Novel electronic states realized by the interplay between Li diffusion and Co$^{\rm 3+}$$/$Co$^{\rm 4+}$ charge ordering in Li$_{\rm x}$CoO$_2$

TL;DR

This study investigates how Li diffusion and charge ordering between Co$^{3+}$ and Co$^{4+}$ ions create novel electronic states in Li$_x$CoO$_2$, revealing phase transitions and disorder effects through magnetization and resistivity measurements.

Contribution

It uncovers the interplay between Li diffusion and charge ordering, demonstrating how rapid cooling induces disordered states with increased resistivity and proposing an electronic phase diagram for Li$_x$CoO$_2$.

Findings

Charge ordering occurs at ~155 K for 0.6≤x≤0.98.

Rapid cooling induces Li diffusion anomalies at ~370 K and 120-130 K.

Disordered Co ion arrangements lead to increased resistivity and insulating states.

Abstract

Measurements of dc magnetization ($M$) and electrical resistivity ($\rho$) have been carried out as a function of temperature ($T$) for layered oxide Li$_{\rm x}$CoO$_2$ (0.51$\leq$$x$$\leq$1.0) using single crystal specimens. After slow cool of the specimens down to 10 K, both of the $M$($T$) and $\rho$($T$) curves are found to exhibit a clear anomaly due to the occurrence of Co$^{\rm 3+}$$/$Co$^{\rm 4+}$ charge ordering (CO) at $T_{\rm S}$$\sim$155 K for 0.6$\leq$$x$$\leq$0.98 (at $T_{\rm S}$$\sim$180$-$190 K for 0.5$\leq$$x$$\leq$0.55). After rapid cool of the specimens, additional anomalies are observed related to the onset of Li diffusion at $T_{\rm F1}$$\sim$370 K and$/$or $T_{\rm F2}$=120$-$130 K. Due to phase mixing with compositions of nearly LiCoO$_2$ and Li$_{\rm 2/3}$CoO$_2$, the specimens with 0.7$\lesssim$$x$$\lesssim$0.9 show anomlies both at $T_{\rm F1}$ and $T_{\rm F2}$. For 0.6$\lesssim$$x$$\lesssim$0.9, the resistivity measured after rapid cool is found to be fairly larger than that measured after slow cool below $T_{\rm S}$. The enhanced resistivity can be explained by the scenario that disordered Co$^{\rm 3+}$$/$Co$^{\rm 4+}$ arrangements, which have been observed and revealed to have an insulating electronic structure contrasting to the regular CO state in the previous scanning tunneling microscopy measurements [Phys. Rev. Lett. {\bf 111}, 126104 (2013)], are realized due to the formation of an amorphous-like structure of Li ions after rapid cool via the interlayer Coulomb coupling. An electronic phase diagram for 0.5$\leq$$x$$\leq$1.0 is proposed.