Role of doping and dimensionality in the superconductivity of NaxCoO2

We report a complete analysis of the formal Co3+/(4+) oxidation state in NaxCoO2, in the interval 0.31 <= x <= 0.69. lodometric titration and thermoelectric power confirm that a direct relationship between the Na content and the amount of Co3+ cannot be established in this system. Creation of a significant amount of oxygen vacancies accompanies Na-ion deintercalation, keeping the formal Co valence at 3.45+ for x <= 0.45. In light of new thermoelectric power data which reveals important differences between the hydrated (superconducting) and nonhydrated (nonsuperconducting) samples, we propose here that water plays an important "chemical" role beyond that of a spacer between the CoO2 layers.