Valence and Na content dependences of superconductivity in NaxCoO2•yH2O

In order to acquire knowledge of the general superconducting phase diagram of sodium cobalt oxyhydrate, various samples with relatively large amounts of Na+ ions were synthesized by a modified soft-chemical process in which a NaOH aqueous solution was added in the final step of the procedure. From these samples, a superconducting phase diagram was determined for a section of cobalt valence of similar to+3.48, which was compared with a previously obtained one of similar to+3.40. In addition, it was proven that the superconductivity was significantly affected by the isovalent exchange of Na+ and H3O+, implying that the e(g)(') bands play an important role in the superconductivity. Furthermore, the high-field magnetic susceptibility measurements for one sample indicated an upper critical field much higher than 8 T.