Anomalous transport properties of Co-site impurity doped NaxCoO2

The effect of substitution of Cu, Zn, Ga, Mn, and Ni for Co in layered NaxCoO2 (x similar to 0.85) on its resistivity (rho), magnetization (M), and thermopower (S) has been investigated. At low temperature, rho remains metallic (dp/dT>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, rho(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 (theta) depend on the nature of impurity. A small enhancement in magnetization above 280 K and an increase of theta with Cu doping are the signatures of superior Na-vacancy ordering in the Cu-doped samples. In contrast to p and M, S changes systematically with doping and temperature, and does not show any anomalous behavior around 280 K. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2978212]