Tuning Co Valence State in Cobalt Oxyhydrate Superconductor by Postreduction

We report a successful tuning of Co valence state in cobalt oxyhydrate superconductor via a facile postreduction using NaOH as reducing agent. The change in Co valence was precisely determined by measuring the volume of the released oxygen. The possible hydronium incorporation was greatly suppressed in concentrated NaOH solution, making the absolute Co valence determinable. As a result, an updated superconducting phase diagram was obtained, which shows that the superconducting transition temperature increases monotonically with increasing Co valence in a narrow range from +3.58 to +3.65.