Superconductivity and Cobalt Oxidation State in Metastable Na(x)CoO(2-delta)*yH2O (x ~ 1/3; y ~ 4x)
We report the synthesis and superconducting properties of a metastable form of the known superconductor NaxCoO2*yH2O (x ~ 1/3, y ~ 4x). Instead of using the conventional bromine-acetonitrile mixture for sodium deintercalation, we use an aqueous bromine solution. Using this method, we oxidize the sample to a point that the sodium cobaltate becomes unstable, leading to formation of other products if not controlled. This compound has the same structure as the reported superconductor, yet it exhibits a systematic variation of the superconducting transition temperature (Tc) as a function of time. Immediately after synthesis, this compound is not a superconductor, even though it contains appropriate amounts of sodium and water. The samples become superconducting with low Tc values after ~ 90 h. Tc continually increases until it reaches a maximum value (4.5 K) after about 260 h. Then Tc drops drastically, becoming non-superconducting approximately 100 h later. Corresponding time-dependent neutron powder diffraction data shows that the changes in superconductivity exhibited by the metastable cobaltate correspond to slow formation of oxygen vacancies in the CoO2 layers. In effect, the formation of these defects continually reduces the cobalt oxidation state causing the sample to evolve through its superconducting life cycle. Thus, the dome-shaped superconducting phase diagram is mapped as a function of cobalt oxidation state using a single sample. The width of this dome based on the formal oxidation state of cobalt is very narrow - approximately 0.1 valence units wide. Interestingly, the maximum Tc in NaxCoO2*yH2O occurs when the cobalt oxidation state is near 3.5. Thus, we speculate that the maximum Tc occurs near the charge ordered insulating state that correlates with the average cobalt oxidation state of 3.5.