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

Abstract

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

8 pages, 5 figures, to appear in Phys. Rev. B