Chemical composition and crystal structure of superconducting sodium cobalt oxide bilayer-hydrate

Abstract

Sodium cobalt oxide bilayer-hydrate, Na_xCoO₂·yH₂O (x ≈ 0.35), was characterized by chemical and structure analyses. The formal valence of Co was found to be +3.4 from redox titration, which was much lower than +3.65 that had been estimated from the Na content, x. The deviation from the charge neutralization was considered to be compensated by the presence of oxonium ions in the CoO₂ galleries, which was evidenced by Raman spectroscopy. Therefore, the composition of the superconducting phase was determined to be Na_0.343(H₃O)_0.237CoO₂·1.19H₂O. Structural studies based on Raman spectroscopy and synchrotron X-ray powder diffraction suggested that the oxonium ions occupy the same crystallographic sites as the Na⁺ ions.