Synthesis of NaxAy(H3O)zCoO2 ⋠wH2O (A=Mg, Ca, Sr and Ba) superconductors via ion exchange

We have synthesized NaxAy(H3O)zCoO2⋅wH2O (A=Mg, Ca, Sr and Ba) superconductors via ion exchange at ambient conditions. The extent of incorporation of alkaline-earth elements was found to be related to the similarity in size between the guest ion and the host one. In the case of A=Ca, about half of the sodium was successfully replaced by calcium. All the synthesized samples were shown to be superconducting with the Tc from 3.1 to 4.5 K.