Na atomic order, Co charge disproportionation and magnetism in Na$_{x}$CoO$_{2}$ for large Na contents

TL;DR

This study synthesizes and characterizes four stable phases of Na-rich Na$_{x}$CoO$_{2}$, revealing charge disproportionation, magnetic fluctuations, and phase-dependent magnetic ordering, advancing understanding of magnetism in layered cobaltates.

Contribution

It provides detailed insights into Na ordering, Co charge states, and magnetic behavior in Na$_{x}$CoO$_{2}$ for large Na contents, including evidence of charge disproportionation above 300 K.

Findings

Co disproportionates into non-magnetic Co$^{3+}$ and magnetic Co$^{3.5+}$ sites above 300 K

Ferromagnetic fluctuations observed in all phases above 100 K

Magnetic order appears below 100 K for $x ext{ } extgreater ext{ } 0.75$ at 22 K

Abstract

We have synthesized and characterized four different stable phases of Na ordered Na$_{x}$CoO$_{2}$, for $0.65<x<0.8$. Above 100 K they display similar Curie-Weiss susceptibilities as well as ferromagnetic $q=0$ spin fluctuations in the CoO$_{2}$ planes revealed by $^{23}$Na NMR data. In all phases from $^{59}$Co NMR data we display evidences that the Co disproportionate already above 300 K into non magnetic Co$^{3+}$ and magnetic $\approx $Co$^{3.5+}$ sites on which holes delocalize. This allows us to understand that metallic magnetism is favored for these large Na contents. Below 100 K the phases differentiate, and a magnetic order sets in only for $x\gtrsim 0.75$ at $T_{N}=$22 K. We suggest that the charge order also governs the low $T$ energy scales and transverse couplings.