Phase formation and phase stability of KMnO4-treated (Na,K)CoO2.yH2O

TL;DR

This study explores how KMnO4 treatment affects the phase formation and stability of (Na,K)CoO2.yH2O, revealing conditions for superconductivity and the effects of environmental humidity on phase stability.

Contribution

It introduces a method using aqueous KMnO4 to synthesize and analyze superconductive cobalt oxyhydrate phases with controlled composition and stability.

Findings

Superconductivity observed at Tc=3.2-4.6K.

Higher KMnO4/Na ratios lead to K+ substitution and non-superconductive phases.

Superconductive phase is unstable in ambient air but reversible with humidity control.

Abstract

We have first used the aquesous KMnO4 solutions to de-intercalate and oxideze gamma-phase of Na0.7CoO2 and successfully form the superconductive phase of cobalt oxyhydrate hydrates (Na,K)xCoO2.yH2O with Tc=3.2-4.6K based on the magnetization measurements. The higher molar ratio of KMnO4/Na used to treat Na0.7CoO2 results in more removal of Na+ and leads to a partial or even complete replacement of K+ for Na+. The low molar ratio of KMnO4/Na forms a superconductive phase with the c-axis ca. 19.6 angstrom, whereas the high molar ratio of KMnO4/Na forms a non-superconductive phase with teh c-axis ca. 13.9 angstrom. The superconductive 19.6 angstrom phase is unstable with respect to the ambient air in terms of losing water molecule from the structure; nevertheless, the dehydration/hydration process is reversible when storing the sample in a chamber with sufficient humidity.