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We report the synthesis and superconducting properties of a metastable formof the known superconductor NaxCoO2*yH2O (x ~ 1/3, y ~ 4x). Instead of usingthe conventional bromine-acetonitrile mixture for sodium deintercalation, weuse an aqueous bromine solution. Using this method, we oxidize the sample to apoint that the sodium cobaltate becomes unstable, leading to formation of otherproducts if not controlled. This compound has the same structure as thereported superconductor, yet it exhibits a systematic variation of thesuperconducting transition temperature (Tc) as a function of time. Immediatelyafter synthesis, this compound is not a superconductor, even though it containsappropriate amounts of sodium and water. The samples become superconductingwith low Tc values after ~ 90 h. Tc continually increases until it reaches amaximum value (4.5 K) after about 260 h. Then Tc drops drastically, becomingnon-superconducting approximately 100 h later. Corresponding time-dependentneutron powder diffraction data shows that the changes in superconductivityexhibited by the metastable cobaltate correspond to slow formation of oxygenvacancies in the CoO2 layers. In effect, the formation of these defectscontinually reduces the cobalt oxidation state causing the sample to evolvethrough its superconducting life cycle. Thus, the dome-shaped superconductingphase diagram is mapped as a function of cobalt oxidation state using a singlesample. The width of this dome based on the formal oxidation state of cobalt isvery narrow - approximately 0.1 valence units wide. Interestingly, the maximumTc in NaxCoO2*yH2O occurs when the cobalt oxidation state is near 3.5. Thus, wespeculate that the maximum Tc occurs near the charge ordered insulating statethat correlates with the average cobalt oxidation state of 3.5.Comment: 22 pages, 9 figures, 1 tabl

physical review. b, condensed matter and materials physics

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Superconductivity and cobalt oxidation state in metastableNaxCoO2−δ∙yH2O<…