Role of Doping and Dimensionality in the Superconductivity of NaxCoO2 (original) (raw)

We report a complete analysis of the formal Co 3+/4+ oxidation state in Na x CoO 2 , in the interval 0.31 ≤ x ≤ 0.67. Iodometric titration and thermoelectric power confirm that a direct relationship between the Na content and the amount of Co 3+ 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. To the light of new thermoelectric power data which reveals important differences between the hydrated (superconducting) and non-hydrated (non-superconducting) samples, we propose here that water plays an important "chemical" role beyond that of a spacer between the CoO 2 layers.