Phase Diagram of Superconducting NaxCoO2·yH2O

We discovered that oxonium (H 3 O + ) ion content is a key parameter to the understanding of unique superconductivity in sodium cobalt oxyhydrate. As Na + ions are substituted by H 3 O + ions, keeping the Co valence constant, the superconducting phase is transformed into a magnetically ordered phase, and then into a hitherto hidden superconducting phase. Furthermore, the superconducting phase previously discovered is suppressed by the magnetic phase under application of a magnetic field. These facts indicate that the superconductivity is induced by magnetic interactions which are substantially governed by the oxonium ion content.