Charge ordering in charge-compensated $Na_{0.41}CoO_2$ by oxonium ions

TL;DR

This study demonstrates that immersing Na0.41CoO2 in water induces charge ordering by reducing Na content and intercalating oxonium ions, aligning the Co valence with that of Na0.5CoO2, revealing the importance of Co valence in charge ordering.

Contribution

It reveals that water immersion causes charge ordering in Na0.41CoO2 through oxonium ion intercalation, a novel method to induce charge ordering in this material.

Findings

Charge ordering appears after water immersion.

Co valence shifts from +3.59 to +3.5.

Charge ordering similar to Na0.5CoO2 observed.

Abstract

Charge ordering behavior is observed in the crystal prepared through the immersion of the $Na_{0.41}CoO_2$ crystal in distilled water. Discovery of the charge ordering in the crystal with Na content less than 0.5 indicates that the immersion in water brings about the reduction of the $Na_{0.41}CoO_2$. The formal valence of Co changes from +3.59 estimated from the Na content to +3.5, the same as that in $Na_{0.5}CoO_2$. The charge compensation is confirmed to arise from the intercalation of the oxonium ions as occurred in the superconducting sodium cobalt oxide bilayer-hydrate.\cite{takada1} The charge ordering is the same as that observed in $Na_{0.5}CoO_2$. It suggests that the Co valence of +3.5 is necessary for the charge ordering.