Oxidation State of Cobalt in the NaxCoO2-d yH2O Superconductor

TL;DR

This study accurately measures the oxidation state of cobalt in the NaxCoO2 yH2O superconductor, revealing a lower value than expected, which suggests the presence of oxygen vacancies or protons and a hole-doped lattice structure.

Contribution

The paper provides precise redox analysis of cobalt's oxidation state in NaxCoO2 yH2O, challenging previous assumptions and proposing a new understanding of its doping mechanism.

Findings

Cobalt oxidation state is approximately 3.46.

The oxidation state is lower than expected based on Na content.

The phase is likely hole-doped rather than electron-doped.

Abstract

Here we present results of accurate wet-chemical redox analyses, revealing that the oxidation number of cobalt in the newly discovered CoO2-layer superconductor, NaxCoO2 yH2O (x ~ 0.36, y ~ 1.3) is ~ 3.46. This value is significantly lower than the one (~ 3.64) expected on the basis of the value determined for the Na to Co stoichiometry ratio, x. The "lower-than-expected" value for the Co oxidation state may be alternatively explained by presence of oxygen vacancies or excess protons. Moreover, the result implies that rather than an electron-doped Co(IV) lattice the phase should be considered as a hole-doped Co(III) lattice.