Thermal and Electrical Properties of gamma-NaxCoO2 (0.70 < x < 0.78)

TL;DR

This study investigates the thermal and electrical properties of Na$_{x}$CoO$_{2}$ for $x$ between 0.70 and 0.78, revealing phase separation, magnetic transitions, and sensitivity of properties to sodium content.

Contribution

It provides detailed measurements of specific heat and resistivity across different sodium concentrations, clarifying previous inconsistent results and identifying phase separation and magnetic instability regions.

Findings

Magnetic transitions at 22 K and 9 K for $x=0.78$

Resistivity shows a steep decrease at $T_c$ and a bend at $T_b$ (~120K)

Physical properties are highly sensitive to sodium content, with phase separation at higher $x$ and magnetic instability near $x=0.70$

Abstract

We have performed specific heat and electric resistivity measurements of Na$_{x}$CoO$_{2}$ ($x=0.70$-0.78). Two anomalies have been observed in the specific heat data for $x=0.78$, corresponding to magnetic transitions at $T_{c}=22$ K and $T_{k}\simeq 9$ K reported previously. In the electrical resistivity, a steep decrease at $T_{c}$ and a bending-like variation at $T_{b}$(=120K for $x=0.78$) have been observed. Moreover, we have investigated the $x$-dependence of these parameters in detail. The physical properties of this system are very sensitive to $x$, and the inconsistent results of previous reports can be explained by a small difference in $x$. Furthermore, for a higher $x$ value, a phase separation into Na-rich and Na-poor domains occurs as we previously proposed, while for a lower $x$ value, from characteristic behaviors of the specific heat and the electrical resistivity at the low-temperature region, the system is expected to be in the vicinity of the magnetic instability which virtually exists below $x=0.70$.