Electronic states realized by the interplay between Li diffusion and Co3+/Co4+ charge ordering in LixCoO2

Abstract

Measurements of dc magnetization $(M)$ and electrical resistivity $(\ensuremath{\rho})$ have been carried out as a function of temperature $(T)$ for layered oxide ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2} (0.51\ensuremath{\le}x\ensuremath{\le}1.0)$ using single-crystal specimens. After slow cooling of the specimens down to 10 K, both of the $M(T)$ and $\ensuremath{\rho}(T)$ curves are found to exhibit a clear anomaly due to the occurrence of ${\mathrm{Co}}^{3+}/{\mathrm{Co}}^{4+}$ charge ordering (CO) at ${T}_{\mathrm{S}}\ensuremath{\sim}155\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ for $0.6\ensuremath{\le}x\ensuremath{\le}0.98$ (at ${T}_{\mathrm{S}}\ensuremath{\sim}180--190\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ for $0.5\ensuremath{\le}x\ensuremath{\le}0.55)$. After rapid cooling of the specimens, additional anomalies are observed related to the onset of Li diffusion at ${T}_{\mathrm{F}1}\ensuremath{\sim}370\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and/or ${T}_{\mathrm{F}2}=120--130\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Due to phase mixing with compositions of nearly ${\mathrm{LiCoO}}_{2}$ and ${\mathrm{Li}}_{2/3}{\mathrm{CoO}}_{2}$, the specimens with $0.7\ensuremath{\lesssim}x\ensuremath{\lesssim}0.9$ show anomalies both at ${T}_{\mathrm{F}1}$ and ${T}_{\mathrm{F}2}$. For $0.6\ensuremath{\lesssim}x\ensuremath{\lesssim}0.9$, the resistivity measured after rapid cooling is found to be fairly larger than that measured after slow cooling below ${T}_{\mathrm{S}}$. The enhanced resistivity can be explained by the scenario that disordered ${\mathrm{Co}}^{3+}/{\mathrm{Co}}^{4+}$ 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 [K. Iwaya et al., Phys. Rev. Lett. 111, 126104 (2013)], are realized due to the formation of an amorphouslike structure of Li ions after rapid cooling via interlayer Coulomb coupling. An electronic phase diagram for $0.5\ensuremath{\le}x\ensuremath{\le}1.0$ is proposed.