Structural and electronic properties of the triangular lattice system NaxCoO2

The structural and electronic properties of the triangular lattice system NaxCoO2 with nominal compositions of 0.7 <= x <= 1 annealed at temperatures from 923 to 1123 K have been explored by means of x-ray diffraction and through measurements of electrical resistivity, thermoelectric power and magnetization. All of the properties for the single-phase specimens are classified into two groups according to the annealing temperature, and a significant correlation exists between the transport and magnetic properties. The first-order phase transition and the irreversible transition for the transport properties, likely due to the ordering of Na ions and to the metastability in the local structure for the polycrystalline specimens, respectively, are also pointed out.