Anomalous transport properties of Co-site impurity doped Na_xCoO₂

Abstract

The effect of substitution of Cu, Zn, Ga, Mn, and Ni for Co in layered Na_xCoO₂ (x∼0.85) on its resistivity (p), magnetization (M), and thermopower (S) has been investigated. At low temperature, p remains metallic (dp/d>0) for Cu, Zn, and Ga impurities up to a maximum doping of 15%, while for Mn and Ni a metal-insulator transition occurs at 3% and 6% doping, respectively. For the Cu-doped samples, p(T) shows a strong anomaly and a hysteresis around 280 K where a first-order structural phase transition occurs due to long-range ordering of the Na-vacancy clusters. Well above the antiferromagnetic transition temperature, the magnetic susceptibility exhibits the Curie–Weiss law. The Neel temperature is insensitive to impurity while both the Curie constant and Weiss constant (θ) depend on the nature of impurity. A small enhancement in magnetization above 280 K and an increase of