T$, $M$$-$$T$ and $C$$/$$T$$-$$T$ curves for $x$=0.66 with thermal hysteresis, indicating the first-order charactor of the transition. The transition at $T_{\rm S}$$\sim$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 $T_{\rm F}$, which is $\sim$130 K for $x$=0.46$-$0.71. $T_{\rm F}$ is found to agree with the temperature at which the motional narrowing in the $^7$Li NMR line width is observed, indicating that the Li ions stop diffusing and order at the regular site below $T_{\rm F}$. The ordering of Li ions below $T_{\rm F}$$\sim$130 K is likely to be triggered and stabilized by the charge ordering in CoO$_2$ layers below $T_{\rm S}$.