Revised superconducting phase diagram of hole-doped Nax(H3O)zCoO2•yH2O -: art. no. 247007

We have studied the superconducting phase diagram of NaxCoO2.yH(2)O as a function of electronic doping, characterizing our samples both in terms of Na content x and the Co valence state. Our findings are consistent with a recent report that intercalation of H3O+ ions into NaxCoO2, together with water, acts as an additional dopant, indicating that Na substoichiometry alone does not control the electronic doping of these materials. We find a superconducting phase diagram where optimal T-C is achieved through a Co valence range of 3.24-3.35, while T-C decreases for materials with a higher Co valence. The critical role of dimensionality in achieving superconductivity is highlighted by similarly doped nonsuperconducting anhydrous samples, differing from the superconducting hydrate only in interlayer spacing.