Na order and Co charge disproportionation in Na$_{x}$CoO$_{2}$

TL;DR

This study uses NMR techniques to reveal sodium order and cobalt charge disproportionation in Na$_{x}$CoO$_{2}$, showing that only a small fraction of cobalt ions are non-magnetic Co$^{3+}$, with charge and magnetic properties highly correlated.

Contribution

It provides detailed NMR evidence of Na order and Co charge disproportionation in Na$_{x}$CoO$_{2}$, highlighting the correlation between magnetic shifts and charge states.

Findings

Na order is confirmed in each phase.

Only about 25% of Co are non-magnetic Co$^{3+}$.

Magnetic shift scales linearly with quadrupolar frequency.

Abstract

We have synthesized and characterized different stable phases of sodium cobaltates Na$_{x}$CoO$_{2}$ with sodium content $0.65<x<0.80$. We demonstrate that $^{23}$Na NMR allows to determine the difference in the susceptibility of the phases and reveals the presence of Na order in each phase. $^{59}$Co NMR experiments give clear evidence that Co charge disproportionation is a dominant feature of Na cobaltates. Only a small fraction ($\approx$ 25%) of cobalts are in a non-magnetic Co$^{3+}$ charge state whereas electrons delocalize on the other cobalts. The magnetic and charge properties of the different Co sites are highly correlated with each other as their magnetic shift $K_{ZZ}$ scales linearly with their quadrupolar frequency $nu_Q$. This reflects the fact that the hole content on the Co orbitals varies from site to site. The unusual charge differentiation found in this system calls for better theoretical understanding of the incidence of the Na atomic order on the electronic structures of these compounds.