Strong correlation between y'⁄x and superconductivity of Na_x(H₃O)_zCoO₂·y'H₂O

Abstract

We prepared single-phase Na_x(H₃O)_zCoO₂·y'H₂O with varying amount of Br₂ volume in acetonitrile, and examined the Na⁺ content (x), H₃O⁺ content (z), H₂O content (y') and T_c. By changing the amount of Br₂ during preparation, the amount of x in Na_x(H₃O)_zCoO₂·y'H₂O was controlled from 0.282 to 0.359. It was found that superconductivity is not correlated with Co valence but with y'/x, that is, the coordination number of Na⁺ with H₂O, and secondly with x⁄x+z, that is, the ratio of Na⁺ ion in the (Na⁺ + H₃O⁺) site, suggesting that shielding effect of Na_x(H₃O)_zCoO₂·y'H₂O governs superconductivity.